According to this energy.gov web page, I consume about 130,000,000 Btu's each year. 130 million Btu's translates to about 38,000 kWh. A 1500 watt hair dryer running for an hour uses 1.5 kWh, so 38,000 kWh is a lot of hair-drying, like 25,000 hours worth.
An important consequence of my energy consumption is the carbon dioxide generated as a result. A really neat and easy-to-use calculator at nature.org tells me that I'm personally responsible for 30 tons of carbon dioxide every year through my energy use (assuming I drive a gasoline-powered car; more on this shortly). The average American is responsible for 27 tons, and the world average is 5.5 tons. I'm quite a carbon dioxide hog.
My energy use is divided into several portions -- transportation, heating, electricity -- the big three -- and some others.
In this post, I'll look at my transportation energy use, and what I can do with transportation to reduce my carbon dioxide footprint.
Transportation can be divided into several portions itself, including getting myself around, and getting the stuff I buy transported from where it is made to where I buy it (often across an ocean and then by land across a continent). In this post, I'll focus just on the transportation energy to get myself around.
Assuming I have a gasoline-powered car, and drive it, say, 12,000 miles each year, and get an average of 30 mpg, I use 400 gallons of gas per year. One gallon of gas produces 18 lbs of carbon dioxide, so my driving produces 7200 lbs; rounding up to 8000, this is 4 tons. So of my 30 tons, 4 tons results from driving.
About a year ago, we purchased an electric vehicle (EV), a Nissan Leaf. A question is whether driving an EV can significantly reduce the carbon dioxide produced by getting me around.
Let's assume I drive the same 12,000 miles with the Leaf. The Leaf averages about 4.8 miles per kWh (if I don't drive it aggressively or excessively fast), which means the Leaf consumes 2500 kWh in one year. For each generated kWh of electrical energy, about 1.25 lbs of carbon dioxide are produced, which means that the carbon dioxide produced by the electricity generation for the Leaf is 3125 lbs, or about 40% of the carbon dioxide my gasoline car produces. This is disappointing. I was hoping to find that the Leaf made my driving carbon dioxide generation essential nothing.
However the above overly-simplistic calculation cuts the Leaf short. For local driving, especially in traffic jams that are common in Massachusetts during commute time, the Leaf really shines. Let's see how taking that into account changes the carbon dioxide calculation.
In a traffic jam, especially during the first few miles when the engine is warming up, my gasoline car gets as little as 5 to 7 mpg (it has a display that shows mileage history, so I'm pretty confident in this number). The Leaf, in the meantime, gets as much as 7 or 8 miles per kWh under these conditions.
Let's say that one fourth of my driving is in congested traffic, or 3000 miles per year, at, say, 7 mpg for the gasoline-powered car. This corresponds to about 425 gallons of gasoline, resulting in 7700 lbs of carbon dioxide. So in slow traffic, the gasoline-powered car produces more carbon dioxide than it would all year if it never encountered traffic jams (should we be so lucky! :-) ).
Meanwhile, in traffic jams, the Leaf gets (conservatively) 7 miles per kWh. Driving 3000 miles at this mileage causes the Leaf to use only 425 kWh, which corresponds to about 530 lbs of carbon dioxide -- about a quarter of a ton. By comparison, the gasoline-powered car generates nearly 4 tons!
For the remaining 9000 miles of non-traffic jam driving, the gasoline-powered car produces 5400 lbs, or more than 2.5 tons, while the Leaf produces about 2400 lbs, or about 1.25 tons.
The bottom line, then, is that the gasoline-powered car's yearly output of carbon dioxide is 4 + 2.5 equals 6.5 tons, while the Leaf puts out 0.5 + 1.25 tons equals 1.75 tons, for a net reduction of about 5 tons!
So of my 30 tons of annual carbon dioxide output, probably a realistic figure is that about six tons would result from driving my gasoline-powered car. I drive the Leaf almost all of the time, and by doing so, my annual carbon dioxide output is five tons less than the calculator claims I produce. Thus my total yearly carbon dioxide output when I drive the Leaf is 25 tons, not 30.
It sounds like driving an EV makes sense if you're concerned about the rising carbon dioxide. Even better, it sounds like driving less, and especially avoiding traffic jams, would help significantly.
The next post will discuss some unfortunate things we've done to lock ourselves into high energy usage and carbon dioxide output.
Friday, May 27, 2016
Wednesday, May 25, 2016
Foray into Solar -- What Would It Take to Run the Basement Dehumidifiers on Solar?
A post a while back asked whether our basement dehumidifiers could be run off a renewable source of electricity, specifically solar panels. This idea seems to make sense since the dehumidifiers run mostly in the summer when solar intensity is at its peak.
Based on what we know from the foray into solar reported in the past few posts, the real question is: "What would it take to run the dehumidifiers off of batteries, and to recharge the batteries using solar?"
We can answer this in two parts: (a) how much battery capacity is needed to run the dehumidifiers? and (b) how much solar capacity is needed to recharge the batteries to allow the system to run 24/7?
Using what we've learned in the previous several posts, the place to start is to figure out how much energy the dehumidifiers use in 24 hours, then figure out what batteries are needed to hold this amount of energy (with some to spare to cover cloudy or excessively humid days), and then figure out how much energy solar panels need to collect to recharge the batteries fully.
The Kill-A-Watt measurements of the dehumidifiers estimate that two units consume 7+ kWh per day. Let's say 8 kWh to be on the safe side. Just to provide some perspective, this is like a 1500 watt hair dryer running continuously for 5 to 6 hours.
The 8 kWh, or 8000 watt-hours is distributed over 24 hours. Let's assume evenly. This means that the power consumed by two units is 8000 watt-hours / 24 hours equals 333 watts, or 167 watts per unit.
To figure out how much battery capacity we need, it is necessary to figure out the current that will be drawn from the batteries. To get current, divide power by voltage: 333 watts / 120 volts equals 2.7 amps. Round up to 3 amps to be on the safe side.
Three amps. Not much at all, eh? Here's where I think it's easy to get lulled into a sense of false security.
The 3 amps is AC. Recall that the AC current is produced by a DC-to-AC converter, and that typically the current is much more substantial on the DC side. Assuming a 12-to-120 volt converter, the current on the DC side is ten times the current on the AC side. Accordingly, we need 30 amps running continuously from our batteries.
So will our 250 amp-hour battery do it? Clearly not -- 250 amp-hours / 30 amps equals 8.33 hours means that our battery-powered dehumidifier system will run for about 8 hours. This suggests that we need 3, maybe 4 (or more), batteries for a system that runs 24/7. Given that each battery costs $500, we're already up to $2000, and we haven't yet even added the solar panels!
Damn the cost! Full speed ahead, even if it is out of curiosity about how many solar panels are needed for this system.
To fully replenish 8000 watt-hours per day, and given that there is only about 4 hours of suitable solar intensity, we need to generate about 8000 watt-hours / 4 hours equals 2000 watts. In other words, our solar panels need to generate about 2000 watts. Before running out and installing a 2000 watt solar system, recall that the 2000 watts is the actual power you need, not the "rating" by the manufacturer. The "rating" needs to be significantly more than the actual power, probably something like 3000 watts.
Recall that you can now get solar panels that are rated to produce 400 watts (the fictitious number, not the actual amount). The actual amount will be probably, under the best circumstances, 300 watts. We need 2000 actual watts, which means that we need 2000 watts / 300 watts per panel equals 6.6 panels. Since no one will sell you 0.6 of a panel, round up to 7. Heck, why not round up to 8?
Now recall that each panel is $1000, making the cost of the solar system upward of $8000. You'll also need a charge controller and some pretty hefty wiring, and probably a permit from the town to install this. It probably is also advisable, and most likely legally required, to have the entire system set up professionally.
BTW, out of curiosity, how much current would be coming from the solar panels? Let's say they are all in parallel, so the output voltage is much like the Harbor Freight system -- ranging between 13 to 18 volts. 2000 watts / 13 volts equals 153 amps! This level of current will make your hair stand on end if you are anywhere near it, and heaven help you if you get a short. The good news is that it is possible to reduce the current by increasing voltage, and to increase voltage we put some of the solar panels in series. Let's say we create two series of four panels each (now you see why I was keen on going with 8 panels ;-) ). The voltage of each series is now 4 x ( 13 to 18 volts ) equals 52 to 72 volts. With the increased voltage, the current is less: 2000 watts / 52 volts = 40 amps. This is still a very substantial amount of current, but much safer than 150 amps (for reference, the circuit to the typical electric kitchen range can handle 50 amps).
Will this solar configuration replenish the batteries? An issue is that we have 12 volt batteries, but the solar system is now configured so the voltage across its output cables is 52 to 72 volts.
Recall that's where the charge controller comes in. It will smooth out the voltage, and produce an output of about 55 volts and 40 amps. To match up the charge controller voltage with the batteries, we put them in series as well. In a series configuration, the four 12 volt batteries combine to create one 48 volt battery bank. To charge the batteries the voltage from the charge controller needs to be a bit higher than the battery voltage (in the Harbor Freight system, the battery is 12 volts, while the charge controller output is 14 volts), so applying the 55 volts from the charge controller to the 48 volt battery configuration will actually be just what is needed to charge the batteries.
So to answer the question, "Will this solar configuration replenish the batteries?" Assuming 4 hours of suitable sunlight x 55 volts x 40 amps gives us a bit more than 8000 watt-hours. We're in business! That's assuming we want to spend $12,000 to $14,000 to run two basement dehumidifiers.
A consideration: producing 1 kWh of conventional electricity creates 2 lbs of carbon dioxide. 8 kWh per day then is responsible for 16 lbs of carbon dioxide each day. Over 100 days, we're up to 1600 lbs, or nearly a ton. Thus we can reduce our electricity carbon footprint substantially -- in my case, by 15% -- by using the solar system. I'm not including the carbon dioxide generated by the manufacture of the batteries, solar panels and related gear, and the energy involved in the installation. But even so, maybe $12-$14K is worth it after all.
If I'm interested in reducing my carbon footprint, is this the place to start? Actually not. Transportation is a much bigger component of my energy usage. The next post will go into some detail about how to reduce my transportation carbon footprint.
Based on what we know from the foray into solar reported in the past few posts, the real question is: "What would it take to run the dehumidifiers off of batteries, and to recharge the batteries using solar?"
We can answer this in two parts: (a) how much battery capacity is needed to run the dehumidifiers? and (b) how much solar capacity is needed to recharge the batteries to allow the system to run 24/7?
Using what we've learned in the previous several posts, the place to start is to figure out how much energy the dehumidifiers use in 24 hours, then figure out what batteries are needed to hold this amount of energy (with some to spare to cover cloudy or excessively humid days), and then figure out how much energy solar panels need to collect to recharge the batteries fully.
The Kill-A-Watt measurements of the dehumidifiers estimate that two units consume 7+ kWh per day. Let's say 8 kWh to be on the safe side. Just to provide some perspective, this is like a 1500 watt hair dryer running continuously for 5 to 6 hours.
The 8 kWh, or 8000 watt-hours is distributed over 24 hours. Let's assume evenly. This means that the power consumed by two units is 8000 watt-hours / 24 hours equals 333 watts, or 167 watts per unit.
To figure out how much battery capacity we need, it is necessary to figure out the current that will be drawn from the batteries. To get current, divide power by voltage: 333 watts / 120 volts equals 2.7 amps. Round up to 3 amps to be on the safe side.
Three amps. Not much at all, eh? Here's where I think it's easy to get lulled into a sense of false security.
The 3 amps is AC. Recall that the AC current is produced by a DC-to-AC converter, and that typically the current is much more substantial on the DC side. Assuming a 12-to-120 volt converter, the current on the DC side is ten times the current on the AC side. Accordingly, we need 30 amps running continuously from our batteries.
So will our 250 amp-hour battery do it? Clearly not -- 250 amp-hours / 30 amps equals 8.33 hours means that our battery-powered dehumidifier system will run for about 8 hours. This suggests that we need 3, maybe 4 (or more), batteries for a system that runs 24/7. Given that each battery costs $500, we're already up to $2000, and we haven't yet even added the solar panels!
Damn the cost! Full speed ahead, even if it is out of curiosity about how many solar panels are needed for this system.
To fully replenish 8000 watt-hours per day, and given that there is only about 4 hours of suitable solar intensity, we need to generate about 8000 watt-hours / 4 hours equals 2000 watts. In other words, our solar panels need to generate about 2000 watts. Before running out and installing a 2000 watt solar system, recall that the 2000 watts is the actual power you need, not the "rating" by the manufacturer. The "rating" needs to be significantly more than the actual power, probably something like 3000 watts.
Recall that you can now get solar panels that are rated to produce 400 watts (the fictitious number, not the actual amount). The actual amount will be probably, under the best circumstances, 300 watts. We need 2000 actual watts, which means that we need 2000 watts / 300 watts per panel equals 6.6 panels. Since no one will sell you 0.6 of a panel, round up to 7. Heck, why not round up to 8?
Now recall that each panel is $1000, making the cost of the solar system upward of $8000. You'll also need a charge controller and some pretty hefty wiring, and probably a permit from the town to install this. It probably is also advisable, and most likely legally required, to have the entire system set up professionally.
BTW, out of curiosity, how much current would be coming from the solar panels? Let's say they are all in parallel, so the output voltage is much like the Harbor Freight system -- ranging between 13 to 18 volts. 2000 watts / 13 volts equals 153 amps! This level of current will make your hair stand on end if you are anywhere near it, and heaven help you if you get a short. The good news is that it is possible to reduce the current by increasing voltage, and to increase voltage we put some of the solar panels in series. Let's say we create two series of four panels each (now you see why I was keen on going with 8 panels ;-) ). The voltage of each series is now 4 x ( 13 to 18 volts ) equals 52 to 72 volts. With the increased voltage, the current is less: 2000 watts / 52 volts = 40 amps. This is still a very substantial amount of current, but much safer than 150 amps (for reference, the circuit to the typical electric kitchen range can handle 50 amps).
Will this solar configuration replenish the batteries? An issue is that we have 12 volt batteries, but the solar system is now configured so the voltage across its output cables is 52 to 72 volts.
Recall that's where the charge controller comes in. It will smooth out the voltage, and produce an output of about 55 volts and 40 amps. To match up the charge controller voltage with the batteries, we put them in series as well. In a series configuration, the four 12 volt batteries combine to create one 48 volt battery bank. To charge the batteries the voltage from the charge controller needs to be a bit higher than the battery voltage (in the Harbor Freight system, the battery is 12 volts, while the charge controller output is 14 volts), so applying the 55 volts from the charge controller to the 48 volt battery configuration will actually be just what is needed to charge the batteries.
So to answer the question, "Will this solar configuration replenish the batteries?" Assuming 4 hours of suitable sunlight x 55 volts x 40 amps gives us a bit more than 8000 watt-hours. We're in business! That's assuming we want to spend $12,000 to $14,000 to run two basement dehumidifiers.
A consideration: producing 1 kWh of conventional electricity creates 2 lbs of carbon dioxide. 8 kWh per day then is responsible for 16 lbs of carbon dioxide each day. Over 100 days, we're up to 1600 lbs, or nearly a ton. Thus we can reduce our electricity carbon footprint substantially -- in my case, by 15% -- by using the solar system. I'm not including the carbon dioxide generated by the manufacture of the batteries, solar panels and related gear, and the energy involved in the installation. But even so, maybe $12-$14K is worth it after all.
If I'm interested in reducing my carbon footprint, is this the place to start? Actually not. Transportation is a much bigger component of my energy usage. The next post will go into some detail about how to reduce my transportation carbon footprint.
Tuesday, May 24, 2016
Foray into Solar -- System Sizing
In the previous two posts, I've introduced my Harbor Freight 45 watt solar power system and some of the things I've learned by experimenting with it. A key question, of course, is whether it is a practical backup for when the power goes out.
The quick answer is no. Essentially, the Harbor Freight system is a 12 volt battery with a solar recharger. The power output is limited to pretty much what a 12 volt battery can put out. Clearly a house requires more power than you'd get from a 12 volt battery.
However, in a pinch, a 12 volt battery can keep some essentials running. For instance, an LED light (10 watts), a cable modem (9 watts) and router (2.5 watts), and a laptop charger (100 watts) and a cell phone charger (20 watts), for a total of about 140 watts.
Since the solar panels in the best case produce 36 watts (not counting any losses in the charge controller and a DC-to-AC converter), what will really be going on is a draining of the 12 volt battery. The question is then: how long can this system run before the battery runs out?
There's a wide range of batteries in terms of (a) the chemistry they are based on (lead-acid, lithium ion, etc.), (b) their voltage, (c) how much energy they will hold, (c) how far down they can be safely drained, (d) whether they are good at putting out large bursts of current, (e) how many times they can be charged before they degrade, among other parameters. My system uses a 12 volt 18 amp-hour sealed lead-acid battery. It's not the ideal for solar applications (a future post on batteries will explain this), but it's what I have at the moment.
So how long will this battery last with the above load? Before we can answer this, we need to understand what "18 amp-hours" means.
In short (pun intended), the "amp-hour" rating of a battery indicates how much current you can draw for a given period of time. For instance, if your load draws one amp, an 18 amp-hour battery will support this load for 18 hours. For a load of 2 amps, the time the battery will last is 9 hours. To determine how long the battery will last while running the essentials, we need to figure out how much current they will draw.
To get current, divide power by volts. For the LED light, the current draw is 10 watts / 12 volts, or .83 amps. For the AC items, we add up the watts -- let's say 150 watts to include the loss in the DC-to-AC converter -- and divide by the 120 volts, which give us 1.25 amps on the AC side.
An important thing to note about the AC devices, however (and I found this out the hard way by blowing out my ammeter fuse and frying some wires), is that the current on the DC side is vastly larger. What seems like a trivial amount of current at 120 volts increases on the DC side to a level that deserves a lot of respect. The reason: the power on the AC side equals the power on the DC side equals 120 volts x 1.25 amps equals* 150 watts equals 12 volts x 12.5 amps on the DC side! For those not familiar with current, 12.5 amps is... well I'll skip using the expletive... is a hefty amount of current. My ammeter limits the current it will measure to 10 amps, which is why I had to buy a new fuse for it. Also you need some pretty heavy gauge wires to carry 12.5 amps.
So in total on the DC side, our essential devices will draw about 13 amps, which will drain the battery in about an hour and a half. This is fine if the power outage is relatively short, but for extended outages like we've had in Sudbury after ice storms or blizzards, this system is woefully inadequate.
Note that the bulk of the load is the 100 watt laptop charger, so if we use that sparingly, we reduce our AC power load to about 50 watts, which reduces the current draw by the AC devices to less than half an amp on the AC side, but on the DC side it is still a very hefty 5 amps. With the LED light drawing another amp, the total current draw is 6 amps, which will stretch the battery out to 3 hours. Still not particularly practical.
By the way, is there any benefit to having the solar panels involved in the system at all, or are they just playthings for people like me who like to tinker? Well, recall that the charge controller applies 14 charging volts to the battery, and in bright, sunny conditions, the current is about 2 amps. Under these conditions, the battery is taking in 14 volts x about 2 amps, which is about 30 watts (recall that the solar panels are putting out about 36 watts, so our numbers here check out). If we're drawing 6 amps, and putting back 2 amps, the net load is 4 amps, so our battery will last perhaps 4.5 hours. Keep in mind that 4.5 hours is about the maximum duration of sunlight in Sudbury in mid-summer sufficient to produce the 2 amps. Most likely, the solar charging system will average significantly less than 2 amps over the course of the 4.5 hours, which means that the actual time the battery will last will probably be less than 4 hours. But that's certainly better than 3 hours.
So now it should be somewhat clearer what's required for a backup system that will last for days, even if all we want to run is the small set of "essentials". For extended periods without sun, we'll be drawing 5 amps, and occasionally 12.5 amps to charge our laptop, which means we'll need a much bigger battery (actually probably a bank of batteries). For the brief periods with sun, we'll want sufficient solar power to fully recharge our batteries so they last until the sun comes up again. So how many batteries do we need, and how many solar panels do we need?
The amp-hour thing comes in handy for answering these questions. Let's say we dispense with the laptop and use our phone for all internet access via WiFi, so our current draw is a constant 5 amps. With an 18 amp-hour battery, we saw that our essentials will run for 3-4 hours. If we want our system to run for say, 48 hours, we need 5 amps x 48 hours which equals about 250 amp-hours. The good news is that you can get a single battery that has a capacity of 250 amp-hours. No need for multiple batteries.
So how many solar panels are needed to keep this battery charged? The power draw on the battery at 5 amps would be 5 amps x 12 volts = 60 watts. However remember that in Massachusetts mid-summer you can count on only about 4.5 hours of sufficient sunlight, so the solar panels need to put out a lot more to fully recharge the battery. Here's how much more: The power draw for one day is 60 watts x 24 hours, or about 1500 watt-hours. The solar panels need to replenish the battery with 1500 watt-hours over, say, 4 hours. This means that the solar panels need to generate about 400 watts when the sun is shining on them. The Harbor Freight panels put out only 36 watts, so we're talking 12 or so Harbor Freight units, or a smaller number of more powerful solar panels. More good news: You can get a single solar panel that allegedly produces 400 watts (but remember what we learned about the "rating" and what the panel actually produces). A "400 watt" panel probably produces only 300 watts, but this gets us in the ballpark. You might want to go with two panels for a total of 600 actual watts, which in reality will be significantly less because of clouds and changing angle of the sun.
So voila! You can create a backup system for your essentials that should run indefinitely assuming the sun shines sufficiently to recharge your battery.
Less good news is that a 250 amp-hour battery is currently around $500. Even less good news is that a "400 watt" solar panel is about $1000. Keep in mind you'll also need a significantly higher capacity charge controller (the Harbor Freight version I have handles about 30 amps of current; two "400 watt" solar panels may produce in excess of 50 amps). $2500+ might be a bit more than what you'd be willing to pay for a backup system for powering a handful of essentials for a couple of days. Consider that for $500 you can get a 7000 watt gasoline-powered generator that can probably run your fridge, a flat-screen television and charge all the laptops in your house simultaneously.
In defense of the battery/solar approach, your solar backup system can run 24/7 without annoying your neighbors, which you probably can't say about the gasoline version. But the bottom line is that a solar backup system is not a particularly practical approach at today's prices. And it is hard to imagine solar panel and battery prices will ever go low enough to beat the fossil fuel generator approach.
* Technically, AC power equals AC voltage x AC current x cos (phase difference). I'm really only interested in the worst possible case, so I'm assuming that the phase difference is 0 degrees, i.e., peak voltage and current occur at the same time.
Monday, May 23, 2016
Foray into Solar, Continued -- What "45 Watts" Really Means When It Applies to Solar Panels
In the previous post, I discussed how the kit I bought did not make it clear that a battery is needed to make the system function properly. Without a battery, you end up with flickering lights and appliances that don't run properly. Another thing the kit does not make clear is that you're not going to get the advertised power.
The manufacturer calls my system a "45 Watt Solar Panel Kit", but the system actually produces significantly less power. This post explains how this can be the case.
My system is configured as illustrated here:
The three solar panels are connected in parallel through a junction, feeding their power to the charge controller. The charge controller smooths out the incoming power, and applies it to the battery to charge it.
The charge controller also dispenses power from the battery to run lights and other appliances.
During operation, the solar panels erratically produce 13-18 volts and a maximum current of about 2 amps. This means that the maximum total power coming from the solar panels (assuming the maximum volts and amps occur simultaneously) is about 18 volts x 2 amps, which is 36 watts. But the kit claims that the solar panels produce 45 watts. "Watts" the deal?
It turns out that the way the panels apparently are rated is by multiplying the open circuit voltage (i.e., the voltage across the lines coming from the junction of the panels when the lines are disconnected from the charge controller) and the short circuit current (the current from the solar panels when the lines from the junction are shorted). The open circuit voltage is the maximum voltage you can get from the panels, while the short circuit current is the maximum current you can get. These are two separate extreme cases, which means you don't ever get this maximum voltage and maximum current at the same time.
The open circuit voltage turns out to be about 20 volts, while the short circuit current is about 2.2 amps. Multiplying these two values you get 44 watts, which is fairly close to 45 watts. I find this rating approach deceptive, because you'll never actually get 45 or even 44 watts.
What you'll actually get becomes more clear when you look at the graph of volts vs. amps for the solar panels:
The blue curve shows the relationship between current and voltage for the solar panels. The current starts at 2.2 amps at the short circuit state (0 volts), and reaches zero at the open circuit state (20 volts).
The orange curve shows actual power output. The maximum power point of 36 watts is somewhere around 18 volts where the current is 2 amps (18 volts x 2 amps = 36 watts). This is markedly lower than the advertised power output of 45 watts.
The important lesson here is when considering solar panels, be sure you know what the power rating is referring to -- the actual power you'll get, or a fictitious level that combines the open circuit voltage and the short circuit current. This is especially important if you're putting solar panels on your roof or filling a field with them. It's one thing to find out that your "45 watt" kit produces only 36 watts. It's quite another thing if your 7500 watt system turns out to produce significantly less.
Next I'll address if my system would be a practical power source during a power outage, and if not, what would be needed?
The manufacturer calls my system a "45 Watt Solar Panel Kit", but the system actually produces significantly less power. This post explains how this can be the case.
My system is configured as illustrated here:
The charge controller also dispenses power from the battery to run lights and other appliances.
During operation, the solar panels erratically produce 13-18 volts and a maximum current of about 2 amps. This means that the maximum total power coming from the solar panels (assuming the maximum volts and amps occur simultaneously) is about 18 volts x 2 amps, which is 36 watts. But the kit claims that the solar panels produce 45 watts. "Watts" the deal?
It turns out that the way the panels apparently are rated is by multiplying the open circuit voltage (i.e., the voltage across the lines coming from the junction of the panels when the lines are disconnected from the charge controller) and the short circuit current (the current from the solar panels when the lines from the junction are shorted). The open circuit voltage is the maximum voltage you can get from the panels, while the short circuit current is the maximum current you can get. These are two separate extreme cases, which means you don't ever get this maximum voltage and maximum current at the same time.
The open circuit voltage turns out to be about 20 volts, while the short circuit current is about 2.2 amps. Multiplying these two values you get 44 watts, which is fairly close to 45 watts. I find this rating approach deceptive, because you'll never actually get 45 or even 44 watts.
What you'll actually get becomes more clear when you look at the graph of volts vs. amps for the solar panels:
The orange curve shows actual power output. The maximum power point of 36 watts is somewhere around 18 volts where the current is 2 amps (18 volts x 2 amps = 36 watts). This is markedly lower than the advertised power output of 45 watts.
The important lesson here is when considering solar panels, be sure you know what the power rating is referring to -- the actual power you'll get, or a fictitious level that combines the open circuit voltage and the short circuit current. This is especially important if you're putting solar panels on your roof or filling a field with them. It's one thing to find out that your "45 watt" kit produces only 36 watts. It's quite another thing if your 7500 watt system turns out to produce significantly less.
Next I'll address if my system would be a practical power source during a power outage, and if not, what would be needed?
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