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.
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