Ah, spring. The weather is warming up, the sun is shining, and a young man’s fancy turns to solar panels. Well, perhaps not all young men, but now that the weather is nice, the Evergreen development just south of us has been finishing up their solar installations. It doesn’t take much to get me to look at solar panels again. They look cool, they’re pretty green, and they’re basically magic, which is all appealing.
Figuring out how big a solar panel you need for a given project can be complicated, though. Just because a panel is rated for 15 Watts doesn’t mean that you’ll get 15 Watts out of it continuously. I’ll take you through the steps to discover what you’ll actually need.
First, you’ll need to use something like a Kill-A-Watt (cheap right now at about $22) to figure out how many kWh your device uses over the course of a day. For example, my cell phone charger uses 0.5 Watts over a period of 6 hours for a total of 3 Wh (Watt-hours) per day of use. However, I only use it about once a week, so the overall use is 0.43 Wh per day. My laptop uses 15 Watts while it’s on and I use it for about 8 hours a day. This adds up to 120 Wh a day.
Next, use an insolation map to figure out how much sunlight you’ll get in an average day. If you’re worried about a minimum value (like if you were planning to go totally off the grid), then you’ll want to find a winter map so that you’ll know that you should always get that much sun. This map shows how many “sunlight-hours” you get during the day. Each sunlight-hour is equivalent to one hour of direct sunlight. For example, here in central Indiana, we get about 10 hours of sunlight a day. However, much of it is at an angle so this is the equivalent of 4.3 sunlight-hours. These numbers assume that your solar panel isn’t shaded and is angled according to your latitude. If that’s not true, you’ll have to adjust this number down.
Using your insolation number, you can figure out what you’ll need as a Watt rating for your device. My cell phone charger needs 0.43 Wh per day. I get 4.3 sunlight-hours per day, so I need a solar panel with a Watt rating of 0.01. My laptop, on the other hand, needs 120 Watts per day, so its minimum requirement is 28 Watts.
Now you can start looking for solar panels. You can often find lists organized by Watt, which makes it easier. Depending on your needs, you may want to build in a safety factor. For example, if I don’t want to have to worry about my laptop running out of batteries after a couple of rainy days, I might want to get a panel rated for more like 40 or 50 Watts. In my case, I’m not going totally off-grid (like I would if I were hiking, for example), so it wouldn’t bother me to have to plug stuff back in under that sort of circumstance.
A 1-Watt panel runs about $30, so a full kit with battery and inverter would probably cost about $100. That would be enough to charge both my cell phone and Maggie’s. That only replaces about 13 cents’ worth of electricity, so the payback is on the order of 750 years, but it’s a reasonably affordable way to play around with solar. A 30-Watt panel and inverter runs about $400 and would allow me to run my laptop off-grid. That has a much more reasonable payoff of about 100 years.
Obviously, solar panels at this scale are never going to pay for themselves. However, the convience, greenness, or just plain coolness might make them worth it for you.