#### svetz

##### Works in theory! Practice? That's something else

These steps will help you to figure out how much solar you need to recharge batteries. Say, for example, you're going hiking for a few days and want to know how much solar you'll need to charge your critical devices.

Suppose you already have panels? How much power do they generate? Panels are rated in

So, if you have 2 100W panels and insolation map says 5 hours for your area, that's 2 x 100 x 5 x .8 = 800 watt hours per day

See the Battery FAQ to get information on properly sizing batteries to store your solar power.

- Measure how much power your devices use under
*actual*conditions with a watt meter. So for a laptop or cell phone battery, let the device discharge and then recharge it with the meter measuring it. Record that number and how long it took to recharge. For this exercise, let's assume it took 150 watts and two hours to charge; you guessed it, that's 150W x 2 hrs = 300 Wh. - Next, you need to know how much usable sunlight you have. The sun might shine for 12 hours a day, but you'll need an insolation map to get the usable hours. 5 isn't too bad a number in the summer at low latitudes on clear days. If you slept in and are hiking around a couple of hours everyday during 9-3 to get to the next camp site you'll have to subtract that out of your usable hours.
- Now take the watt-hours you need and divide by the number of hours of sunlight, so say 300wh/5h = 60w.
- Inverters have losses, so divide that by 80%. So, for 300wh you'd need at least 75W of solar panel and that assumes a perfectly clear day all day. But you probably won't have that, perhaps you'll have some clouds or rain. You'll either have to do without under those conditions or budget a little extra -how much is up to you.
- Next you'll need an inverter to convert the DC power from the panel into AC so your laptop's charger can convert it back into DC. You know your device draws 150W but you'll want to add a little more to be safe. So you'll need an inverter that can continuously supply at least 150 x 120% = 180 watts.

**How much power do my panels generate?**Suppose you already have panels? How much power do they generate? Panels are rated in

*watts*at a very specific set of lab conditions you'll probably never see. Without getting into really complicate equations, folks use an insolation map. This will give you the amount of "usable" sunlight in hours per day for the average day throughout the year, here's the forumla: Number of Panels x Watts/panel * hours from insolation map * losses.So, if you have 2 100W panels and insolation map says 5 hours for your area, that's 2 x 100 x 5 x .8 = 800 watt hours per day

See the Battery FAQ to get information on properly sizing batteries to store your solar power.

**Not in the U.S.?**Get your insolation map here (Link courtesy of brassmonkey001).
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