Instructables

Solar power questions?

I have this same solar panel 5 watt 12 volt panel 

http://store.bruntonoutdoor.com/portable-power/panels-and-chargers/solarflat-trade-5-12v-orange/

And a 400 watt power inverter. What type of lightweight battery should i get? (12 volt, something amp battery)


orksecurity3 years ago
There are three numbers you need to look at.

One is watts. Watts is volts times amps. To get 400 watts out of the power inverter, you would need to put in 400 watts, or (400/12 = 33.3) amps. Actually, a bit more than that, due to inefficiencies in the inverter.

Another is watt-hours -- how long you can keep a given amount of power running before the battery runs down and the inverter shuts off. A bigger battery can hold more watt-hours. If you draw less power from the inverter, it will draw less power from the battery -- in very rough numbers, a 100W load can run 4 times as long as a 400W load using the same battery, or conversely you can use a smaller/lighter battery and get the same run time.

The third concern: As you said, the solar panel only puts out 5 watts in full sun (less in partial sun). Ignoring inefficiencies, that means you'd need to charge for (400/5 = 80) hours for every hour that you draw 400 watts back out. Even in high summer, 80 hours of daylight is on the order of 4 days. Again, this suggests that you really want to plan on using much, much less than 400 watts. (Cut it down to 10 watts, and -- ignoring inefficiencies -- you'll only need two hours charging per hour of use.)

Note that you would get somewhat better efficiency -- and save some weight -- by ditching the inverter and instead carrying devices designed to run directly from 12V.

You can save even more weight by discarding anything which draws enough power to require this kind of power. It's a campout. Leave the electronics at home.

Batteries are only 50% efficient.
If you put in 80 Amp-Hours, you can only expect 40 amp-hours back.

A
Exact number depends on the battery, but yes, that's a good point and probably a good rule of thumb.

So double the charging time.

Whether any of this is practical will depend on how much power you actually want to draw, how long, how often. I still think dodging the whole question and either leaving the electronics home or carrying spare batteries -- or recharging from a car, if you aren't on a long through-hike -- sounds like a better bet.
+1
iceng3 years ago
My friend who has an iPad said on a single charge you can run 11 Hrs 12 Watt.

A