Voltage, amps, and watts

The same formula applies whether it is AC or DC: watts = amps X volts. Since the voltage differs by a power or 10 it makes it easy. Just multiply AC amps by 10 to figure DC amps. It takes .833 amps to power a 100 watt light bulb at 120 volts AC. Run it for an hour and you pay for .1 KWH on your electric bill. It takes 8.33 amps to power the same light bulb at 12 volts DC. Run it for an hour and you have used 8.33 amps from your battery, excluding any inverter loss.

lbrjet wrote:
Run it for an hour and you have used 8.33 amps from your battery, excluding any inverter loss.

8.33 amp-hours

Wow! Those are some great responses. We do a lot of dry camping in the high country of Wyoming, in the most secluded places that we can find. It's high enough and cool enough that an AC is not needed. So, we crank up the generator for breakfast and dinner; this is for coffee pot, microwave, induction cooktops, and charging batteries. Other than that, it's strictly battery. We've been doing it this way for a lot of years but I've never really cared about usage. Now, I'm looking into solar panels and want all the info I can get. Thanks for all the help.

Here is a simple flow chart.

Budget-->Energy Audit-->Battery bank size-->number of watts-->PWM or MPPT. What ever type of controller is chosen, make sure it has adjustable set points and a temperature probe that is on the battery.

If you use solar as a battery charger, one rule of thumb is between 60 and 150 watts of panels per 100 amp-hours of storage. The smaller the battery bank the higher the wattage needed (per 100 amp-hours). Here is a link to the rather special spreadsheet which includes an energy audit, that N8GS has created to help size solar battery charging systems!

solar spreadsheet by N8GS

If you full time or use an inverter lots, then populate the entire unshaded area of the roof with panels. I'm considering replacing my awning with solar panels.

For a nice explanation of solar, try this link:

Golden rules of solar