Solar Questions

Do the calculation in amps and amp hours, not watts and watt hours. That is because there are lots of power losses in the system, so you will underestimate if you use watts unless you keep track of those. There aren't really any significant current losses so that approach works better and simpler. Also batteries are rated in amp hours.

Separate your loads into AC and DC. If it is a 12V DC load use or measure the current draw. A clamp on DC multimeter is handy for these measurements. Under $40 on Amazon and well worth having in your travelling toolkit.

Multiply the current by the hours or fractions of hours per day you are running that load to get amp hours. Loads like the water pump really only run a few minutes in a while day so no need to overestimate.

If it is an AC load convert it to DC amps at 12V as follows

AC watts X 10/.85 = DC amps.

The X 10 multiplier is to convert from 120V to 12V. The .85 is an approximation of the inverter conversion efficiency.

As you can see even a small AC load takes a large amount of energy to operate, so avoid AC loads as much as possible. Once you have the DC amps to the inverter needed to run each AC load multiply by the hours or fractions of hours to get DC amp hours, same as with the DC loads.

Add up all the calculated amp hour loads to get total daily amp hours.

If you want to be able to run from your batteries for one day, multiply by 2 and get batteries of at leasr that capacity. The 2x multiplier is to limit the max depth of discharge of the battery to 50%, which is the most you will want to pull out of a lead acid battery if you want it to last. If you want to go 2 days without recharging then double that.


If you want to run on solar for long periods you might want even more battery capacity in case you run into a few cloudy days or you're camped under a tree canopy, but you will find that the battery size gets excessive quickly if you are trying to run a lot of gear and appliances. Better to bring a generator for those situations.

You could also consider converting to lithium batteries but that is another discussion.

To determine how much solar you need, start by calculating the size needed to maintain your battery on an average day in your area/time of year you are camping For summer use in the west with a portable solar module tilted south you can expect about 5 hours of full sun daily. In the east around 4. If you are roof mounting the solar modules horizontally then assume around 4 and 3 hours respectively. If you plan on camping into the late fall then solar performance starts dropping pretty quickly, especially for a horizonal roof mount.

To calculate solar module size you can use this formula

load amp hours per day / sun hours per day × 12 × 1.5 equals watts of solar.

The 1.5 takes into account system losses and an adjustment for the way solar modules are tested and rated, the discussion of which I won't bore you with here.

Sizing your battery and solar module to you loads this way works fine for most folks, just be sure to have an alternate charging source if you boondock for long periods in areas that get cloudy weather.

Don't forget to get a solar charge controller and install properly sized wire with a fuse at the battery + terminal to protect the solar charge circuit. If you get a standard solar module designed for 12V battery operation a simple PWM charge controller will be fine.

Wiring and fusing should be sized for current at least 50% higher than the max short circuit rating of the solar module to handle peak output from cloud reflections and avoid nuisance fuse blows. That is 20A for 12 gauge, 30A for 10 gauge, 40 for 8 gauge. If you're using a portable module with long cables go at least one size larger yet to limit voltage drops.