Free tool

RV solar sizing calculator

The formula the makers publish: daily watt-hours divided by peak sun hours, corrected for real-world losses. The hard part is knowing your daily watt-hours, so the tool walks you through it, and the peak sun hours come with the numbers for your part of the country.

The answer updates as you type. Nothing is sent anywhere.

What you use in a day

How to get it: multiply each item's label watts by the hours you run it, and add them up. A few LED lights all evening is about 30, the furnace fan is 45 to 60 watts while it runs, the water pump 50 to 110 while it runs, a 12-volt television 35 to 50. If you run 120-volt gear through an inverter, the inverter's own draw adds to the total. A battery monitor that counts amp-hours is the best source: its daily amp-hours times 12.8 is your watt-hours.

Where you camp

Peak sun hours are not hours of sunshine: they are the equivalent hours per day at full panel output, and the whole country runs from about 3 to 6.5 of them. The US average is 4.98. If you want your exact location, NREL's PVWatts calculator gives it free.

Where the formula comes from

The sizing method is the one the panel and battery makers publish: daily watt-hours divided by peak sun hours gives the panel watts. Renogy states it as total solar wattage needed equals daily watt-hours divided by peak sun hours, and Battle Born states the same formula with a 2,400 watt-hour example that yields 600 watts at 4 peak sun hours. The derate factor, the share of nameplate power that actually reaches the batteries, is NREL's: the PVWatts method defaults to a 0.77 derate, and this tool applies it rather than pretending nameplate watts arrive at the battery.

On top of the derate, the makers recommend margin for imperfect days. Battle Born recommends adding 20 to 30 percent solar capacity when space allows, and Renogy rounds its worked example up from 200 to 250 or 300 watts. The tool shows the raw answer and the margined one.

What the answer means

The result is a starting point for shopping, not a guarantee. Panels flat on an RV roof face the sky rather than the sun, so winter and northern latitudes cut the harvest below the rated hours. Shade, dirt on the panels, and a partially drained battery all cut it further. If the answer says 400 watts, buying 400 watts of panel and expecting a full recharge every day in Oregon in December will disappoint you; the same array in Arizona will mostly not.

The rule of thumb worth knowing: Battle Born's guideline of roughly 200 watts of solar per 100 amp-hours of lithium battery capacity, which is a sanity check on the sizing answer rather than a substitute for it.

Where to go next

The battery runtime calculator answers the other half: how long the bank carries you between charges. The watts to amps calculator converts any appliance label into the draw it becomes on the battery.

Sources