How Much Solar Do I Need for My RV? Sized for the Month You Camp In

The month decides it, not the rig. On 1,500 Wh a day and a lithium bank, Quartzsite in December needs about 663 W of panel and the same rig in June needs 243 W. Size on the annual average everyone quotes and a December trip gets roughly half the power it needs — and in Seattle, no array that fits an RV roof will do it at all.

The annual average is nobody’s month

Every guide picks one peak-sun-hours figure — “four to five hours” is the usual phrasing — or an annual average, and divides daily watt-hours by it.

Here is what the months actually look like, from NASA’s monthly climatology:

Place	Annual	December	Sized on annual, December gets
Quartzsite, AZ	5.90	3.11	53% of what it needs
Denver, CO	4.85	2.29	47%
Bend, OR	4.27	1.26	30%
Seattle, WA	3.42	0.85	25%
Anchorage, AK	2.54	0.14	6%

Nobody camps in the annual average. If you go south for the winter, Quartzsite in December is your design month and the annual figure under-sizes you by nearly half. If you only ever go out in July, the annual figure over-sizes you and you have paid for panels you will never use.

The annual average is wrong in both directions. It is only ever right by accident.

Flat to the roof, not tilted to the sun

The second problem is subtler and it runs the same way.

Published sun-hour tables are almost always for a tilted, south-facing plane, because that is how a house array is mounted. An RV’s panels are bolted flat to the roof, and a flat panel collects global horizontal irradiance — a different, lower number.

The gap is widest exactly when it hurts most. In midsummer the sun is high and a flat panel loses little. In December the sun is low, a tilted panel gains a great deal from facing it, and a flat one does not.

One thing that needs no conversion: kWh/m²/day and peak sun hours are the same number. A peak sun hour means an hour at 1,000 W/m², which is the irradiance a panel’s rating is measured at, so 3.11 kWh/m²/day is 3.11 peak sun hours.

Roof area is the real limit

On an RV, panels do not run out because of money. They run out because of space.

A Victron 150 W panel is 1485 × 668 mm — 10.68 square feet each. Denver in December, on 1,500 Wh a day, needs seven of them: 74.7 square feet.

Usable roof is much less than the roof looks. Subtract the air conditioner, the vents, the skylight, the antenna and the fridge vent, and a 25-foot trailer might offer 60 square feet of clear run in awkward strips. The calculator asks for that number rather than assuming it, and it picks the panel that uses the least roof area rather than the one with the most watts — which is why a 190 W panel in the same footprint as a 150 W beats it, even though both “work”.

The derate, and the one number nobody publishes

The sum needs a factor for everything the laboratory rating does not include: soiling, shading, mismatch, wiring loss and panel temperature.

This page uses 0.75, and that is the only number on it that nobody publishes. It is the figure our battery charge time calculator already uses, reused so the two cannot disagree, and it is flagged in the result rather than buried.

What is published is one component of it. Victron gives a temperature coefficient of maximum power of −0.45% per °C, so a panel well above its 25°C rating gives up real output. But Victron publishes no NOCT for these panels, so there is no sourced way to turn an air temperature into a cell temperature — and this page names the coefficient rather than inventing the temperature to apply it to.

Everything else in the calculation comes from a manufacturer or from NASA: charge efficiency, the controller’s 98%, the panel ratings, and the sun.

The two numbers this page does not work out

Solar sizing gets tangled with two neighbouring questions, and keeping them apart is most of what makes the answer usable.

Your daily watt-hours. Everything here scales directly off that one figure, and this page takes it as given rather than guessing at it. If you are working from “we run a 12V fridge, some lights and charge two laptops”, our amp draw calculator turns an appliance list into watt-hours. Get this wrong and every number on this page is wrong by the same proportion — it is the input worth spending ten minutes on.

The battery bank. Panels and batteries answer different questions. The array decides how much you can put back in a day; the bank decides how long you can go without putting anything back, and how much of a cloudy week you can ride out. A big array on a small bank spends the morning full and throws the afternoon away; what size lithium battery for an RV sizes the other half, and the cold that costs you sun in December costs you capacity at the same time.

The third neighbour is the charge controller, and it is the one that can be destroyed by getting it wrong rather than merely disappointing — what size solar charge controller covers it.

When the honest answer is “solar cannot do this”

Ask for Seattle in December and the arithmetic returns 2,425 W of panel across 181 square feet. That is not a recommendation, it is a reductio — and the calculator says so instead of printing it as a target.

What it does instead is tell you what your roof can do. Five 150 W panels in Denver in December return about 1,250 Wh a day against 1,500 Wh of use: a gap of 250 Wh that no amount of panel closes in that month. That gap is the useful number, because it is what the alternator, a pedestal, or a generator has to cover.

Filling the roof and accepting the gap is usually the right call. So is going south.

Model	Rated	Size	Weight	Open circuit	Short circuit	Buy
BlueSolar 95W	95 W at STC, ±3% tolerance	770 x 668 x 30 mm	5.4 kg	20.4 V	5.7 A	Check price(opens in a new tab)
BlueSolar 130W	130 W at STC, ±3% tolerance	1020 x 668 x 30 mm	7.1 kg	24.8 V	6.53 A	Check price(opens in a new tab)
BlueSolar 150W	150 W at STC, ±3% tolerance	1485 x 668 x 30 mm	11.0 kg	22.3 V	8.69 A	Check price(opens in a new tab)
BlueSolar 190W	190 W at STC, ±3% tolerance	1485 x 668 x 30 mm	10.5 kg	25.5 V	9.17 A	Check price(opens in a new tab)
BlueSolar 285W	285 W at STC, ±3% tolerance	1550 x 880 x 35 mm	14.32 kg	38.9 V	9.08 A	Check price(opens in a new tab)

Where this goes wrong

• Sizing on the annual average. Nobody camps in it, and in December it under-sizes by two to four times across most of the country.
• Using a tilted sun-hour figure for a flat roof. Those tables are for house arrays. An RV’s panels do not tilt.
• Measuring the roof rather than the clear space on it. The air conditioner and the vents take the middle of it.
• Buying on watts instead of square feet. Two panels of the same footprint can differ by 40 W; on a full roof that is free power.
• Guessing the daily load. Every figure on this page scales directly off it — our amp draw calculator works it out from your appliances.
• Forgetting the controller. An array that fits the roof can still destroy the controller on a cold morning: what size solar charge controller checks the voltage side.

Questions worth answering