Divide your daily watt-hours by 0.7, then divide that by your peak sun hours. The answer is the size of array you need, in watts. At 1,500 Wh a day and 5 peak sun hours that is 429W, which rounds up to three 200W panels. Our RV solar calculator runs exactly that arithmetic and shows the working, and this page explains why the 0.7 is there and why it is not pessimism.
Two inputs decide everything. What you use in a day, and how much sun you actually get. Get either one wrong and the array is wrong by the same proportion. If you do not have a daily watt-hour figure yet, build one first: see how to build a power budget.
The formula the calculator uses
array watts = (daily watt-hours / 0.7) / peak sun hoursThe 0.7 is system efficiency: everything between the sunlight and the battery that takes a cut. The peak sun hours figure is not daylight hours. It is the number of hours of full-strength sun your location gets, with weaker light converted to an equivalent.
Worked, at 1,500 Wh a day:
- Divide by 0.7: the panels have to make 2,143 Wh.
- At 5 peak sun hours: 2,143 / 5 = 429W of panel.
- Rounded up to 200W panels: 3 panels, 600W of array.
That is a 600W array to cover 1,500 Wh. The gap between 1,500 and 2,143 is 643 Wh a day that never reaches your battery.
Where the missing 30% goes
This is the part people assume is padding. It is not. Take the Renogy RSP-200D, a common 200W 12V panel, and read its own datasheet.
Tolerance. The panel is rated 200W plus or minus 3%. Yours may be a 194W panel that says 200 on the label.
Heat. The 200W figure is measured at 1,000 W/m2 irradiance and a 25C cell temperature. The same datasheet gives a nominal operating cell temperature of 47C and a temperature coefficient for maximum power of -0.37% per degree C. At 47C the cell is 22C above the test condition, so it gives up about 8% of its output. A dark roof in Arizona in July is not a cool place, and panels lose power exactly when the sun is strongest.
The charge controller. A good MPPT controller is efficient but not free. Victron rates the SmartSolar MPPT 100/30 at 98% maximum efficiency, and that is the peak figure, not the all-day average.
Stack just those three: 194W after tolerance, about 178W once the cells are at 47C, about 174W through the controller. You are at 87% of the number on the box before you have accounted for a single thing that is actually your fault.
Then come the losses that are your fault, or at least your roof’s: panels lying flat instead of tilted at the sun, dust and pollen and pine sap, undersized wire on a long run, and shade. Shade is the harsh one. A branch across one corner does not cost you one corner’s worth of output.
70% is the planning figure. A tilted, clean, well-wired array on a cool day beats it. A flat rooftop array in August, partly shaded by an air conditioner shroud, does worse.
Sun hours move the answer more than anything else
Same 1,500 Wh a day, three different places and seasons:
- 5 peak sun hours: 429W needed, 3 panels, 600W of array.
- 3 peak sun hours: 714W needed, 4 panels, 800W of array.
- 2 peak sun hours: 1,071W needed, 6 panels, 1,200W of array.
The load never changed. The array doubled. This is why a system that works beautifully in Utah in June dies in Oregon in December, and why sizing off an annual average is a mistake: you do not need average performance, you need the system to work in the worst month you plan to camp in.
We have not published per-state peak sun hour figures. We did not verify a dataset for them in this pass, and a made-up number here would be worse than no number, because it would silently double or halve your array. NREL’s PVWatts calculator will model a figure for a specific latitude and tilt. Until you have one, the calculator’s slider spans 2 to 7, and 5 is a reasonable placeholder for summer in the western half of the country.
A smaller, honest example
Not everyone needs 1,500 Wh. A van with a portable fridge, lights, and phone charging is a much smaller number.
The Dometic CFX3 45 datasheet lists 1.03 amp-hours per hour at 12V, measured at 90F ambient with the box set to 39F. That is 24.7 Ah a day, or roughly 297 Wh at 12V nominal. Say the rest of your gear adds another 300 Wh. At 600 Wh a day and 5 sun hours:
- 600 / 0.7 = 857 Wh the panels must make.
- 857 / 5 = 171W of panel.
- One 200W panel covers it.
One panel. The reason so many builds end up at 600W or 800W is not the fridge. It is inverters, induction cooking, and the assumption that a laptop and a Starlink dish are rounding errors. They are not.
What more panel will not fix
Solar makes power during the day. It does nothing at 2am, and it does very little through a three-day storm. That is the battery’s job, and it is a separate calculation: see sizing a battery bank.
An array also will not solve a surge problem. A big inverter load draws its current from the battery in an instant, and the panels are irrelevant to whether that works. Victron’s MultiPlus 12/1200, for example, is rated 1,000W continuous at 25C and 900W at 40C. The generator sizing tool covers the surge side of this if you are weighing a generator instead.
And solar will not fix a load you have not measured. Every number above is only as good as the daily watt-hour figure you fed it, and almost everyone’s first estimate is low.
Before you buy
Put your real numbers into the solar calculator, open “show the working”, and check that the array it gives you fits on the roof you actually have. If it does not, the answer is usually to cut the load, not to hope for better sun. Then read the battery bank guide, because an array with nowhere to put its power is just a warm roof.
Anything you intend to wire permanently is worth having checked by a qualified installer. These are planning estimates, not a system design.