How this is calculated
Each load is watts times hours, added into one daily watt-hour figure. The fridge is entered differently because it cycles: compressor watts times 24 hours times the share of the day it actually runs.
Then the day has to be refilled, and a watt of panel does not bank a watt-hour per sun hour. NREL's PVWatts Version 5 carries a default 14 percent system loss, built in its Table 6 from 3 percent shading, 3 percent availability, 2 percent soiling, 2 percent mismatch, 2 percent wiring and smaller terms. Victron rates a LiFePO4 battery at 92 percent round trip. Multiply the two and each watt of panel banks about 0.79 watt-hours per peak sun hour.
The charge controller takes a further cut on top of that, and how much depends on the unit. It is left out here, so read the array figure as a floor.
Battery is the daily figure times your dry-camping days, divided by 10.24 watt-hours per amp-hour: a 12.8 volt LiFePO4 module worked to 80 percent depth, the depth Victron rates at 2,500 cycles.
Worked example. A 45 W fridge running 40 percent of the day (432 Wh), 12 W of lights for 5 hours (60 Wh), a 60 W pump for half an hour (30 Wh), a 25 W roof fan for 8 hours (200 Wh) and a 45 W laptop for 4 hours (180 Wh) come to 902 Wh. At 4.2 peak sun hours that is 902 ÷ 3.32 = 271 W of panel, so two 200 W modules, 176 Ah of battery for two days without sun, and a 30 A controller sized on the 400 W actually fitted.
Anything fed through the inverter costs more than its plate says, so enter those loads high. Take the result to the power station calculator if the plan is a portable unit rather than a wired system.
Quick answers
How many solar panels do I need for a travel trailer?
Two 200 W modules cover a fridge, lights, pump, fan and laptop at 4.2 peak sun hours, and the same two at 3.0 with nothing spare. Satellite internet or air conditioning changes that answer completely.
How much solar does an RV air conditioner need?
Four hours of a 1,600 W rooftop unit is 6,400 Wh, which alone asks for about 1,930 W of panel and 120 square feet of clear deck. Most rigs do not have it.
How many watts of solar fit on an RV roof?
About 16 W per square foot of clear deck, so a 26 ft trailer with half its roof free tops out near 1,600 W.
What size battery goes with RV solar?
Daily watt-hours times your no-sun days, divided by 10.24. A 900 Wh day with two days of reserve is 176 Ah of 12 V LiFePO4.
Does Starlink change the answer?
Yes, because it runs all day. Sized at the top of Starlink's published range a Mini adds 960 Wh and a Standard dish 2,400 Wh — more than the rest of a normal trailer put together.
How do I size an MPPT charge controller for an RV?
Array watts divided by 14.4 V, rounded up to the next standard rating: 400 W is 28 A, so a 30 A unit. The array side is a separate check against module short-circuit current.
What each RV load costs in a day
These are the calculator's starting points. Sort by watt-hours and the order tells the story: anything that runs around the clock, or makes cold air, buries the rest.
| Load | Watts | Hours a day | Watt-hours | Where the watts come from |
|---|---|---|---|---|
| 12 V compressor fridge | 45 | 9.6 running | 432 | Your fridge's plate |
| Roof fan | 25 | 8 | 200 | Your fan's plate |
| Laptop and phones | 45 | 4 | 180 | Charger plate rating |
| LED lighting | 12 | 5 | 60 | Count the fixtures |
| Water pump | 60 | 0.5 | 30 | Pump plate |
| CPAP, blower and humidifier | 35 | 8 | 280 | Your machine's manual |
| Starlink Mini | 40 | 24 | 960 | Spec sheet: 25 to 40 W average |
| Starlink Standard | 100 | 24 | 2,400 | Spec sheet: 75 to 100 W average |
| Rooftop air conditioner | 1,600 | 4 | 6,400 | The unit's plate |
Both satellite rows are sized at the top of the published range, which is the figure to plan on. A dish that averages 40 W outruns a fridge, a fan and a laptop combined.
Air conditioning hides a second problem: the compressor kicks hard at start-up, and that surge sizes the inverter rather than the array, the same trap the well pump calculator deals with.
How much panel the roof will take
Power per unit of area barely varies between modules. On Victron's BlueSolar monocrystalline datasheet a 175 W panel measures 1485 by 668 mm and a 360 W panel 1980 by 1002 mm: 176 and 181 watts per square meter, with the 185 W panel on the same 1485 by 668 frame reaching 186.
This page sizes roof space on the low end of that range, 176 W per square meter, or about 16 watts per square foot. A 100 W module then needs about 6 square feet, a 200 W about 12, a 400 W residential module about 24.
The deck is body length times body width, and roughly half of it is already taken by the air conditioner, vents, skylight, antenna and fridge flue.
Width on the National Network is capped at 102 inches by 23 CFR 658.15, so an 8 ft trailer body is close to the legal maximum and a 26 ft box gives roughly 200 sq ft of deck.
Take the deck from the body, not the towing length: a 7.5 by 2.5 m caravan is about 19 square meters, and about half of that survives the vents, air conditioner and flue.
Sun hours on a flat roof
An RV roof lies flat, and flat is not where the sun-hour figures in most solar guides come from. Over a year the European Commission's PVGIS puts a horizontal plane in central Kansas at 4.65 peak sun hours a day against 5.47 on an optimally tilted one, a penalty near 15 percent. In December and January the gap widens to 2.2 hours flat against 3.9 tilted.
Daily averages on a horizontal plane, from PVGIS using the PVGIS-NSRDB radiation database over 2005 to 2015. The calculator starts at 4.2, which is under the middle of the country and well over the Pacific Northwest.
| Where you park | Year-round | December and January |
|---|---|---|
| Phoenix, AZ | 5.9 | 3.2 |
| Orlando, FL | 5.1 | 3.4 |
| Denver, CO | 4.8 | 2.3 |
| Central Kansas | 4.7 | 2.2 |
| Portland, OR | 3.5 | 1.0 |
| Seattle, WA | 3.4 | 0.9 |
Daily averages on a horizontal plane, from PVGIS using the PVGIS-SARAH2 radiation database over 2005 to 2020. The calculator starts at 3.0, which suits northern and central Europe; Iberia and Greece run half as much again.
| Where you park | Year-round | December and January |
|---|---|---|
| Athens | 5.0 | 2.3 |
| Madrid | 4.9 | 2.2 |
| Berlin | 3.1 | 0.6 |
| Utrecht | 3.0 | 0.7 |
Midwinter is the column that decides whether you carry a generator. A bank sized on the year-round figure runs flat in the first week of December.
Common rigs, start to finish
Every row runs the same base list — the fridge, lights, pump and roof fan from the worked example, 722 Wh — and adds one thing. Two days of dry camping, 200 W modules.
| Pattern | Wh a day | Array at 4.2 sun hours | Array at 3.0 sun hours | 200 W panels | Roof taken | Battery, 2 days | Controller |
|---|---|---|---|---|---|---|---|
| Weekend van or teardrop | 722 | 217 W | 304 W | 2 | 24 sq ft | 141 Ah | 30 A |
| Trailer, full-time, plus a laptop | 902 | 271 W | 380 W | 2 | 24 sq ft | 176 Ah | 30 A |
| Full-time plus CPAP | 1,182 | 356 W | 498 W | 2 | 24 sq ft | 231 Ah | 30 A |
| Full-time plus Starlink Mini | 1,862 | 560 W | 784 W | 3 | 37 sq ft | 364 Ah | 50 A |
| Full-time plus Starlink Standard | 3,302 | 994 W | 1,391 W | 5 | 61 sq ft | 645 Ah | 80 A |
| Full-time plus 4 hours of air conditioning | 7,302 | 2,197 W | 3,076 W | 11 | 135 sq ft | 1,426 Ah | 160 A |
The last row is the useful one. Air conditioning asks for more clear deck than a 26 ft trailer has, which is why full-timers in hot country carry a generator or plug in, and why shade and a roof fan are the cheaper answer.
Battery, controller and the cold
PVWatts allows 2 percent for soiling and nothing at all for snow, both of which a flat panel collects and a tilted one sheds. That is the part of the 14 percent an RV quietly overspends.
Cold shrinks the battery too, and that is the figure most RV plans miss. Victron rates its 12.8 volt LiFePO4 modules at full capacity at 25 degrees Celsius, 80 percent at 0 and half at minus 20, and they must not be charged below 5. Cold-country installs put the bank inside the heated space.
| Cell temperature | 100 Ah module delivers | 200 Ah bank delivers |
|---|---|---|
| 25 °C (77 °F) | 100 Ah | 200 Ah |
| 0 °C (32 °F) | 80 Ah | 160 Ah |
| −20 °C (−4 °F) | 50 Ah | 100 Ah |
- Size the controller on output amps, then check the array side against module short-circuit current — Victron rates its 175 W panel at 9.89 A, and parallel strings add. Have that rating confirmed against the electrical code where the rig is registered.
- PVWatts allows 2 percent for wiring across a whole system, which one long roof-to-controller run can spend on its own. Size the pair in the DC wire size calculator.
- A tilting bracket buys back most of what a flat roof loses in midwinter: in December and January PVGIS puts central Kansas at 3.9 peak sun hours tilted against 2.2 flat.
- One panel shaded by the air conditioner or an awning drags a whole series string down.
Shore power arrives at 120 V, so the inverter, not the array, decides which appliances work away from a post.
Campsite hookups deliver 230 V behind a small breaker, so the inverter, not the array, decides which appliances work off the pitch.
Sources
Satellite draw: Starlink Mini and Starlink Standard specification sheets, 25 to 40 W and 75 to 100 W average, retrieved September 2026. Module area and short-circuit current: Victron Energy BlueSolar Monocrystalline Panels datasheet (175 W at 1485 × 668 mm, 9.89 A Isc; 360 W at 1980 × 1002 mm). Battery capacity by temperature, 92 percent round trip, 2,500 cycles at 80 percent depth and the 14 to 14.4 V charge voltage: Victron Energy 12,8 and 25,6 Volt Lithium-Iron-Phosphate Smart datasheet. System losses: PVWatts Version 5 Manual, NREL/TP-6A20-62641, Table 6. Peak sun hours: European Commission PVGIS monthly radiation tool on the horizontal plane, PVGIS-NSRDB 2005 to 2015 for North America and PVGIS-SARAH2 2005 to 2020 for Europe. Width limit: 23 CFR 658.15. Appliance watts other than the two dishes are editable starting points rather than datasheet values. Checked September 2026.
Go deeper
For a portable unit rather than a wired install, size it on the power station calculator and compare hardware in the RV and van-life power station guide. Working from a battery you own, use what size solar panel to charge a battery. The internet load has Starlink off-grid power; the bank has the battery bank calculator.