How to size an RV or van solar system
Every off-grid electrical system answers four questions: how much energy you use in a day, how much you need to store, how much you can put back each day, and how much power you need at one moment. The calculator above answers each one in that order. Here is what it does and why.
1. Measure daily energy, not watts
Batteries and panels are about energy: watts multiplied by hours. A 45 W fridge that runs 40% of a 24-hour day uses 45 × 24 × 0.4 = 432 watt-hours (Wh). A 1,500 W microwave used for nine minutes uses 225 Wh before inverter losses. Big-wattage appliances matter for the inverter; long-running small ones usually dominate the battery.
Anything that runs through the inverter costs extra, because inverters are roughly 85–95% efficient. RVWatt divides AC loads by the inverter efficiency you set (90% by default) and can add the inverter's idle draw, which on some models is 10–25 W for every hour it is switched on. If you have a battery monitor, a few days of real readings beat any estimate.
2. Size the battery for cloudy days
Battery capacity = daily Wh × days without charging ÷ usable depth of discharge. Lithium iron phosphate (LiFePO4) can routinely use about 80% of its rating or more; AGM should stay above about 50% charge to last. Divide watt-hours by system voltage to get amp-hours. Two days of reserve is a sensible default for most travelers; full-timers in cloudy regions often choose three.
3. Size solar for your worst month
A peak sun hour is one hour of full-strength (1,000 W/m²) sunlight. A panel rated 200 W produces about 200 Wh per peak sun hour under test conditions, and less in real life. The catch is that sun hours change dramatically through the year. These NASA POWER averages for a flat panel show how much:
| Location | Dec | Mar | Jun | Sep |
|---|---|---|---|---|
| Phoenix, AZ | 3.08 | 5.89 | 8.50 | 6.16 |
| Moab, UT | 2.32 | 5.15 | 8.30 | 5.75 |
| Denver, CO | 2.29 | 4.73 | 7.22 | 5.37 |
| Hailey, ID | 1.54 | 4.08 | 7.51 | 5.14 |
| Seattle, WA | 0.85 | 2.85 | 5.73 | 3.91 |
| Miami, FL | 3.44 | 5.53 | 5.68 | 4.96 |
That is why RVWatt sizes to the weakest month among the months you actually travel. Many calculators use a single yearly average, which overstates winter output in northern states several times over.
Real panels then lose energy on the way to the battery. RVWatt multiplies these losses together and shows the result as the system factor:
| Loss | Default | Why |
|---|---|---|
| Panel heat | 10% | Panels lose roughly 0.3–0.4% of output per °C above 25 °C, and roof-mounted panels run hot. |
| Dirt and dust | 3% | Raise it for desert dust or if you rarely clean the roof. |
| Shading | 0% | Vents, a roof box or parking under trees can cost far more than this. |
| Wiring | 3% | Resistance in cable runs and connectors. |
| Mismatch and aging | 3% | Panels rarely produce exactly their rating, and output fades slowly. |
| Charge controller | 3% (MPPT) | PWM controllers waste much more on standard panels. |
| Battery charging | 5% (LiFePO4), 15% (AGM) | Not every watt-hour that goes in comes back out. |
With the defaults the system factor is about 0.76: a 200 W panel in a 5-sun-hour month puts about 200 × 5 × 0.76 ≈ 760 Wh a day into a lithium battery.
4. Count the alternator and shore power
Solar is not your only charger. A DC-DC charger delivers its rated current while you drive: 30 A for an hour on a 12 V system is roughly 340 Wh after losses. Shore power or a generator through a battery charger works the same way. RVWatt subtracts this energy before sizing panels, which is often the difference between a system that fits on a van roof and one that doesn't.
5. Size the controller, inverter and cable
An MPPT controller is rated by the current it pushes into the battery, so its size is array watts ÷ battery voltage, plus 25% headroom, rounded up to a standard size. The inverter must handle every AC load you run at the same time, plus headroom, and its surge rating must cover motor start-up. Cable between battery and inverter carries the most current in the system: RVWatt sizes that current at the inverter's low-battery cutoff (about 10.5 V on a 12 V system), when it draws the most, and picks the smallest copper gauge that both carries the fuse rating (ABYC 90 °C table, single conductor in free air) and keeps voltage drop under 3% over the round-trip length you enter. Use pure copper cable rated 90 °C or higher, not copper-clad aluminum, and go up a size if cables are bundled or run through a hot space.
Worked examples
Weekend van in Moab, Utah (May–October)
A compressor fridge, roof fan, lights, water pump and two phones add up to about 830 Wh a day. Traveling May through October, the weakest month in Moab is October at 4.34 peak sun hours. Solar needed: 830 ÷ (4.34 × 0.757) ≈ 253 W, so two 200 W panels or three 100 W panels. Battery for two days on lithium: 830 × 2 ÷ 0.8 ≈ 2,075 Wh, or about 173 Ah at 12 V, which in practice means a 200 Ah bank. With 400 W of panels, a 50 A MPPT controller covers the array with headroom. No inverter is needed.
Full-time remote worker in Hailey, Idaho
Add a laptop six hours a day, a Starlink Mini, a diesel heater and a little induction cooking, and use climbs to about 2,100 Wh a day, including about 60 Wh of inverter idle draw. Year-round, December is the design month at only 1.54 peak sun hours, so solar alone would need about 1,800 W, more than most vans can fit. A 30 A DC-DC charger with an hour of driving a day brings that to about 1,510 W. Traveling May through September instead, the weakest month is September at 5.14 hours and about 540 W is enough (three 200 W panels). The battery stays the same either way: about 5.3 kWh, or 438 Ah at 12 V. Hailey also has sub-freezing temperatures on record in every month except July and August, so this build should use self-heating lithium batteries or keep them in the heated cabin.
The lesson: winter van life in the northern Rockies runs on the alternator. Solar is the summer workhorse.
Guides
Short, practical pages for the questions people ask most often. Each one uses the same numbers as the calculator and can open it with a starting point already filled in.
- RV and van appliance wattage chart — typical watts and daily watt-hours for fridges, fans, lights and more.
- How much solar to run a 12 V RV fridge — panel and battery size for a compressor fridge, by place and season.
- Solar for a camper van — weekend and full-time van examples, including why winter in the north needs more than solar.
- Solar for a travel trailer — a common 8–10 cu ft 12 V fridge setup, and when a 400 W factory kit is enough.
- Peak sun hours for RV travel — NASA monthly figures for 74 places, and why we size to your weakest travel month.
- LiFePO4 vs AGM batteries for an RV — how much more AGM you need for the same day, charging losses, weight and cold-weather charging.
- What size inverter for an RV or camper van — size from what runs at the same time, check surge, and see the fuse and cable each size needs.
- MPPT vs PWM charge controllers — why PWM collects less from the same panels and how much more array it needs.
- Can you run an RV air conditioner on solar? — honest panel, battery and inverter numbers, and what usually works better.
- How long does a 100Ah lithium battery last? — runtime for common RV loads on one battery, and why big 120 V appliances hit the current limit first.
- How much solar to charge a 200Ah battery — panel watts by place and month, and whether 400 W covers your day.
- Running a CPAP in an RV or van on battery — nights per battery with and without the humidifier and inverter, and the solar it adds.
- How we calculate this — NASA sun data, the losses we count, the November 2026 audit, and the real-rig check.
Frequently asked questions
How many solar watts do I need for my van?
Divide your daily watt-hours by the peak sun hours of your worst travel month, then divide by a system factor for real-world losses (about 0.76 with an MPPT controller and lithium batteries). A van using 1,000 Wh a day in a 4-sun-hour month needs about 1,000 ÷ (4 × 0.76) ≈ 330 W. In winter at northern latitudes the same van can need three to five times that, which is why winter travelers lean on alternator charging.
What is a peak sun hour?
One peak sun hour is one hour of sunlight at 1,000 watts per square meter, the intensity solar panels are rated at. A location's daily solar energy in kWh per square meter equals its peak sun hours. RVWatt uses NASA POWER's 2001–2025 monthly averages for a flat, horizontal surface, which matches panels mounted flat on a roof.
Why does RVWatt size for the worst month instead of the yearly average?
Because batteries run flat in the worst month, not the average one. In Hailey, Idaho a flat panel gets about 7.7 peak sun hours in July but only about 1.5 in December. Sizing to the annual average would leave you short for months. Uncheck the months you don't travel and the calculator sizes to your weakest remaining month.
How much battery do I need for 2 days off-grid?
Multiply your daily watt-hours by 2 and divide by the usable depth of discharge: 0.8 for LiFePO4 or 0.5 for AGM. Divide by system voltage to get amp-hours. At 1,000 Wh a day on 12 V lithium that is 1,000 × 2 ÷ 0.8 ÷ 12 ≈ 208 Ah.
LiFePO4 or AGM?
LiFePO4 gives you about 80–100% of its rating, weighs much less per usable kWh and is rated for thousands of cycles, but most cells must not be charged below freezing. AGM is cheaper up front and tolerates cold charging, but you should only use about half its rating. For full-time use LiFePO4 is often cheaper per usable kWh over its life.
Can I charge lithium batteries below freezing?
Usually not. Most LiFePO4 batteries must not be charged below 0 °C (32 °F) because it can plate lithium and permanently damage the cells; many BMSs block charging below that temperature. If you camp in the cold, use self-heating batteries or mount the bank inside the heated living space. RVWatt flags locations where sub-freezing temperatures have been recorded in your travel months.
How do I size the charge controller?
An MPPT controller's rating is its output current into the battery. Divide array watts by battery voltage and add 25% headroom: 400 W ÷ 12 V × 1.25 ≈ 42 A, so a 50 A controller. Also check the controller's maximum PV input voltage against your panels' open-circuit voltage in the coldest weather; that depends on how you wire the panels and is not covered here.
What size inverter do I need?
Add the watts of every AC appliance you might run at the same time, add 25% headroom, and check that the inverter's surge rating covers motor start-up (fridges, A/C and pumps can briefly draw several times their running watts). RVWatt assumes short, high-power appliances like kettles, coffee makers, hair dryers and blenders run one at a time, so the inverter is sized for whichever is bigger: everything else running together, or the biggest of those on its own. Count the inverter's idle draw too, about 8–25 W for every hour it is on. Use a pure sine wave inverter for electronics and induction cooktops.
Does alternator charging really make a difference?
Yes. A 30 A DC-DC charger running one hour a day delivers roughly 340 Wh on a 12 V system, about what 100 W of panels deliver on a 4.5-sun-hour day, and it works in winter and under trees. Enter your typical driving hours and the calculator subtracts that energy before sizing solar.
Is this calculator electrical advice?
No. RVWatt gives planning estimates from published formulas and public climate data. Wire sizes are a starting point from the ABYC 90 °C copper ampacity table and a 3% voltage-drop limit. Follow your equipment manuals and use the larger size when they differ, fuse every positive conductor near the battery, and have a qualified installer check high-current wiring. Use of the site is subject to our Terms of Use.
Data and methodology
Sun hours are NASA POWER all-sky surface shortwave irradiance (ALLSKY_SFC_SW_DWN), monthly climatology for January 2001 to December 2025, on a horizontal surface, which is how most RV and van panels are mounted. NASA's solar data has about 1° (roughly 100 km) resolution, so mountain valleys, coastlines and local weather patterns can differ; if you know your local value, choose "Custom peak sun hours". Tilting portable panels toward the sun produces more than these figures in winter. Temperatures are NASA POWER's monthly minimum air temperature (T2M_MIN, the coldest reading in each month over the period, not the average low), used only for the cold-charging warning. Cable ampacity follows the ABYC E-11 table for 90 °C copper conductors outside engine spaces; bundled or enclosed cables need derating. Everything runs in your browser; nothing you enter is sent anywhere.
Last reviewed: November 2026.