Planning an RV solar system: five steps to the right size
The most common question is “How many watts do I need?” – but the right answer starts with consumption, not with the panel. This guide walks through the planning in five steps and points to the matching calculator at each stage.
Step 1: Determine daily consumption
List all loads with power and daily running time. Cycling appliances (fridge, furnace fan) only count with their actual running time. 120 V appliances through the inverter cost an extra 10–15% in losses plus idle consumption.
Power consumption calculator – with presets for typical appliances
Step 2: Define travel season and location
Irradiation varies enormously between December and July – in Germany by a factor of ten, in the northern US by a factor of three to four. A system that suffices in July is hopelessly undersized in October – and a system designed for winter is pointlessly large in summer. Choose the worst month in which you regularly want to boondock as the design month.
- Summer campers (May–September): 5–7 peak sun hours per day in most of the US
- Shoulder seasons (March/April, October): 3–5 peak sun hours depending on the region
- Winter in the north: 2–3 peak sun hours – better plan a DC-DC charger or shore power here
The NREL PVWatts calculator provides monthly solar radiation (kWh/m²/day = peak sun hours) for any US address.
Step 3: Calculate panel wattage
Power [W] = consumption [Wh] × (1 + reserve) ÷ (peak sun hours × performance ratio)The performance ratio of 0.6–0.75 for flat-mounted panels is realistic: panel temperature, dirt, controller, cables and partial shading from roof equipment together cost a quarter to a third of the rated power. If you can tilt the panels, you gain 20–50% in the shoulder seasons.
Solar panel size calculator – with your own peak sun hours
Step 4: Choose a matching battery
Solar only delivers during the day; the battery bridges the night and rainy days. Two to three days of consumption as usable capacity is a good guideline. Conversely, the solar system must also be able to refill the battery after a spell of bad weather – the solar charge time calculator shows how many sunny days that takes.
Off-grid days calculator – weighs consumption, solar output and battery against each other
Step 5: Charge controller and wiring
- MPPT instead of PWM: from about 150–200 W an MPPT controller gets 10–30% more out, especially in cool weather and with higher-voltage panels.
- Controller size: the controller must handle the short-circuit current and open-circuit voltage of the panels (higher in the cold!) – check the data sheet.
- Wiring: from controller to battery short and thick (voltage drop under 1–2%), otherwise the charge voltage doesn't arrive.
- Fusing: fuse every wire close to the battery; manufacturer specifications and codes are authoritative.
Wire gauge calculator – for the wire controller → battery
Common mistakes
- Planning by roof area instead of by consumption.
- Confusing rated power with continuous power – 200 W deliver about 30 W on the daily average of a summer day.
- Ignoring shading from roof vents, air conditioner or satellite dish – partial shade can knock out a whole panel.
- Not setting battery and controller to LiFePO4.
Sources
Published 09/19/2026.