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Solar Output Calculator: How much power does the solar system on my RV deliver?

200 W on the RV roof deliver around 700 Wh per day in July in central Germany – but only about 70 Wh in December. This calculator shows what your solar system realistically delivers in which month and location, and whether that's enough for your daily consumption.

It is based on typical irradiation values per region and month (Europe) or your own peak sun hours (e.g. from NREL PVWatts), plus a performance ratio that reflects losses from temperature, flat mounting, charge controller and wiring.

Units

Sum of the panels' rated power per the data sheet.

Location and period

Typical irradiation of the region (long-term averages, rounded) – for the US use a custom value.

Daily irradiation in kWh/m² – e.g. from NREL PVWatts for your site. Continental US: winter ≈ 2–4, summer ≈ 5–7 depending on the region.

If given, the calculator shows whether the output covers consumption.

Advanced settings

Losses from panel temperature, dirt, charge controller, cables, partial shading. RV flat: 60–75%.

Result

Solar output per day (custom value)

630 Wh

at 70% performance ratio, flat-mounted

Equals (charging current equivalent)
49.2 Ah/day
Output in the month
18.9 kWh
Peak sun hours (reference)
4.5 h/day
Show calculation
  1. 1Irradiation (custom value): 4.5 kWh/m² per day ≙ 4.5 peak sun hours
  2. 2Output per day: 200 W × 4.5 h × 70% = 630 Wh
  3. 3Output over the period (30 days): 630 Wh × 30 = 18.9 kWh

 

How it's calculated

The rated power of a solar panel (W) applies to an irradiance of 1,000 W/m² under standard test conditions. The daily irradiation total in kWh/m² therefore equals the number of “peak sun hours”: at 4 kWh/m² per day the panel effectively works 4 hours at rated power.

Output [Wh/day] = power [W] × peak sun hours [h/day] × tilt factor × performance ratio
Example July, central Germany, flat: 200 W × 5.2 h × 1.0 × 0.70 = 728 Wh

The performance ratio sums up all losses: high panel temperatures in summer, dirt, charge controller (PWM or MPPT), cable losses and partial shading from roof equipment. For flat-mounted RV systems 60–75% is realistic.

Flat-mounted panels receive roughly the irradiation of the horizontal surface. A panel tilted toward the south (30–35°) gains about 10–15% on an annual average in central Europe – much more in winter, almost nothing in summer.

Typical horizontal irradiation in kWh/m² per day (rounded long-term averages, Europe)
RegionDecemberMarchJuneSeptemberYear
Northern Germany0.42.45.02.9≈ 985 kWh/m²
Central Germany0.52.65.43.2≈ 1,070 kWh/m²
Southern Germany0.72.95.73.5≈ 1,155 kWh/m²
Alps1.03.25.83.8≈ 1,230 kWh/m²
Mediterranean (south)2.04.57.35.2≈ 1,735 kWh/m²

For locations in the US, enter the peak sun hours for your site and month as a custom value – the NREL PVWatts calculator provides monthly solar radiation (kWh/m²/day) for any US address.

Worked example

A 300 W system lies flat on the roof of a camper van in southern Germany. April is calculated at 70% performance ratio; daily consumption is 800 Wh.

Inputs

  • Solar power: 300 W
  • Mounting: Flat on the roof (typical RV)
  • Location: Southern Germany
  • Month: April
  • Daily power consumption (optional): 800 Wh
  • Performance ratio (system efficiency): 70 %
  • System voltage (for the Ah figure): 12 V (LiFePO4 12.8 V)

Result

882 Wh

Solar output per day (April)

Equals (charging current equivalent)
68.9 Ah/day
Output in the month
26.5 kWh
Peak sun hours (reference)
4.2 h/day
Coverage of daily consumption
110%

Calculation

  1. Irradiation (April): 4.2 kWh/m² per day ≙ 4.2 peak sun hours
  2. Output per day: 300 W × 4.2 h × 70% = 882 Wh
  3. Output over the period (30 days): 882 Wh × 30 = 26.5 kWh
  4. Coverage: 882 Wh ÷ 800 Wh consumption = 110%

The variables explained

Solar power (W)
Rated power of the panels under standard test conditions (1,000 W/m², 77 °F / 25 °C cell temperature).
Peak sun hours
Daily irradiation total in kWh/m². Varies greatly by region and season; exact site values come from NREL PVWatts (US) or PVGIS (Europe).
Tilt factor
Extra output of a south-facing tilted surface compared to flat mounting (1.0 = flat).
Performance ratio
Ratio of actual to theoretical output. Includes temperature, controller, cable and soiling losses.
Daily power consumption (Wh)
Optional: sum of all loads per day – see the power consumption calculator.

Common mistakes

  • Planning with the summer output: in winter the same system in northern latitudes delivers only a tenth of the July output.
  • Treating rated power as continuous power: 200 W does not mean 200 W throughout the day – on a summer day it averages about 30 W.
  • Underestimating shading: even one partially shaded cell can greatly reduce the output of a whole panel – watch roof equipment such as the air conditioner or satellite dish.
  • Forgetting that the battery can be full: surplus the battery can no longer absorb is lost. The output is an upper limit, not a guaranteed charging current.
  • Equating PWM and MPPT controllers: an MPPT controller gets noticeably more out, especially in cool weather and with higher panel voltage.

Assumptions and limits

  • Irradiation values are rounded long-term averages per region; the actual weather in a given year deviates considerably (±30% in the monthly average is normal).
  • The tilt factor applies to a southern orientation in central Europe; other orientations reduce the gain.
  • The performance ratio is a blanket value; it does not replace a detailed simulation.
  • The output assumes the battery can absorb the energy (not full, not too cold).
  • Results serve planning and are not a guarantee of output.

Frequently asked questions

How many watts of solar do I need for my RV?

That depends on daily consumption and travel season. For 600–800 Wh per day in summer, 200–300 W is usually enough in the northern US and Germany; if you want to boondock in spring and fall you need more like 400 W and up. The solar panel size calculator works that out backwards for you.

Is tilting the panels worth it?

Hardly in summer (a few percent), considerably in the shoulder seasons and winter (+30 to +50%). For fixed roof systems that's usually impractical; portable folding panels, on the other hand, can be aimed at the sun.

Where do I get exact irradiation values for my location?

For the US, the free NREL PVWatts calculator provides monthly values for any address; for Europe, the PVGIS tool of the European Commission. Enter the value (kWh/m² per day, horizontal) under “Custom value”.

Sources and background