Solar Daily Energy Harvest Estimator
Calculates real-world daily Watt-hours and Amp-hours generated by your solar array. Incorporates flat roof mounting derating and seasonal insolation changes.
Enter the values above and click Calculate to view recommendations and engineering specifications.
Understanding Real-World Solar Derating Factors
Calculation Assumptions
- Total daily harvest = Array Rated Watts × Geographic Peak Sun Hours × System Derate Factor (~75%–80%).
- Derates account for panel soiling, wiring loss, MPPT tracking, and ambient heat.
Engineering Limitations
- Does not predict real-time cloud transient cover or complex tree canopy shading.
A 400W roof array with 5.0 Peak Sun Hours and a 0.78 system derating produces approximately 1,560 Watt-hours (120 Ah at 13V) per sunny day.
A 400W solar panel never produces 400W continuously. Standard Test Conditions (STC) assume 1000 W/m² irradiance at a chilly 25°C (77°F) cell temperature. Under real operating conditions:
- Flat Rooftop Mounting Loss (~10-15%): Solar panels mounted completely flat on a van or trailer roof receive sunlight at non-perpendicular angles for most of the day, reducing morning and afternoon output.
- Thermal Derating (~8-12%): Solar cells in summer sun routinely reach 55°C to 65°C. For every degree above 25°C, crystalline silicon panels lose roughly 0.35% of their power.
- Dust, Wiring & MPPT Conversion (~5-8%): Soiling on the glass, resistance across wiring runs, and the 98% efficiency of the MPPT charge controller introduce minor cumulative losses.
Frequently Asked Questions
How many Watt-hours will a 400W solar array produce in a day?
Daily harvest equals: Array Watts * Peak Sun Hours * System Efficiency. In a region receiving 4.5 peak sun hours per day with an 80% system performance ratio (accounting for heat, dust, wiring, and MPPT conversion losses), a 400W array produces: 400W * 4.5 hours * 0.80 = approximately 1,440 Watt-hours (Wh) per day, or 110-120 Ah at 12V.
What is a 'Peak Sun Hour'?
A Peak Sun Hour is not simply an hour of daylight; it is defined as one hour during which solar solar irradiance averages 1,000 Watts per square meter (1 kW/m²). Most continental US locations receive between 3.5 and 5.5 peak sun hours per day on average over the year.
How much solar power is lost to flat rooftop mounting on an RV or van?
Mounting solar panels horizontally flat on an RV or campervan roof results in an annual energy loss of 15% to 30% compared to panels tilted toward the sun at the optimal latitude angle. Winter harvest is impacted even more severely (up to 50% loss) due to low solar azimuth angles.
How does partial shading affect solar panel output?
Shading just a single solar cell in a series string can reduce the output of the entire module by 50% to 80%. Bypass diodes inside modern solar panels help route current around shaded cell groups, but maintaining unshaded panel placement away from AC units, roof vents, and antennas is critical.