Solar Panel Output Calculator
Estimate how much energy your solar installation will generate. Enter panel wattage, number of panels and peak sun hours for your location to get daily, monthly and yearly kWh output with an estimated annual saving at your electricity tariff.
Last updated: May 2026
Enter panel wattage, number of panels and peak sun hours to estimate output.
Daily kWh = (Wp × panels / 1000) × peak sun hours × efficiency
How to estimate solar panel output
Solar output depends on three variables: the rated power of your panels in peak watts (Wp), the number of hours per day at which irradiance reaches 1,000 W/m² (peak sun hours for your location), and system losses from the inverter, cabling, soiling, and temperature. The standard formula is:
Daily kWh = system size (kWp) × peak sun hours × system efficiency
A 4 kWp system in the Netherlands (2.8 peak sun hours, 80% efficiency) produces approximately 4 × 2.8 × 0.80 = 8.96 kWh per day, or around 3,270 kWh per year. The same system in Spain (5.0 peak sun hours) produces roughly 5,840 kWh per year.
Peak sun hours by location
Annual averages. Actual output varies by season, roof orientation, shading, and local climate. See the full peak sun hours reference for 25 European cities, all 50 US states and 8 world cities.
| Location | Peak sun hours / day | Annual kWh per kWp (at 80% eff.) |
|---|---|---|
| United Kingdom | 2.6 h | 759 kWh/kWp |
| Netherlands | 2.8 h | 817 kWh/kWp |
| Germany | 3.1 h | 905 kWh/kWp |
| France (north) | 3.5 h | 1,022 kWh/kWp |
| Spain / southern Europe | 5.0 h | 1,460 kWh/kWp |
| United States (avg.) | 4.5 h | 1,314 kWh/kWp |
| Australia | 5.5 h | 1,606 kWh/kWp |
Why 80% as a default system efficiency?
Modern string inverters run at 96 to 98% efficiency, but total system losses add up. Cable losses account for 1 to 2%, soiling and shading 2 to 5%, temperature derating 3 to 8% (panels lose about 0.4% efficiency per degree above 25°C), and mismatch losses 1 to 2%. A combined efficiency of 78 to 82% is a realistic average for a well-installed rooftop system in northern Europe. Premium microinverter or optimiser systems can reach 85% by eliminating string shading losses.
Frequently Asked Questions
What is the difference between Wp (peak watts) and actual output?
Wp is the panel's rated output under Standard Test Conditions (STC): 1,000 W/m² irradiance, 25°C cell temperature, and AM 1.5 spectrum. In real installations, cells are warmer and irradiance is rarely at the full 1,000 W/m² for more than a few hours around midday on a clear day. A 400 Wp panel in the Netherlands typically produces 350 to 380 kWh per year (roughly 1 kWh per Wp per year), not 400 kWh. The Wp figure is useful for comparing panels to each other; the output calculator translates it to realistic annual energy.
How does roof orientation and tilt affect solar output?
In the northern hemisphere, south-facing roofs at a tilt matching the latitude (35 to 50 degrees for most of Europe) give maximum annual output. East and west orientations reduce annual yield by roughly 15 to 20% compared to south, but spread generation over more hours of the day, which can reduce peak export to the grid. A flat roof loses about 10% compared to the optimal tilt. North-facing roofs in northern Europe may produce 30 to 50% less than south-facing. Use the calculator with a reduced efficiency (65 to 70%) to estimate output from non-optimal orientations.
Can I use this calculator for a system with a battery?
Yes. The calculator shows total energy generated. Whether you self-consume it immediately, store it in a battery for evening use, or export it to the grid does not change the total kWh produced. A battery only affects when you use the energy and how much you export, not how much the panels generate. To estimate battery payback separately, take the annual kWh from this calculator, estimate how much of that you would actually consume at home versus export, and apply the relevant import tariff and export tariff to find the net saving.
How many panels do I need to cover my electricity use?
Divide your annual electricity consumption (from your energy bill in kWh) by the annual output per panel in kWh. In the Netherlands, a 400 Wp panel generates roughly 340 to 360 kWh per year. A household using 3,500 kWh per year needs approximately 10 panels to theoretically match its consumption. In practice, solar generation peaks in summer and is low in winter, so a grid connection or battery is needed to balance the difference regardless of how many panels you install.