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.
Where this calculator sits in the sizing chain
This daily kWh figure is what the rest of your solar system is built around. Before it means anything, two upstream numbers must be solid: the off-grid cabin sizing guide produces the total watt-hour demand your array has to meet, appliance by appliance, and the peak sun hours reference supplies the irradiance input you enter here. For any off-grid or winter-critical build, the gloomiest-month value from that reference is the figure to use, not the annual average. You are here: estimated array output in daily kWh. Real panels deliver 75 to 85% of their nameplate rating once heat, cable loss, soiling and inverter cut are accounted for, and this calculator applies that correction before passing a number downstream. Three tools read directly from this production figure: the battery bank sizing calculator checks whether the daily yield can refill the bank on a short-day cycle and sizes the Ah reserve accordingly; the charge controller calculator takes the kWp total and adds a cold-weather current margin; and the inverter sizing calculator cross-references peak production against the continuous and surge loads the system must handle. Finally, once annual kWh is known, the solar payback calculator applies your tariff to that figure and returns a realistic return period. Two adjacent tools also draw on this output: the solar panel layout calculator translates the sized array into physical roof area, and solar panels for EV charging compares the daily kWh budget against what your electric car actually consumes.
A note from sizing real systems: the efficiency field is where the gap between nameplate and reality lives. Panels are rated at 25 degrees cell temperature, but on a warm summer afternoon in the Netherlands the cell easily reaches 50 to 60 degrees, cutting output by roughly 10 to 15%. For off-grid planning, run the calculator twice: once on the annual average to check the yearly energy budget, then again on the January or December peak sun hours (around 1 h on a Dutch roof) to see what the array delivers on the days the battery bank actually has to survive.
Frequently Asked Questions
Why does my actual output fall short of the Wp rating every hour of the day?
The Wp rating is tested at Standard Test Conditions: 1,000 W/m² irradiance, a cell temperature of exactly 25 degrees, and a specific lab spectrum. Real panels run hotter than 25 degrees whenever the sun is actually strong (cell temperatures of 50 to 60 degrees are typical on a warm afternoon), and irradiance only hits 1,000 W/m² briefly around solar noon on clear days. The combined effect is that a panel operating in normal conditions delivers 75 to 85% of its nameplate rating, which is why the 80% default in this calculator is not a conservative figure but a realistic one. A 400 Wp panel in the Netherlands produces roughly 340 to 360 kWh per year, not 400 kWh. Use the Wp number to compare panels against each other; use this calculator to translate it to actual 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.
Methodology and sources
This tool estimates the energy a photovoltaic array generates by scaling its rated DC power (kWp) by the daily peak sun hours for your location and a combined system-efficiency factor that accounts for inverter, cabling, soiling and temperature losses.
- Method: system size (kWp) = panel Wp × number of panels / 1000; daily kWh = kWp × peak sun hours × (efficiency / 100); monthly = daily × 30.44; annual = daily × 365.25; estimated saving = annual kWh × tariff.
- Standards and sources: Standard Test Conditions per IEC 60904-3 (1,000 W/m² irradiance, 25°C cell temperature, AM 1.5 spectrum) define the Wp rating; peak-sun-hour figures are drawn from public solar-resource datasets (PVGIS for Europe, NREL NSRDB for the United States). The output relation itself is the standard kWp × peak-sun-hours yield model.
- Assumptions and limits: peak sun hours are annual averages; real output varies with season, roof orientation and tilt, shading and local climate. The default 80% efficiency reflects a well-installed rooftop string system; non-optimal orientation or older equipment warrants a lower value. Savings use a single flat tariff and ignore export/feed-in rates and self-consumption split.
Reviewed and maintained by Rick Oosterling, who builds and wires 12 V, solar and EV systems hands-on. Last reviewed: June 2026. These are planning estimates only; actual generation, tariffs, prices and usage vary, so confirm yields with a site-specific assessment or installer before committing.