What the Solar Panel Output Calculator does
This calculator estimates how much electricity a solar array produces each day, month and year from the panel rating, the number of panels and the peak sun hours at your site, after the losses every real system has. It also works backwards: tell it how many kWh a day you use and it tells you how many panels of that size cover it.
The loss model is the one NREL's PVWatts uses - soiling, shading, wiring, mismatch and the rest, combined multiplicatively to 14.08% by default - plus a temperature loss and the inverter's efficiency. Nothing is looked up for you: sunshine varies too much by site, tilt and season to guess, so you enter the peak sun hours for your location.
How to use it
- Enter the panel rating in watts (the STC figure on the data sheet) and how many panels you have or plan.
- Enter peak sun hours for your site: the average daily solar energy on the panel plane in kWh/m², which equals hours of full 1,000 W/m² sun. A solar resource map or the PVWatts calculator gives it for your address, tilt and orientation.
- Check the inverter efficiency (96% is the PVWatts default) and add a temperature loss if your panels run hot.
- Open the system losses to change any of the ten PVWatts categories - more shading, a dusty site, an older array.
- For a year that follows the seasons, paste twelve monthly peak sun hour values; each month then uses its real number of days.
- Enter your daily use as a target to see how many panels of the chosen rating would cover it on an average day.
Reading the results
Daily kWh is an average-day figure. Cloudy days produce much less and clear summer days more; a single annual peak sun hour value hides that swing, which is why the monthly table is worth filling in for off-grid or battery sizing.
The performance ratio is the share of the nameplate DC energy that survives all losses. Well-designed grid-tied systems usually land around 75-85%; much higher than that suggests a loss has been left out.
Panels for target covers the average day only. Covering the worst month, or days without sun, needs either more panels or storage - see the battery backup calculator.
Worked example: a 4 kW rooftop array
Ten 400 W panels make a 4 kWp array. The site gets 5 peak sun hours a day on average, the inverter is 96% efficient and the PVWatts default losses apply.
Before losses the array would deliver 4 kW x 5 h = 20 kWh. The default losses keep 85.92% (1 - 0.1408), and the inverter keeps 96% of that: 20 x 0.8592 x 0.96 = 16.50 kWh a day, a performance ratio of 82.5%. Over 365 days that is about 6,022 kWh.
The household uses 20 kWh a day. Each panel gives 0.4 x 5 x 0.8592 x 0.96 = 1.65 kWh, so 20 / 1.65 = 12.1, rounded up to 13 panels (5.2 kWp).
Formulas and scoring rules
- Array size
kWp = panel watts x panels / 1000- Combined system loss
L = 1 - (1 - l1) x (1 - l2) x ... x (1 - l10)The PVWatts method. The ten defaults (2, 3, 0, 2, 2, 0.5, 1.5, 1, 0, 3%) give 14.08%.- Daily energy
kWh/day = kWp x PSH x (1 - L) x (1 - temperature loss) x inverter efficiency- Monthly and yearly
month = kWh/day for that month's PSH x days in the month; year = sum of months (or kWh/day x 365)- Panels for a target
panels = ceil(target kWh/day / (panel kW x PSH x performance ratio))
Where peak sun hours come from
Peak sun hours are not hours of daylight. They are the day's total solar energy on a square metre of panel, expressed as the number of hours the sun would need to shine at 1,000 W/m² to deliver it. A June day with 15 hours of light might give 6 peak sun hours; a December day 1.5.
Tilt and orientation change the figure a lot. Use a source that asks for your panel angle and direction, such as NREL's PVWatts or the European Commission's PVGIS, and take the plane-of-array value, not the horizontal one.
Limitations: what the result does not prove
- It does not know your weather. The output is only as good as the peak sun hours you enter; a figure for the wrong tilt or the wrong city can be off by a third.
- Shading from trees, chimneys or neighbouring buildings can cost far more than the default 3% if it falls on a string at midday. A site survey or a shade analysis is the only way to know.
- Temperature is a single flat percentage here. PVWatts models cell temperature hour by hour; in hot climates the real summer loss can reach 10% or more.
- It is not a design for permits or connection. Inverter sizing, string voltages, cable sizing and grid rules are for a qualified installer and your network operator.
Privacy: where your data goes
Everything you paste, type or drop is processed in this browser tab. It is not uploaded, logged, stored or sent to analytics. Session recording and tag-manager scripts are switched off on this page.
Standards and sources
- NREL PVWatts calculator documentation (system losses and derate) - checked 19 Sep 2026
- NREL - PVWatts Calculator
- European Commission JRC - PVGIS photovoltaic geographical information system
Frequently asked questions
How many kWh does a 400 W solar panel produce per day?
About 1.65 kWh on a day with 5 peak sun hours after typical losses (0.4 kW x 5 h x 0.825). With 3 peak sun hours it is nearer 1 kWh; with 6.5 it is about 2.1 kWh. Enter your own site's value to get your figure.
What is a good performance ratio for a solar system?
Most grid-connected systems land between 75% and 85% of their nameplate energy once soiling, wiring, mismatch, temperature and inverter losses are counted. The PVWatts defaults with a 96% inverter give 82.5%. A figure well above 85% usually means a loss has been left out.
Why are my panels producing less than their wattage times the hours of daylight?
Because the panel rating is measured at 1,000 W/m² and 25 °C, and real sunlight is weaker for most of the day. Peak sun hours convert the day's changing light into equivalent full-sun hours, and losses from heat, dirt, wiring and the inverter come off after that.
How many solar panels do I need for 30 kWh a day?
With 400 W panels, 5 peak sun hours and typical losses, each panel gives about 1.65 kWh a day, so 30 / 1.65 = 18.2, rounded up to 19 panels (7.6 kWp). Fewer sun hours need proportionally more; enter your own figures in the target box.
Should I use yearly or monthly peak sun hours?
Use the yearly average to estimate annual savings on a grid-tied system. Use monthly values when the worst month matters - off-grid homes, battery sizing or winter heating - because a system that covers the average day can fall well short in December.
Does the combined loss simply add up the individual losses?
No. Each loss applies to what is left after the previous one, so they multiply: two 10% losses remove 19%, not 20%. PVWatts combines its ten default categories this way to reach 14.08% rather than the 15% a simple sum gives.
Last reviewed by the A2Z.Tools team against the sources listed above.
