Answer first
What this calculator tells you
Estimate a solar array's daily and yearly energy from panels, wattage, sun hours and losses. Compare a proposed system's production against what your household actually uses. Formula: Daily kWh = panels × panel watts ÷ 1,000 × peak sun hours × (1 − system losses). At the worked-example inputs, the energy per day (kwh) is 31. Holding every other input steady, moving number of panels from 16 to 24 moves the result from 24.8 to 37.2.
Transparent method
The formula
Compare a proposed system's production against what your household actually uses.
Worked example
Example inputs
How to interpret the result
A solar array's yield is its rated power times the hours of full-strength sun times what survives wiring, heat and dirt. Twenty 400 watt panels are 8 kilowatts on paper. With 4.5 peak sun hours and 14 percent losses, they make about 31 kilowatt-hours a day. Peak sun hours are not daylight hours. They are the hours of full-strength sun that would deliver the same energy, and they swing with the season.
At the worked-example inputs the energy per day (kwh) is 31. It rises with peak sun hours per day, number of panels and watts per panel and falls as system losses increases.
Real energy use depends on how the appliance or vehicle is actually run: duty cycle, weather, driving style, battery age and your utility's rate structure, including tiers and time-of-use pricing. Treat the result as a planning estimate and check it against your own bill or trip log.
Before you rely on it
What to check
Use sun hours for your own location and for the roof's tilt and direction. A shaded or north-facing roof can produce far less than the default.
The common error
Where people go wrong with solar panel output calculator
Using the panel's rated watts as if it delivered them all day. Ratings are for ideal test conditions, and real output sits well below them for most of the day.
Sensitivity evidence
How number of panels changes the energy per day (kwh)
Holding every other input at the worked-example value, moving number of panels from 16 to 24 moves the energy per day (kwh) from 24.8 to 37.2: a spread of 12.4, or 40% of the worked-example result.
| Number of panels | Energy per day (kWh) | Energy per year (kWh) |
|---|---|---|
| 16 | 24.8 | 9,040.3 |
| 18 | 27.9 | 10,170.4 |
| 20worked example | 31 | 11,300.4 |
| 22 | 34.1 | 12,430.4 |
| 24 | 37.2 | 13,560.5 |
Every input, tested
Which input moves the energy per day (kwh) most
Of the 4 inputs, peak sun hours per day moves the energy per day (kwh) most (6.9 across the range tested) and system losses moves it least (1.4).
| Input | Tested from | To | Energy per day (kWh) at each end | Swing |
|---|---|---|---|---|
| Peak sun hours per day | 4 | 5 | 27.5 to 34.4 | 6.9 (22%) |
| Number of panels | 18 | 22 | 27.9 to 34.1 | 6.2 (20%) |
| Watts per panel | 360 | 440 | 27.9 to 34.1 | 6.2 (20%) |
| System losses | 12.0% | 16.0% | 31.7 to 30.2 | 1.4 (4.7%) |
Two variables at once
Energy per day (kWh) by number of panels and watts per panel
Across the grid the energy per day (kwh) runs from 19.8 to 44.6. Moving number of panels from 16 to 24 shifts it by 12.4 at the middle column, and moving watts per panel from 320 to 480 shifts it by 12.4 at the middle row, so neither is the bigger lever here.
| Number of panels \ Watts per panel | 320 | 400 | 480 |
|---|---|---|---|
| 16 | 19.8 | 24.8 | 29.7 |
| 18 | 22.3 | 27.9 | 33.4 |
| 20 | 24.8 | 31 | 37.2 |
| 22 | 27.2 | 34.1 | 40.9 |
| 24 | 29.7 | 37.2 | 44.6 |
The highlighted cell is the worked example: 31.
Step by step
The worked example, input by input
| Input | Value used | What it means |
|---|---|---|
| Number of panels | 20 | Enter the number of panels used in this calculation. |
| Watts per panel | 400 | Enter the watts per panel used in this calculation. |
| Peak sun hours per day | 4.5 | Average daily hours of full-strength sun for your location and season. |
| System losses | 14.0% | Wiring, inverter, heat and dirt. Around 14 percent is a common planning default. |
| Energy per day (kWh) | 31 | |
| Energy per year (kWh) | 11,300.4 | |
Inputs, definitions and assumptions
Number of panels
Enter the number of panels used in this calculation. The prefilled worked-example value is 20.
Watts per panel
Enter the watts per panel used in this calculation. The prefilled worked-example value is 400.
Peak sun hours per day
Average daily hours of full-strength sun for your location and season. The prefilled worked-example value is 4.5.
System losses
Wiring, inverter, heat and dirt. Around 14 percent is a common planning default. The prefilled worked-example value is 14.0%.
How to use this calculator
- 1Verify the inputs. Gather number of panels, watts per panel, peak sun hours per day and system losses from your own documents; the prefilled values are examples.
- 2Save a baseline. The worked example puts the energy per day (kwh) at 31. Store your own version of it as Scenario A.
- 3Test one change. Start with peak sun hours per day, the input with the biggest effect here: moving peak sun hours per day from 4 to 5 takes the energy per day (kwh) from 27.5 to 34.4, a swing of 22% of the worked-example figure.
- 4Check the extremes. At half the example peak sun hours per day (2.3) the energy per day (kwh) is 15.5; at double (9) it is 61.9.
People also ask
Frequently asked questions
How do you calculate solar panel output?
Daily kWh = panels × panel watts ÷ 1,000 × peak sun hours × (1 − system losses). Enter system losses in percent (14 means 14%). At the worked-example inputs the energy per day (kwh) is 31.
What does the solar panel output result mean?
Compare a proposed system's production against what your household actually uses. At the worked-example inputs the energy per day (kwh) is 31. It rises with peak sun hours per day, number of panels and watts per panel and falls as system losses increases.
How much does number of panels change the energy per day (kwh)?
Holding every other input at the worked-example value, moving number of panels from 16 to 24 moves the energy per day (kwh) from 24.8 to 37.2, a spread of 12.4.
What are the limits of this solar panel output calculator?
Real energy use depends on how the appliance or vehicle is actually run: duty cycle, weather, driving style, battery age and your utility's rate structure, including tiers and time-of-use pricing. Treat the result as a planning estimate and check it against your own bill or trip log. The tables on this page test number of panels only from 16 to 24; a value outside that range is not tabulated here.
Which input moves the energy per day (kwh) most in the solar panel output calculator?
Ranked by how far each moves the energy per day (kwh) across the range tested: peak sun hours per day (6.9, 22%), number of panels (6.2, 20%), watts per panel (6.2, 20%) and system losses (1.4, 4.7%).
If I double peak sun hours per day in the solar panel output calculator, does the energy per day (kwh) double?
Doubling it from 4.5 to 9 takes the energy per day (kwh) from 31 to 61.9, which is 2.00 times the worked-example figure. So the result scales almost exactly in proportion. Halving it to 2.3 gives 15.5.
How much does watts per panel matter in the solar panel output calculator?
The worked example uses 400. Holding every other input at its worked-example value, moving watts per panel from 360 to 440 takes the energy per day (kwh) from 27.9 to 34.1, a swing of 20% of the worked-example figure.
How much does peak sun hours per day matter in the solar panel output calculator?
The worked example uses 4.5. Holding every other input at its worked-example value, moving peak sun hours per day from 4 to 5 takes the energy per day (kwh) from 27.5 to 34.4, a swing of 22% of the worked-example figure.
How much does system losses matter in the solar panel output calculator?
The worked example uses 14.0%. With the other inputs left at the worked example, moving system losses from 12.0% to 16.0% takes the energy per day (kwh) from 31.7 to 30.2, a swing of 4.7% of the worked-example figure.
Which inputs change the energy per year (kwh) in the solar panel output calculator?
At the worked-example inputs it is 11,300.4. Number of panels takes it from 10,170.4 to 12,430.4, watts per panel takes it from 10,170.4 to 12,430.4, peak sun hours per day takes it from 10,044.8 to 12,556 and system losses takes it from 11,563.2 to 11,037.6.
How do I measure my real gas mileage?
Fill the tank, note the odometer, drive normally, then fill again. Miles since the first fill divided by the gallons added is your mileage for that tank. Average a few tanks.
Why do solar panels produce less than their rated watts?
Ratings come from lab conditions of strong sun and cool panels. Real roofs face heat, dirt, shade, wiring losses and an angled sun, so a planning loss of around 14 percent is a common allowance.
How long will a battery really last?
Runtime falls as the load rises and as the battery ages, cools or drains deeply. The calculator shows the best case at your chosen usable depth, so plan on somewhat less.
Sources and evidence
Free Calculators Online is independent and is not affiliated with or endorsed by the source organizations. Educational estimates only.