Solar Panel Calculator
A home using 900 kWh a month in a location with 4.5 peak sun hours needs about a 8.2 kW system, which is roughly 21 panels rated at 400 watts each. This solar panel calculator turns your monthly electricity use into the three numbers that matter most when you plan a rooftop array, the system size in kilowatts, the number of panels, and the estimated energy the system produces over a full year. Enter your average monthly kilowatt-hours from a recent utility bill, adjust the peak sun hours for your region, pick the wattage of the panels you are considering, and set a realistic system loss figure. The tool then spreads your monthly usage across the average number of days in a month, works out the usable sunlight after losses, and rounds the panel count up so the array covers your whole need rather than falling short. It gives you a fast, honest first estimate before you talk to an installer, so you walk into that conversation already knowing the ballpark size of the project and what it should generate.
Quick answer
System size in kilowatts equals your daily energy need divided by the usable sun, which is peak sun hours multiplied by the fraction of power left after losses.
What this tells you
- •System size in kilowatts equals your daily energy need divided by the usable sun, which is peak sun hours multiplied by the fraction of power left after losses.
- •Peak sun hours are not daylight hours. They measure how many hours of full-strength 1000 watt per square meter sunlight your location averages per day.
- •Panel count rounds up, so a system that needs 20.3 panels worth of capacity is shown as 21 panels to fully cover the load.
- •System losses of about 20 percent are typical and cover inverter conversion, wiring resistance, heat, shading, and dust on the glass.
- •Higher wattage panels reduce the number of panels but not the total kilowatts, since capacity is what determines output.
- •Estimated annual production projects a full year of output from the system size, local sun hours, and the same loss figure used to size the array.
How to Use
- 11. Enter your average monthly electricity use in kilowatt-hours. Take it from a recent utility bill or average the last twelve months for the most accurate figure.
- 22. Set the peak sun hours for your area. Use 4.5 as a rough national middle value, lower for cloudy northern regions and higher for sunny southern ones.
- 33. Choose the wattage of the panels you are considering. Most modern residential panels fall between 350 and 500 watts, and 400 watts is a common default.
- 44. Set the system loss fraction. The default of 0.20 represents a typical 20 percent loss, raise it for heavy shading or a hot climate and lower it for an ideal install.
- 55. Read the panel count as your primary result, then check the system size in kilowatts, your daily energy need, and the estimated annual production underneath.
How It Works
Formula
System kW = (monthly kWh / 30.44) / (peak sun hours * (1 - loss)), Panels = ceil(System kW * 1000 / panel watts)The calculator first converts monthly use to a daily figure by dividing by 30.44, the average number of days in a month across a full year. It then divides that daily need by the usable sun, which is the peak sun hours multiplied by the delivered fraction of power after losses. The result is the system size in kilowatts. To get panel count, it converts kilowatts to watts, divides by the wattage of a single panel, and rounds up with a ceiling function so the array is never undersized. Estimated annual production takes the system size in kilowatts, multiplies by peak sun hours and 365 days, and applies the same loss fraction, giving a realistic year of output rather than a theoretical maximum.
Calculation note: values are processed in the order shown above, using the current input units.
Worked Examples
Average single-family home
900 kWh divided by 30.44 days is 29.57 kWh per day. Dividing that by 4.5 sun hours times the 0.8 delivered fraction gives an 8.21 kW system. That is 8213 watts, divided by 400 watt panels and rounded up, which is 21 panels producing roughly 10,792 kWh across the year.
Sunny southern climate
The same 900 kWh usage in a place with 6 peak sun hours needs a smaller system because each panel produces more each day. The system drops to 6.16 kW and 16 panels, even though the annual production target stays close to the same, because more sun does more work per panel.
High wattage panels
A larger home using 1200 kWh a month needs a 10.95 kW system. Choosing 500 watt panels instead of 400 watt panels keeps the panel count at 22 rather than a higher figure, which can matter when roof space is tight.
Shaded roof with higher losses
A modest 700 kWh usage still needs a large 8.21 kW system here because the roof has only 4 sun hours and a heavy 30 percent loss from shading. Higher losses force a bigger array to hit the same daily need, which shows why an honest loss figure matters.
Solar System Size by Monthly Usage
Approximate system size and panel count at 4.5 peak sun hours, 400 watt panels, and 20 percent losses.
| Monthly kWh | Daily kWh | System Size | Panels (400 W) |
|---|---|---|---|
| 500 | 16.4 | 4.6 kW | 12 |
| 700 | 23.0 | 6.4 kW | 16 |
| 900 | 29.6 | 8.2 kW | 21 |
| 1100 | 36.1 | 10.0 kW | 25 |
| 1300 | 42.7 | 11.9 kW | 30 |
| 1500 | 49.3 | 13.7 kW | 35 |
These figures are planning estimates. Local sun hours, panel wattage, roof orientation, and shading all change the real numbers, so treat this table as a starting point rather than a quote.
What actually decides how many panels you need
The single biggest driver of your solar array size is how much electricity you use, followed closely by how much usable sun your location gets. Two homes with identical roofs can need very different systems if one uses 600 kWh a month and the other uses 1400 kWh. That is why the calculator starts with your monthly usage rather than your roof area. Pull an average from a full year of bills if you can, since summer air conditioning and winter heating can swing a single month far from your true average.
Peak sun hours are the part most people get wrong. They do not mean the number of hours the sun is up. A peak sun hour is one hour of sunlight at the full reference intensity of 1000 watts per square meter, and a location that has ten hours of daylight might only average four or five peak sun hours once you account for the sun's angle, weather, and season. Regional peak sun hour maps are widely published, and using the right figure for your area is the difference between a system that covers your bill and one that falls short.
System losses are the quiet tax on every solar array. The panels are rated under ideal lab conditions, but in the real world you lose power to inverter conversion, wiring resistance, heat that reduces panel efficiency, dust and pollen on the glass, and any shading from trees or chimneys. A well-designed system in a mild climate might lose 12 to 15 percent, while a shaded roof in a hot region can lose 25 percent or more. The 20 percent default here is a sensible middle ground, but adjusting it to match your real conditions makes the estimate far more useful.
Common mistakes
- Confusing peak sun hours with daylight hours. A location with 12 hours of daylight usually averages only 4 to 6 peak sun hours, and using the higher number badly undersizes the array.
- Sizing from a single month's bill. Air conditioning and heating make one month a poor guide, so average a full year of usage for a realistic figure.
- Ignoring system losses. Skipping the loss fraction or setting it to zero produces a system that looks smaller on paper but cannot cover your real load.
- Assuming more panels always means more output. What matters is total kilowatts, so two 500 watt panels produce the same as roughly two and a half 400 watt panels.
- Forgetting roof space and orientation. The calculator sizes for energy need, but a north-facing or small roof may not physically fit the panel count it returns.
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