Solar Panel Wattage Calculator
Enter your solar panel specs to see daily, monthly and yearly energy output - or switch to the "size my system" mode and enter your annual electricity use to find the total wattage you need. Peak sun hours, system efficiency and bill-offset percentage are all adjustable. The math panel shows every step so you can verify the result against your own electricity bill.
Formula
Worked example
A 10-panel array of 400 W panels with 4.5 peak sun hours and 85% efficiency: system capacity = 400 x 10 / 1,000 = 4 kWp. Daily output = 4 x 4.5 x 0.85 = 15.3 kWh. Annual output = 15.3 x 365 = 5,585 kWh.
What is solar panel wattage and why does it matter?
The wattage printed on a solar panel (for example, "400 Wp" or "400 W peak") is the maximum power it produces under Standard Test Conditions (STC): 1,000 W/m2 of sunlight, a cell temperature of 25 C, and a specific air-mass spectrum. In real installations, panels produce less than their rated wattage most of the time because the sun is not always at full intensity, panels heat up above 25 C, and there are always some wiring and inverter losses. Knowing the watt-peak rating is the starting point for every energy estimate, but you also need to account for local sunlight, orientation and system losses to get a realistic output figure.
How to calculate solar panel output
The core formula is: Daily output (kWh) = System capacity (kWp) x Peak sun hours x System efficiency. "Peak sun hours" is not the number of daylight hours - it is the number of hours per day when sunlight averages 1,000 W/m2 (the same intensity as STC). A location that receives 4 kWh of solar radiation per m2 per day is said to have 4 peak sun hours. System efficiency captures all the losses between the panel rating and the electricity reaching your meter: inverter conversion loss (typically 4-6%), cable and connection loss (1-3%), temperature derating (3-8% for every 10 C above 25 C), and soiling or shading (2-10%). A well-sited clean system typically achieves 80-87% overall efficiency. Multiply the kWp by peak sun hours and by the efficiency fraction (for example 0.85 for 85%) and you get the usable kWh per day.
How to size a solar system for your electricity use
Find your annual consumption in kWh from 12 months of electricity bills (the US average household is about 10,500 kWh per year). Decide what fraction you want solar to cover - 100% is the full offset, but 80% is common where grid export is limited. Divide the target annual kWh by 365 to get your daily target, then divide that by (peak sun hours x system efficiency) to get the required kWp. Finally, divide the kWp by the individual panel wattage (in kW) and round up to find the panel count. The "size my system" mode in this calculator handles all those steps for you.
Factors that affect real-world solar output
Location and orientation: a south-facing roof at a 30-35 degree tilt maximises output in the Northern Hemisphere; east- or west-facing arrays typically produce 15-20% less. Shading: even a small shadow on one cell can cut the output of an entire string significantly; micro-inverters and DC optimisers reduce this penalty. Temperature: silicon panels lose about 0.3-0.5% of output for every degree Celsius above 25 C, so panels in hot climates produce noticeably less than their STC rating. Panel age: most panels degrade at roughly 0.5% per year, so a 25-year-old panel produces about 87-90% of its original rated output. Weather variability: the chart in this calculator shows approximate seasonal swings for a Northern Hemisphere location; actual month-to-month variation depends on local cloud cover.
Typical solar output by system size
| System size (kWp) | Panels (400 W each) | Daily output (kWh) | Annual output (kWh) | Homes covered (approx.) |
|---|---|---|---|---|
| 1 | 3 | 1.5 | 555 | Small apartment |
| 2 | 5 | 3.1 | 1,111 | Studio or 1-bed flat |
| 4 | 10 | 6.1 | 2,223 | Small house |
| 6 | 15 | 9.2 | 3,334 | Medium house |
| 8 | 20 | 12.2 | 4,446 | Large house |
| 10 | 25 | 15.3 | 5,557 | Large house or small business |
| 15 | 38 | 22.9 | 8,336 | Small commercial |
| 20 | 50 | 30.6 | 11,115 | Medium commercial |
Estimated daily and annual output at 4.5 peak sun hours per day and 85% system efficiency. Actual output varies with location, panel quality, shading and orientation.
Frequently asked questions
What is a peak sun hour?
A peak sun hour is one hour in which sunlight intensity averages 1,000 W/m2, the same intensity used for Standard Test Condition panel ratings. It is not the same as a daylight hour - a location might have 14 hours of daylight but only 4.5 peak sun hours, because the sun is weaker early and late in the day. You can find your local peak sun hours from NREL's PVWatts database or similar tools for your country.
How many solar panels do I need for a 2,000 kWh monthly bill?
A 2,000 kWh per month use equals 24,000 kWh per year. At 4.5 peak sun hours per day and 85% system efficiency you need 24,000 / (365 x 4.5 x 0.85) = about 17.2 kWp. At 400 W per panel that is 43 panels. Fewer sun hours or lower efficiency push that number higher; more sun hours or better equipment reduce it.
What is a good system efficiency to use?
Use 80-85% for a standard residential string-inverter system in a temperate climate. Use 85-90% for a micro-inverter or DC-optimiser system with no significant shading. Use 75-80% if panels will be partially shaded, installed in a very hot climate, or are older. The default in this calculator is 85%, which is a reasonable mid-point for a new, well-sited residential system.
How accurate is this calculator?
This calculator uses the standard industry formulas for solar output estimation. It gives a good ballpark figure for planning purposes, but real output depends on your exact roof orientation, any shading, local micro-climate, and the specific panels and inverter you install. For a precise estimate, use NREL PVWatts with your exact address, or ask your installer for a simulation using software such as PVSyst or Aurora Solar.
Does temperature affect how many panels I need?
Yes. Panels are rated at 25 C cell temperature. In hot climates the cell temperature can reach 50-70 C in summer, and most silicon panels lose 0.3-0.5% of output per degree Celsius above 25 C. This temperature derating is part of the system efficiency figure. If you live in a very hot region, use a lower efficiency value (such as 78-80%) to account for this, or look for panels with a low temperature coefficient on the datasheet.
What is the difference between kW and kWh for solar?
kW (kilowatts) is a measure of power - the rate of energy production at any given moment. A 4 kWp solar array can produce up to 4 kW when conditions are perfect. kWh (kilowatt-hours) is a measure of energy - the total amount produced or consumed over time. If that 4 kWp array runs at full power for one hour it generates 4 kWh. Your electricity bill is in kWh, so that is the figure you need to match to size your system.