How to use this solar panel calculator
- Pick your direction. Leave the top toggle off to size a system from your electricity use. Tick it to work backwards from a daily output goal instead.
- Enter your average monthly use in kWh. If you do not know it, tick the second toggle and enter your monthly bill plus your rate per kWh. The tool divides one by the other to get your kWh.
- Enter your peak sun hours per day. Most regions sit between 4 and 6, so 5 is a fair starting point.
- Enter the panel wattage you are shopping for. Home panels are commonly around 400 W.
- Cost per watt is optional. The 2.75 default stands in for a typical installed price. Swap in the number from a real quote if you have one.
System size, panel count, daily production, annual production and rough cost all update as you type.
The sum behind the usage estimate
Four steps, and none of them are hard. Your monthly kWh is divided by 30.4, the average month length, to give a daily figure, and that daily figure divided by your peak sun hours times 0.8 gives the system size in kilowatts. Panel count is the size in watts over your panel wattage, rounded up. Daily output runs the same sum backwards. Times 365 for the year.
At 900 kWh a month, 5 peak sun hours and 400 W panels, you get 7.40 kW, 19 panels, 29.6 kWh a day and 10,806 kWh a year.
Where that 20 percent losses factor comes from
Nobody gets full nameplate wattage off a roof. The nameplate number is a lab measurement taken at Standard Test Conditions, set out in IEC 60904-3:2019as 1,000 W per square metre of light, a cell temperature of 25 °C and the AM1.5 reference spectrum. Roof panels often run at 50 °C or hotter in summer. Silicon loses power as it heats up, so the lab figure is a ceiling, not a forecast.
The 0.8 factor here is one blended derate covering everything between the panel face and your meter.
NREL's PVWatts splits the same ground into ten named line items. The defaults are soiling 2%, shading 3%, snow 0%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate rating 1%, age 0% and availability 3%. Those do not add up. PVWatts multiplies them instead, using 100 × (1 − ∏(1 − Li/100)), which lands on 14.08 percentrather than the 15 percent you get by summing.
Here is the bit most solar pages miss. That 14.08 percent leaves out heat and inverter losses on purpose, because PVWatts models both of those separately from hourly weather data. So a flat 20 percent covers more ground than PVWatts' 14.08 percent does. It absorbs two things PVWatts keeps outside its loss list entirely.
Target output mode works differently
Tick the top toggle and the logic flips. You hand the tool a daily kWh target and it sizes the array to hit it. Each panel is credited with its wattage times peak sun hours, divided by 1,000, and your target is divided by that and rounded up.
No 0.8 factor lives in this path. Target mode sizes panels at full rated output on purpose, since you have already named the number you want out of the array. Want that target to survive real-world derating? Divide it by 0.8 before you type it in. A 30 kWh goal becomes 37.5 kWh, which moves the answer from 15 panels to 19 at 400 W and 5 sun hours.
Why the panel count and the kW figure disagree
Panel count rounds up to a whole panel. The kW, output and cost figures do not.
At 900 kWh a month the readout says 7.40 kW and 19 panels, yet 19 panels rated at 400 W is 7.6 kW of hardware, and the gap is the half panel that got rounded away. Multiply panel count by panel wattage before you set this against an installer's proposal. That is the number their quote gets written against. Cost carries the same rounding. So it sits a little under what you would pay for the array shown.
What this tool cannot see
Peak sun hours decides whether the answer is any use, and the calculator takes your word for it. It has no location data, no weather history, no roof shape and no shading model. Phoenix runs about 6.5 peak sun hours a day and Seattle about 3.5, close to a two to one spread in panel count for the same household.
Roof pitch, which way it faces, a chimney, a neighbour's tree and winter snow all move the answer too. Treat the output as a sanity check on a quote, not a design.
Nothing you type is stored. There is no file input and no download here, just numbers in the page.
Estimate, not a quote: These figures are a planning guide only. Real output and pricing depend on your roof angle, shading, orientation, local weather, equipment and installer. Always get a site-specific quote before buying.
Frequently asked questions
How many solar panels do I need?
It depends on your electricity use, your peak sun hours and the panel wattage. A home using 900 kWh a month at 5 peak sun hours needs roughly a 7.4 kW system, which the tool reports as 19 panels rated at 400 W each.
What is a peak sun hour?
One hour of sunlight at 1,000 watts per square metre. It packs a whole day of shifting light into one number, so 5 peak sun hours is not 5 hours of daylight. Most regions average 4 to 6 across the year.
Why does this calculator assume 20 percent losses?
It is a blended figure. Heat, inverter conversion, wiring, dirt, mismatch and panel tolerance all sit inside that one multiplier. PVWatts itemises a similar list at 14.08 percent, but that total leaves out heat and inverter losses, which it handles separately.
What does target output mode do differently?
It runs the maths backwards. You give it a daily kWh goal and it returns the panel count needed to reach it. Panels are sized at full rated wattage, and no losses factor is applied.
Is the cost estimate a real quote?
No. It multiplies system size in watts by your cost per watt and stops there. Incentives, roof repairs, permits, inverter choice and labour are all missing, so read it as a rough order of magnitude.
Why do 19 panels not match the 7.40 kW shown?
Panel count rounds up to a whole panel while the kW figure keeps its decimal. Nineteen 400 W panels are 7.6 kW of installed hardware, so use panel count times wattage when you compare against a proposal.