How many solar panels do I need?
The honest answer is "it depends on three numbers you already half-know": how much electricity you use, how much sun your roof gets, and how big each panel is. Most solar calculators bury those numbers, gate the result behind your email and phone number, then hand you to a salesperson. This one shows the maths, keeps every input editable, and never asks who you are. Here is the whole method.
System kW = annual kWh ÷ (365 × peak sun hours × derate)
Number of panels = round up( system kW × 1000 ÷ panel watts )
For a monthly figure, swap in monthly kWh ÷ (30 × peak sun hours × derate). The peak sun hours are the equivalent hours per day of full 1000 W/m² sun at your location — roughly 3 in the cloudy Pacific Northwest, 4–5 across most of the US and Europe, and 6–8 in the desert Southwest or much of India and the Middle East. The derate (about 0.75) is the fraction of the panel's lab rating you actually keep after inverter, heat, soiling, wiring and mismatch losses.
So if you want to know what size solar system you need, or how much solar power you need to run a house, you are really asking how many kilowatts cover your yearly kWh — and then how many panels make up those kilowatts. A US home using about 10,800 kWh a year at 4.5 sun hours and 0.75 derate works out to roughly 8.8 kW, or about 22 panels of 400 W. Change any input above and the panel count, the system size and the share of your usage offset all update instantly.
How many solar panels per kW (panels required for 1 kW)?
This part is pure arithmetic and does not depend on where you live: a kilowatt is 1000 W of panels, so just divide 1000 by the panel wattage.
| Panel wattage | Panels per kW | Panels for 5 kW |
|---|---|---|
| 100 W | 10 | 50 |
| 200 W | 5 | 25 |
| 300 W | 3.3 → 4 | 17 |
| 400 W | 2.5 → 3 | 13 |
| 450 W | 2.2 → 3 | 12 |
Location does not change the panels per kilowatt; it changes how many kilowatt-hours that kilowatt produces, which is what the next section is about.
How much energy will solar panels produce?
Flip the calculator to its second mode and it becomes a solar panel output calculator: tell it how many panels (or how many kW) you have and it estimates the kWh you can expect per day, month and year. Each photovoltaic (PV) panel is just a stack of solar cells, and the energy it makes is the same NREL PVWatts idea used by the US government tool, reduced to the part that matters for a quick estimate.
Daily kWh = (panels × panel watts ÷ 1000) × peak sun hours × derate
Monthly kWh = daily × 30 | Annual kWh = daily × 365
To estimate solar panel output for a single panel, set the panel count to one. How much electricity does one solar panel produce per day? A 400 W panel at 4.5 peak sun hours and 0.75 derate makes about 1.35 kWh a day — roughly 40 kWh a month and 490 kWh a year. The optional tilt factor lets you nudge that up or down for orientation and angle; a south-facing array at the ideal tilt sits at 1.00, while an east- or west-facing roof or a far-from-ideal angle might be 0.80–0.90.
How many solar panels fit on my roof?
Sizing tells you how many panels you need; roof area tells you how many you can fit. Open the roof-fit box in Mode A, enter your usable area, and the tool floor-divides it by a single panel's footprint (about 1.7 m² or 18 ft² for a typical 60-cell residential module) to count panels that physically fit. As a rule of thumb, 1 kW of modern panels needs roughly 5–6 m² (55–65 ft²) of clear roof. If the panels that fit fall short of the panels you need, you can close the gap with higher-wattage modules, by accepting a smaller offset, or by adding ground-mounted panels.
Peak sun hours by location
This single number drives every result, so it is worth getting roughly right. The table below lists annual-average peak sun hours so you can pick a starting point, then fine-tune in the calculator. Tap a column heading to sort. For a system you want to cover your usage year-round, size against your worst month (often December in the north), which can run 30–50% below the annual average.
| Location | Region | Peak sun hrs/day |
|---|
Sources: US states and DC from NREL NSRDB / PVWatts (US government, public domain). World cities from the Global Solar Atlas (© World Bank / ESMAP / Solargis, CC BY 4.0). Values are rounded annual averages for orientation; your exact address can differ by 10–25%. Use the calculator's "Custom" option with your own figure for anywhere not listed.
A worked example
Take a home in Texas using 1,100 kWh a month, panels rated 400 W, 5.0 peak sun hours and a 0.75 derate:
- System size: 1,100 × 12 = 13,200 kWh/yr ÷ (365 × 5.0 × 0.75) = 9.64 kW
- Panels: 9.64 × 1000 ÷ 400 = 24.1 → 25 panels
- Daily production: (25 × 400 ÷ 1000) × 5.0 × 0.75 = 37.5 kWh/day ≈ 1,125 kWh/month
That covers essentially all of the 1,100 kWh monthly use on an annual-average basis. Push the same array into a cloudy December at 3.4 sun hours and daily output falls to about 25.5 kWh/day — which is exactly why the annual average flatters winter, and why people who want true year-round coverage size against the worst month instead.