Solar Panel Size Calculator
Solar Panel Size Calculator
How many kilowatts of panels your house needs: from your daily units, the peak sun hours where you live and honest system losses — with the panel count and the roof area they take.
Array size
10 kWh a day, 5 peak sun hours, 14% losses, 590 W panels of 2.58 m², 20% spacing
Array size from daily energy
- E
- energy used in a day, in kWh (monthly units ÷ 30.42)
- PSH
- peak sun hours: the day’s solar energy on the array in kWh/m²
- losses
- system derate — soiling, shading, wiring, inverter, availability
- ⌈ ⌉
- rounded up: you cannot buy a fraction of a panel
Worked example
10 kWh a day, 5 peak sun hours, 14% losses, 590 W panels of 2.58 m², 20% spacing
Effective sun = 5 × (1 − 0.14) = 4.30 equivalent full-output hours
Array = 10 ÷ 4.30 = 2.33 kW
Panels = ⌈2326 ÷ 590⌉ = 4, an installed 2.36 kW
Which generates about 10.15 kWh a day (309 units a month) and needs 12.4 m² of roof
Array needed for 10 kWh a day at different peak sun hours
| Peak sun hours | Array size | 590 W panels |
|---|---|---|
| 3.0 — a cloudy winter month, or a badly shaded roof | 3.88 kW | 7 panels |
| 4.0 — north India in December, monsoon weeks | 2.91 kW | 5 panels |
| 5.0 — a fair year-round average for much of India | 2.33 kW | 4 panels |
| 6.0 — Rajasthan, Gujarat, much of the Gulf | 1.94 kW | 4 panels |
| 7.0 — the top of the MNRE range, best months only | 1.66 kW | 3 panels |
Sizing a rooftop solar array
The whole calculation is one division: the energy you use in a day, divided by the energy one kilowatt of panels produces in a day. A kilowatt of panels produces its rating multiplied by the peak sun hours, minus the system’s losses. At five peak sun hours and 14% losses, one kilowatt gives 4.30 kWh a day, so a 10 kWh household needs 2.33 kW — 4 panels of 590 W, an installed 2.36 kW, on about 12.4 m² (133 sq ft) of roof.
Peak sun hours. This is not how long the sun is up. It is the day’s solar energy falling on the array in kWh per m², which equals the number of hours of full 1,000 W/m² sunshine that would deliver the same energy. India’s ministry (MNRE) puts most of the country at 4–7 kWh/m² per day; the Gulf sits at the upper end. Both vary by month and by how your roof faces, so look your own site up on the Global Solar Atlas rather than taking a national average.
Losses are not optional. NREL’s PVWatts defaults to 14% and lists where it goes: soiling 2%, shading 3%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate tolerance 1% and availability 3%, combined multiplicatively. In India and the Gulf, dust is the one to watch — an unwashed array in a dusty month loses far more than 2% — and heat is the other: a panel’s rating is measured at 25 °C and output falls about 0.29% for every degree above that on the Jinko panel used as the example here, so a cell 30 °C hotter gives about 8.7% less. Panels also age: the median measured degradation across nearly 2,000 systems is 0.5% a year, so the same array yields about 8.95 kWh a day in year 25.
Grid-tied or off-grid. A grid-tied system can be sized to the annual average, because the grid absorbs the surplus at noon and covers the shortfall at night — in India that is net metering, where the rules and the export price are set state by state. An off-grid system cannot: it needs enough array to refill the battery on a poor day as well as run the house, which usually means 1.3 to 2 times this figure, plus the battery itself. Size that battery with the solar battery bank calculator, the controller between panels and battery with the solar charge controller calculator, and the inverter with the inverter/UPS size calculator.
Frequently asked questions
How many solar panels do I need for my home?
Divide your daily units by the peak sun hours and the derate: 10 kWh a day at 5 peak sun hours and 14% losses needs 2.33 kW, which is 4 panels of 590 W.
How much roof area does 3 kW of solar need?
A 590 W panel covers 2.58 m², so panels alone take 4.4 m² per kW and about 5.2 m² per kW once 20% is allowed for spacing and access. Three kilowatts is six such panels — 18.6 m².
What are peak sun hours?
The day’s solar energy on the array in kWh/m². Five peak sun hours means the array receives as much energy as five hours of full 1,000 W/m² sun, spread across the whole day.
What system losses should I use?
14% is NREL’s PVWatts default and a reasonable starting point. Increase it for a dusty site, a hot roof, long cable runs or any shading; a well-kept, unshaded system can beat it.
Do solar panels lose output over time?
Yes, slowly. The median rate measured across nearly 2,000 systems is 0.5% a year, so a panel is typically at about 88% of its original output after 25 years, which is why module power warranties are written for 25 or 30 years.
Is a bigger array better for an off-grid house?
Usually yes. Off-grid, the array has to both run the house and refill the battery after bad weather, so it is normally sized 1.3–2× the grid-tied figure.
Related calculators
References
- Ministry of New and Renewable Energy, Government of India. Solar Overview: most parts of India receive 4–7 kWh per m² per day.
- World Bank / ESMAP / Solargis. Global Photovoltaic Power Potential by Country, 2020, and the Global Solar Atlas (globalsolaratlas.info) for the long-term average at your own site.
- Dobos AP. PVWatts Version 5 Manual. NREL/TP-6A20-62641, 2014. Default losses: soiling 2%, shading 3%, snow 0%, mismatch 2%, wiring 2%, connections 0.5%, light-induced degradation 1.5%, nameplate 1%, age 0%, availability 3%, combined multiplicatively to 14%.
- Jordan DC, Kurtz SR. Photovoltaic degradation rates — an analytical review. Progress in Photovoltaics 2013;21(1):12–29: median 0.5% per year across nearly 2,000 measured rates.
- JinkoSolar. Tiger Neo N-type 72HL4-(V) 580–605 W datasheet, model JKM590N: 590 W, Voc 52.63 V, Isc 14.13 A, Vmp 43.71 V, Imp 13.50 A, temperature coefficient of Voc −0.25%/°C, 2,278 × 1,134 mm, 22.84% efficient.