Solar PV Sizing Calculator (kWp, Panels, Battery)
Size a residential solar PV system from monthly electricity use and rooftop area: kWp, panel count, battery Ah, and inverter rating for Nepal/India/Pakistan.
Demand and rooftop
Panel and irradiance
Storage
How this works
We start from monthly consumption, derive a daily figure, then size the array against the local peak-sun-hours allowance with an 85% system efficiency. The battery covers a user-specified number of autonomy days at 60% depth-of-discharge:
dailyKwh = monthlyKwh / 30
pvKwp = dailyKwh / peakSunHours / 0.85
panelCount = ceil(pvKwp × 1000 / panelWattage)
needArea = panelCount × panelAreaFt2
fitsRoof = needArea ≤ rooftopAreaFt2
batteryAh = (dailyKwh × batteryDays × 1000)
/ (systemVoltage × 0.6)
inverterKw = pvKwp × 1.1Peak sun-hours is the locality's annual average daily solar yield. 4.5 covers Kathmandu and most of north India; 5.0–5.5 applies to the Indian deserts and southern Pakistan.
Worked example
300 kWh/month, 800 ft² rooftop, 540 W panels (~22 ft² each), 4.5 peak sun-hours, 2 days battery, 48 V system:
- Daily =
300 / 30 = 10 kWh - PV kWp =
10 / 4.5 / 0.85 ≈ 2.61 kWp - Panels =
ceil(2,614 / 540) = 5 panels - Required area ≈
5 × 22 = 110 ft²— fits the 800 ft² roof. - Battery Ah ≈
(10 × 2 × 1000) / (48 × 0.6) ≈ 694 Ah - Inverter ≈ 2.87 kW (round up to 3 kW)
Sources
- PVsyst design rules; MNRE / AEPC residential rooftop guidelines (India / Nepal)
FAQ
What is 'peak sun-hours' and why does it matter?
Peak sun-hours is the locality's annual-average daily solar yield expressed as the number of hours per day that the sun delivers 1000 W/m² — the panel's rated test condition. The Kathmandu valley and most of north India sit at about 4.5; the Indian deserts and southern Pakistan reach 5.0 to 5.5; cloudy hill stations may drop to 3.5. The PV array sizing is inversely proportional to this number, so getting it right matters — overstating peak sun-hours undersizes the array and the system underdelivers in winter.
Why is the system efficiency only 85 percent?
A panel's STC rating is measured at 25 °C, perfectly clean glass, and a brand-new inverter. Real installations lose 5–8 percent to elevated cell temperature (panels run at 50–60 °C in summer), 3–5 percent to inverter conversion, 2–3 percent to soiling, and 1–2 percent to DC and AC cabling. Combined, an 85 percent system-yield ratio is the conservative residential figure. Premium inverters and microinverters can edge this to 87–88 percent, but assuming higher than 85 risks chronic underdelivery.
Why does the battery use 60 percent depth-of-discharge?
Discharging a lead-acid battery deeper than 50–60 percent on every cycle dramatically shortens its life (a 50 percent DoD design typically lasts 7–10 years; an 80 percent DoD bank degrades in 3–4 years). 60 percent is the standard residential compromise. Modern LiFePO₄ chemistries can run safely at 80–90 percent DoD — for a lithium bank, divide the calculator's battery Ah by 1.4 to size more aggressively.
Why does the inverter need to be 110 percent of the array kWp?
On clear, cool days a panel can briefly output 5–10 percent above its STC rating thanks to clipped clouds (the 'cloud-edge' effect) and lower-than-rated cell temperatures. An inverter sized exactly at the array kWp will clip these peaks and waste energy. The 1.1 oversizing factor is the standard residential allowance. Some installers go higher (1.2–1.3) when the array faces east + west or sits at a steep tilt where peaks are gentler.
What if my rooftop is too small?
When the required area exceeds your available rooftop, you have three choices. First, accept partial coverage: reduce monthly kWh until the array fits and use the grid for the rest. Second, switch to higher-efficiency panels (mono-PERC at 22 percent vs polycrystalline at 17 percent gives ~25 percent more watts per ft²). Third, extend the mounting structure with a tilt-mounted ground or carport array. Regulators in Nepal and India allow rooftop solar up to your sanctioned load, but not all installers will design for partial coverage.
Does this size grid-tied or off-grid systems?
Both. The PV array kWp, panel count, and inverter rating apply to either configuration. The battery Ah figure is meaningful only for off-grid or hybrid installations — a pure grid-tied (net-metering) system needs no battery, so set 'battery autonomy' to a small value (0.1 day) and ignore the Ah output. For a hybrid system, the battery Ah is sized for the autonomy days you specify, typically 1–2 days for a critical-load backup.