1. Start with an honest load assessment
Every correct PV design starts with energy, not power. Collect at least twelve months of utility bills and record the monthly consumption in kWh. Twelve months matters because air-conditioning, heating and irrigation loads swing seasonally, and sizing on a single summer bill routinely oversizes an array by 20–30%.
If bills are unavailable, build a load table: appliance, rated watts, hours per day, quantity. Daily energy = Σ (W × h × qty) ÷ 1000. Add a 10% miscellaneous allowance for phantom loads, routers, chargers and lighting that never appears on a survey sheet.
2. Convert irradiation into peak sun hours
Peak Sun Hours (PSH) is simply the daily plane-of-array irradiation expressed in kWh/m²/day — numerically identical to the number of hours at 1000 W/m². Most of India sits between 4.5 and 5.8 PSH; northern Europe is closer to 2.6–3.2.
Use annual average PSH for grid-tied sizing and the worst-month PSH for off-grid sizing. Designing an off-grid system on annual averages guarantees a December blackout.
3. Apply a realistic performance ratio
Performance Ratio (PR) bundles every real-world loss: temperature derating, soiling, mismatch, DC and AC cable losses, inverter efficiency, and downtime. A well-built rooftop system lands at PR 0.75–0.80; a utility-scale plant with tracking and good O&M can reach 0.82–0.85.
Array size (kWp) = Daily energy (kWh) ÷ (PSH × PR). For 20 kWh/day at 5.2 PSH and PR 0.78: 20 ÷ (5.2 × 0.78) = 4.93 kWp — round up to 5.0 kWp.
4. Select modules and check the roof
Divide the array size by the module wattage to get module count: 5000 W ÷ 550 W = 9.1, so 10 modules. Always round up, never down — you cannot install a fraction of a panel.
Area check: monofacial mono-PERC modules need roughly 6.5–7.5 m² per kWp including walkway and inter-row shading clearance. A 5 kWp array therefore needs about 35 m² of unshaded roof, not the 25 m² of bare module area.
5. Verify with an annual yield estimate
Annual generation (kWh) = kWp × PSH × PR × 365. Compare that with the annual consumption from step 1. If generation exceeds consumption by more than the net-metering cap in your jurisdiction, resize downward — exported units are frequently paid at a fraction of the retail tariff.
Finally, sanity-check the specific yield (kWh/kWp/year). Anything above 1700 or below 1100 in a temperate climate signals an input error somewhere in the chain.