A solar panel's wattage rating (Wp, "peak watts") is measured under laboratory Standard Test Conditions — a fixed 1000 W/m² of sunlight. Real sunlight varies all day and by season, so solar designers use peak sun hours: the number of hours per day, if sunlight were a constant 1000 W/m², that would deliver the same total energy your location actually receives. A place with "5 peak sun hours" might have 12 hours of daylight, but the equivalent full-intensity exposure is just 5 hours — this is the number solar designers actually use, not raw daylight length.
Your daily energy need (in watt-hours) must be supplied by your panels' rated wattage × the site's sun hours × your system's real-world efficiency: Daily Load = Wp × Sun Hours × Efficiency. Rearranged to solve for the panel wattage you need: Wp = Daily Load / (Sun Hours × Efficiency).
| Quantity | Formula / Typical Value |
|---|---|
| Required panel wattage | Wp = Daily Load (Wh) / (Sun Hours × Efficiency) |
| Number of panels | ⌈Required Wp / Panel Wattage⌉ |
| Typical system efficiency | 75–85% (inverter + wiring + soiling + temperature losses) |
| Typical peak sun hours (India) | 4–6 hrs/day depending on region and season |
Once you know the required total wattage, the next steps in a full system design are the Battery Backup Sizing calculator (how much storage you need), the MPPT string sizing calculator (how to wire your panels safely), and the kWh Generation calculator (checking your chosen array actually produces enough energy).
First find your required total wattage: Wp = Daily Load (Wh) / (Sun Hours × System Efficiency). Then divide that by your chosen panel's wattage rating and round up to get the number of panels.
Peak sun hours are the equivalent number of hours per day at a constant 1000 W/m² (the standard test condition panels are rated at) that would deliver the same total solar energy your location actually receives. They are almost always fewer than the number of daylight hours, since real sunlight intensity varies through the day.
75–85% is typical for a well-designed system, accounting for inverter conversion losses (~95-97%), wiring resistance, panel soiling/dust, and reduced panel output at higher operating temperatures. Older or poorly-maintained systems can run lower; use a conservative 70-75% if unsure.
Add up (power in watts) × (hours used per day) for every appliance/device you intend to run, or check your utility bill's monthly kWh usage and divide by 30 for a rough daily average.
Solar irradiance maps and government/NREL-style solar resource databases publish average daily peak sun hours by region and month; many solar panel and inverter manufacturers also provide regional lookup tools. Always use your area's worst-month (typically monsoon/winter) figure for a conservative, reliable design.
For an off-grid or backup-critical system, size for your worst realistic month (often the cloudiest/shortest-day month) so the system still meets your needs year-round; for a grid-tied system offsetting an annual bill, average annual sun hours is a reasonable basis since the grid covers any shortfall.
No — the Wp rating is measured under ideal laboratory Standard Test Conditions (1000 W/m², 25°C cell temperature, specific spectrum). Real installed panels rarely hit their full rated output continuously, which is exactly why the system efficiency (derating) factor is applied separately in this calculation.
Because required panel wattage depends on sun hours and system efficiency as well as load. A house in a sunnier region, or with a more efficient system, needs less panel capacity than an identical-load house in a cloudier region or with an older, lossier setup.
Yes, a common practice is adding 10-25% margin above the calculated minimum to account for panel degradation over time (typically 0.5-0.8% output loss per year), unexpected load growth, and to maintain performance on below-average sunlight days.
Use the Battery Backup Sizing calculator to size your storage (if off-grid or hybrid), the MPPT calculator to design a safe series/parallel panel wiring configuration, the Charge Controller calculator to size your controller, and the kWh Generation calculator to confirm your chosen array actually produces enough energy for your location.
Yes significantly — even partial shading on part of an array can disproportionately reduce total output (especially with older non-bypass-diode panel strings), so a shaded installation may need meaningfully more panel capacity than this calculator's unshaded estimate, or shade-tolerant equipment (microinverters/optimizers).
Battery Backup Sizing • MPPT String Sizing • Solar kWh Generation • Solar Inverter Sizing • All Calculators