Solar Array Sizing Calculator
Array kWp, module count, inverter rating, roof area and payback from your monthly bill and local peak sun hours.
Consumption
System
Net metering
Payback ignores subsidy, module degradation of about 0.5 % a year, and tariff escalation. Tariff escalation usually shortens real payback; degradation lengthens it slightly.
Want this system quoted?
Send your figures across and we will come back with a proper design and price, including current subsidy.
| Project | Circuit ref | ||
| Prepared by | Date | ||
| Checked by | Date |
Solar Array Sizing Calculator · kWp = E / (PSH · PR) · EnergyCalcHQ · energycalchq.com
Preliminary calculation. The figures behind it are representative values for the stated conditions, not a substitute for the current edition of the standard or the manufacturer's published data. Verify before issuing for construction. Not a substitute for a qualified engineer or a protection study.
For page numbers, keep Headers and footers ticked under More settings in the print dialog.
The formula
kWp = daily kWh / (peak sun hours × performance ratio)Three inputs, and two of them are routinely got wrong.
Peak sun hours is not daylight hours
Peak sun hours is the number of hours of 1000 W/m² equivalent irradiance the site receives in a day — not how long the sun is up. A location with 12 hours of daylight typically has 4.5 to 5.5 peak sun hours, because early morning and late afternoon sun arrives at a shallow angle and through more atmosphere.
The values in the selector are annual averages. Design for the worst month if the system must carry the load year-round — in most of India that is the monsoon, and it can be 30–40 % below the annual average.
Performance ratio is where the losses live
A module rated 550 Wp never delivers 550 W in the field. The performance ratio bundles every loss between the sunlight and the meter:
| Loss | Typical |
|---|---|
| Temperature — cells run 25–30 °C above ambient | 8 – 12 % |
| Soiling — dust, bird droppings, pollen | 2 – 5 % |
| Inverter conversion | 2 – 3 % |
| DC and AC wiring | 2 – 3 % |
| Mismatch and tolerance | 2 – 3 % |
That lands at 0.75 to 0.80 for a well-built rooftop system. A dusty site without regular cleaning can fall to 0.70. Anyone quoting you a PR above 0.85 for a rooftop is being optimistic.
Temperature is the biggest single loss and it is why the same array makes more energy on a cold clear February day than a hot hazy May one, despite the longer May daylight.
Why the inverter is smaller than the array
A DC:AC ratio above 1 is deliberate, not a mistake. The array reaches its full rating for only a few hours around noon on clear days. Sizing the inverter for that peak means it runs at poor part load the rest of the time, and costs more.
At a ratio of 1.2, you clip a small amount of midday output a few days a year and gain better inverter efficiency for the whole rest of the year. 1.1 to 1.3 is the normal range. Go higher only where the array faces east or west and never reaches full output.
Roof area
Roughly 6.5 m² per kWp for current mono PERC modules laid flat-ish, or about 9–10 m² per kWp on a tilted mounting structure where you must leave inter-row spacing to avoid self-shading.
Shading matters more than area. A single module shaded by a water tank or a parapet can pull down its whole string. Walk the roof at 9 am and 4 pm before you commit to a layout.
What this calculator does not cover
- String sizing. Modules per string depends on the inverter MPPT window and the coldest expected temperature, since Voc rises as temperature falls.
- Net metering rules. Most state DISCOMs cap sanctioned export at some fraction of your connected load.
- Subsidy. Residential rooftop subsidy changes often, and differs by country and region — check the current rates before quoting payback to a customer.
Peak sun hours is not daylight
PSH is the number of hours of full 1 kW/m² sunshine that would deliver the same energy as the whole day actually does. A twelve-hour day with a hazy morning and a cloudy afternoon might be 4.5 peak sun hours. Using daylight hours instead overstates generation by a factor of two or more.
What the performance ratio is hiding
PR bundles every loss between the module label and the meter, and it is worth knowing the shape of it because two of the terms are things you control:
| Loss | Typical |
|---|---|
| Module temperature | 8–15 % |
| Soiling | 2–10 % |
| Inverter conversion | 2–4 % |
| DC and AC wiring | 2–3 % |
| Mismatch and tolerance | 2 % |
| Downtime | 1–2 % |
Temperature is the largest and the least avoidable — a panel rated at 25 °C runs at 60 °C on an Indian roof in May. Soiling is the one you can actually fix, and in a dusty location it is the difference between a good system and a disappointing one.
Check the string voltage, not just the kilowatts
The array has to suit the inverter’s MPPT window as well as its power rating. Panel voltage rises as temperature falls, so the critical case is a cold clear morning at first light: a string sized on nominal voltage can exceed the inverter’s maximum input and shut it down, or worse.
Work the string length from the module’s open-circuit voltage at your lowest expected temperature, not from its rated voltage.
Why the array is bigger than the inverter
A DC:AC ratio around 1.2 is deliberate. Full output happens for very few hours a year, so an inverter sized to catch that peak sits underused the rest of the time — and inverters are least efficient at low load. Slightly clipping the top of a handful of days costs less than the larger unit, and it lifts generation on every ordinary day.
Questions people ask
- How big an array do I need for 900 units a month?
- About 7.7 kWp. Nine hundred units a month is 30 units a day, and kWp = daily kWh / (peak sun hours × performance ratio) — so at 5.0 peak sun hours and a performance ratio of 0.78 that is 7.7 kWp, which is 14 modules of 550 Wp and roughly 50 m² of unshaded roof. The roof area is the constraint that bites first on most houses, not the money.
- Is peak sun hours the same as hours of daylight?
- No, and confusing the two oversizes or undersizes everything downstream. Peak sun hours is how many hours of 1000 W/m² equivalent irradiance a site receives in a day, not how long the sun is up. A location with 12 hours of daylight typically has 4.5 to 5.5 peak sun hours, because early morning and late afternoon sun arrives at a shallow angle and through more atmosphere. For a real site, the figure from Global Solar Atlas or PVGIS for your coordinates beats any regional average.
- What performance ratio should I use?
- 0.75 to 0.80 for a clean, well-ventilated roof. The performance ratio bundles every loss between the sunlight and the meter: module temperature, soiling, wiring, inverter efficiency, mismatch and downtime. A module rated 550 Wp never delivers 550 W in the field, and the performance ratio is where that honesty lives — a calculator that assumes 0.9 is selling you a smaller array than the load needs.
- Should I size on the annual average or the worst month?
- On the worst month, if the system has to carry the load year-round. The figures in the location selector are annual averages, and in most of India the worst month is the monsoon, which can run 30 to 40 per cent below that average. A grid-tied system that simply banks surplus in the good months can be sized on the annual figure; an off-grid or backup system sized that way will fall short for weeks at a time.
- Why is the inverter rated lower than the array?
- Deliberately, and it is standard practice rather than a compromise. A DC to AC ratio of 1.1 to 1.3 is typical, because an array reaches its nameplate kWp only in a narrow band of conditions — cool, clear, sun square to the modules — and an inverter sized for that peak sits underused for the rest of its life. Clipping a few hours a year at the top costs less than the larger inverter would, and inverters run at their best efficiency nearer full load anyway.