Solar panel sizing:
from daily kWh to kW and number of panels

Start from what you use, not what you can fit

The right size for a solar system begins with your electricity use, not with the roof. Look at your bills, work out the units you use in a day and decide how much of that you want solar to cover. Roof area, budget and utility rules then decide whether that target is achievable.

Sizing matters because an undersized array leaves you paying for most of your power, while an oversized one may produce more than your meter credits at a good rate or than your roof or sanctioned load allows.

An energy audit before you commit is cheap and often profitable. Swapping old lights and fans for efficient ones can cut daily use by a few units and shrink the array you need, which usually costs less than the extra panels.

The sizing formula

Peak sun hours are not hours of daylight. They compress the day's sunshine into the number of hours at full intensity. A site with ten hours of daylight might have five peak-sun hours.

Use the winter or monsoon figure if you want to be sure of covering the bad months, and the annual average if you are content to import a little in those months and export in the sunny ones.

Worked example: a home using 20 kWh a day

Rounding up to ten panels, the array's rated output is 5.4 kW, which at 4.0 effective hours would produce about 21.6 kWh a day, comfortably covering the 20 kWh.

Panel count and roof area

Once you have the kW, convert to panels by dividing by the module rating, then check the roof. A 540 W module is about 2.3 m by 1.1 m, roughly 2.6 m² each, so ten panels need around 26 m² of unshaded, usable surface, plus walkways and clearances.

Orientation and tilt also matter. In India, south-facing panels at a tilt close to the site's latitude generally produce the most over a year. East-west layouts give a flatter output through the day with somewhat lower totals.

Panel ratings are changing quickly, with higher-wattage modules now common. A higher rating means fewer panels for the same kW but not necessarily a smaller roof requirement per kW, because larger panels are physically larger. Compare area per kW across datasheets rather than looking only at wattage.

Adjusting the size for real conditions

• Shading from trees, water tanks or neighbouring buildings can cut output sharply; one shaded panel can limit a string.
• Dust and monsoon cloud lower the yearly average; use a conservative peak-sun figure for cloudy regions.
• Future loads: an air conditioner, an EV charger or a second storey can raise demand, so leave headroom in the inverter and the roof plan.
• Sanctioned load and utility rules often cap the rooftop capacity you can connect, so check the limit with the utility.
• Net metering rules affect how much export is credited, which influences whether to size for 100% of use.

Temperature also lowers output. Panels lose a fraction of a percent of power for each degree above 25 °C of cell temperature, and rooftop panels in a hot summer can run 20 to 30 degrees above air temperature. That loss is one reason system efficiency is taken as 75 to 85% and not much higher.

Checking the sizing against a bill

A sanity check with your bill takes two minutes. Annual output of a well-sited system is roughly its kW rating times 1,300 to 1,600 kWh in much of India. For a 5 kW system, that is 6,500 to 8,000 kWh a year, or about 18 to 22 kWh a day, which matches the 20 kWh daily target from the example.

Compare that with your annual consumption. If your bills show 7,300 kWh over twelve months, the 5 kW system would cover essentially all of it on paper. In practice, timing matters: output is concentrated in the middle of the day, and the grid fills the gap at night, with net metering balancing the units over the billing period.

If the check does not match, look for the reason before you buy. A designer who proposes 8 kW for a 20 kWh a day home may be allowing for future loads, or may be padding the quote, and you are entitled to ask which.

Grid-tied, hybrid and off-grid sizing

A grid-tied system sends surplus to the utility and uses the grid at night, so the array size follows your daytime and annual energy needs. An off-grid system with batteries must also cover the nights and cloudy days, which means a larger array and a battery bank sized for a chosen number of days of autonomy.

As a first estimate for batteries, multiply the daily energy you want to use from storage by the days of autonomy and divide by the usable depth of discharge and the round-trip efficiency. Battery sizing has more variables, so ask an installer to confirm the design.

A short checklist before you buy

1. 1Take twelve months of bills and find the average and peak daily use.
2. 2Choose the coverage target: all of your use, or only the daytime share.
3. 3Use the sizing formula with a realistic peak-sun figure for your city and a system efficiency of about 0.75 to 0.80.
4. 4Check roof space, shading and structural suitability with the installer.
5. 5Confirm utility and subsidy conditions before you sign.

The result should be treated as a first estimate. A site survey, a shading analysis and the manufacturer's data sheet refine it, and the installer should explain how their design matches the number you worked out.