Solar panel size calculator
Work backwards from your monthly electricity use to the kW your roof needs, the number of panels and the area they take.
| Breakdown | Value |
|---|
Estimate. The method and sources are on this page — check them against your own case before you act.
How it is calculated
A solar system is sized to the energy you use, not to the roof you have. The calculator reverses the generation formula: units first, then kW, then panels.
Daily units = monthly units ÷ 30 · kW = daily units ÷ (sun hours × performance ratio) · Panels = ceil(kW × 1000 ÷ panel Wp)
Convert the bill to daily units
300 units a month is about 10 units a day. If your use is seasonal, use a year-average month so the system is not oversized for one hot month.
Divide by real generation per kW
One kW generates about sun hours × performance ratio units a day — roughly 3.4 units at 4.5 sun hours and a 0.75 ratio. So 10 units a day needs about 3 kW.
Convert kW to panels and roof
At 550 Wp a panel, 3 kW needs six panels, and a rooftop array of this size takes roughly 300 sq ft including gaps and walkways.
Sizing assumptions and rules of thumb
| Item | Value used | Why |
|---|---|---|
| Monthly units | 300 units (editable) | Your bill's kWh for a typical month. |
| Peak sun hours | 4.5 hours/day (editable) | Conservative all-India average used for generation. |
| Performance ratio | 0.75 (selectable) | Heat, dust, wiring and inverter losses. |
| Panel rating | 550 Wp (editable) | Module count = system kW ÷ module kW, rounded up. |
| 1 kW in a day | About 3–4 units | 4.5 sun hours × 0.75 gives 3.4 units. |
| Roof per kW | About 100 sq ft / 9–10 m² | Includes spacing and walkways, not just panel area. |
| 30-day month | Daily units = monthly ÷ 30 | A simplifying assumption for the average day. |
| Unit | 1 unit = 1 kWh | The same unit your discom bills. |
What most pages get wrong
Size to units, not to roof size
A roof that fits 5 kW does not mean you should buy 5 kW. The system should match the units you actually buy from the discom, otherwise the extra generation is exported at a lower credit.
More kW does not always mean more saving
Under net metering, units exported beyond your consumption are usually credited at a lower rate or banked, not paid at your retail tariff. Match the system to your use.
Real output is about three-quarters of the nameplate
A 3 kW array does not make 3 kW all day. Heat, dust, inverter conversion and wiring leave about 70–80% of the theoretical sun-hours figure.
Plan for 100 sq ft per kW
A 3 kW system needs roughly 300 sq ft of shadow-free roof, including access gaps. Shading on even one panel can drag down a whole string.
Frequently asked questions
What size solar system do I need for 300 units a month?+
How many units does a 1 kW solar system generate per day?+
How much roof area does a 1 kW solar system need?+
Should I add extra capacity for the future?+
Does the calculator account for roof direction and shading?+
How do I get my monthly units?+
Sources, dates and a changelog.
Every number this calculator uses is either an input you control or a value from the sources below. The formula is compiled and tested at build time, so the page cannot ship with a broken calculation.
Sources
- PM Surya Ghar: Muft Bijli Yojana — National Portal (MNRE)
- Ministry of New and Renewable Energy — rooftop solar programmes
- Checked by hand, not scraped
This page
- Last verified: 29 October 2026
- Next review: 29 April 2027
- Vertical: Solar & Energy
- Build tests: 3 passed
- Changelog: 1 entry
How we handle errors
- Spotted a mistake? Report it — we correct and log it.
- Corrections appear in the changelog below.
- Read the methodology and the disclaimer.