Your electricity bill already holds almost everything needed to size a rooftop solar system. Three numbers matter most: units consumed, your rate per unit, and your sanctioned load.
1. Units consumed (kWh)
A unit is one kilowatt-hour (kWh): the energy a 1,000-watt appliance uses in one hour. Your bill lists the units you used in the billing period.
If you are billed every two months, divide by two to get a monthly figure. Usage changes with the seasons, so take an average across summer and winter bills where you can.
2. Your rate per unit (₹)
Your bill has fixed charges (for your connection) and energy charges (for the units you use). Many tariffs also charge more per unit as your usage rises.
The simplest way to find your real rate: divide the total bill amount by the units on the same bill. For example, ₹3,000 for 400 units works out to ₹7.50 per unit.
3. Sanctioned load (kW)
This is the maximum load your electricity company has approved for your connection. It is a useful guide to the inverter size you need. Your electricity company may also link how much rooftop solar you can connect to your sanctioned load, so check its rules.
Putting the numbers together
Take a home that uses 400 units a month:
- 400 units ÷ 30 days = about 13.3 units a day.
- In much of India, 1 kWp of solar panels produces about 5 units a day on average, before losses. After typical losses (we use 78% system efficiency), that is about 3.9 units a day per kWp.
- 13.3 ÷ 3.9 = about 3.4 kWp of panels needed.
- With 635 Wp Tenzor Stellar modules, 6 modules give 3.81 kWp, producing roughly 446 units a month.
Generation varies with your location, roof direction, shading and weather, so treat this as a starting estimate. A site visit confirms the final design.
Try it with your own bill
The Tenzor solar calculator does this for you. Enter your units or your bill in rupees and it recommends the Tenzor inverter, the number of panels, the roof area and your likely savings.