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Solar system size calculator: kW from a monthly bill

Enter a monthly electricity bill and this calculator suggests a rooftop solar size in kilowatts. It divides the bill by an average tariff to get units, spreads them over the month, divides by peak sun hours and a performance ratio, then rounds up to the next half kilowatt. You can change the tariff.

Calculator

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How it is calculated

  1. Units per month = monthly bill ÷ average tariff per unit.
  2. Units per day = units per month ÷ days per month.
  3. Raw size in kilowatts = units per day ÷ (peak sun hours × performance ratio).
  4. Round up to the next half-kilowatt step, then keep the result between the minimum and maximum size.

Assumptions

  • Average tariff: ₹7 per unit (kWh), one blended figure. You can type your own tariff in the calculator and the size follows it.
  • Peak sun hours: 4.5 equivalent full-sun hours per day, one figure for most of India with no adjustment for your location.
  • Performance ratio: 0.8, an allowance for inverter, wiring, soiling and temperature losses.
  • Month length: 30 days. The bill is treated as one month of units at the single tariff, so fixed charges, slabs and taxes are not separated out.
  • Rounding: the size is rounded up to the next 0.5 kW step, never below 1 kW and capped at 100 kW.
  • Not a design: roof area, shading, direction, sanctioned load and the utility's net-metering limits are not considered.

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How we verify data

What the estimate means

The result is a talking-point guess for a first conversation with a customer. It uses the same sizing constants Bharat Solar Venture uses when it suggests a capacity from a bill. It is not a design: roof area, shading, direction, sanctioned load and the utility's net-metering limits are all left out.

How to read the result

The suggested size is rounded up so the customer is not under-sized. A bill that implies a size just above a step moves to the next half kilowatt. Very small bills show the minimum size, and very large ones show the maximum, and the page says so when that happens.

What to do with it

Use the size as the starting capacity for a site visit, then check the indicative subsidy for that size. Once the size is agreed, price it in a quotation. Share the result as a link so the customer can see the same numbers and assumptions you saw.

Frequently asked questions

Why does the calculator ask for a tariff?

A bill is a rupee amount, but solar size depends on units. The tariff converts one into the other. The default is a blended average, so if the customer's real rate is higher or lower, type their figure and the size will follow.

Is this a design or a quotation?

No. It is a first estimate to start a conversation. A real quotation needs a site visit, a rooftop check, the sanctioned load and the customer's utility rules. Use quotation software to price the system once the size is agreed.

Why does the size move in half-kilowatt steps?

Installers usually quote in half-kilowatt steps, and the calculator rounds up so the customer is not under-sized. A bill that implies a size just over a step moves to the next step.

Use this inside Bharat Solar Venture

Quick Quote turns a customer's monthly bill into a draft quotation priced from your own price book. Create an account to try it.

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