How to read your electricity bill before you go solar

Before anyone can tell you what size of solar plant you need, they should read your electricity bill properly. Most people look at the amount and the due date. The useful information is in the small boxes around them.

This guide follows the WBSEDCL bill, which covers most of West Bengal. CESC bills in Kolkata look different but carry the same basic information.

1. The billing period

Start here, because everything else depends on it. Many home bills in West Bengal cover three months, while most commercial and industrial bills are monthly. Compare a quarterly bill with a monthly figure and you'll get the plant size wrong by a factor of three. We have seen it happen.

2. Units consumed

This is the number that matters most for solar. It's the present meter reading minus the previous one, sometimes multiplied by a meter factor on bigger connections. If the meter was replaced during the period, the bill can show two or three meter lines, and you need to add them all up.

One bill isn't enough. Collect twelve months if you can. Consumption in Bengal climbs sharply in summer when fans and air conditioners run, and a single bill can make a plant look far too big or too small.

3. Sanctioned load or contract demand

Homes and small shops have a sanctioned load in kW. Larger connections have a contract demand in kVA. Under West Bengal net metering this figure caps the size of your solar plant, so check it early. If it's too low, an application to increase it has to go in before the solar application.

4. Tariff category and time-of-day blocks

Your category (domestic, commercial or industrial, LT or HT) sets the rate per unit. Many commercial and industrial connections are also billed by time of day, and the bill shows units separately for each block:

• Normal: 6 am to 5 pm
• Peak: 5 pm to 11 pm
• Off-peak: 11 pm to 6 am

Solar generates in the normal block, and under the 2025 rules its surplus can only flow down into cheaper blocks. So the split between these three lines tells us how much of your bill solar can actually reach. There's more on that in sizing a factory solar plant.

5. Fixed or demand charges

Every bill carries a charge that doesn't depend on units: a fixed charge on smaller connections, a demand charge on larger ones. On HT bills the demand charge is usually billed on the higher of your actual maximum demand or a set share of your contract demand.

Solar reduces the units you buy. It doesn't remove this part of the bill, and anyone who promises your bill will drop to zero has forgotten about it. Our calculator assumes the fixed part stays, which is why its saving is lower than a simple units-times-rate sum.

6. Electricity duty

Duty is a state levy charged as a percentage on top of energy and fixed charges. On some industrial connections we've decoded, the duty rate steps up once monthly consumption crosses a threshold. That has a pleasant side effect: a plant that pulls a month back below the step lowers the duty rate on the whole bill, not just on the units it replaces.

7. Rebates

Look for the amount payable within the due date and, on many bills, a slightly lower figure for paying online within the due date. Paying online and on time is one of the easiest savings there is, with or without solar.

8. What to send us

Clear photos of your last three bills (twelve is better), with the consumer number, category, units, load and any time-of-day lines readable. That's enough for a first sizing, and it's what we use to build the "your current cost of power" page in every proposal. Send them on WhatsApp.

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