How to Calculate Your Solar Requirement From Your EB Bill

If you're planning to install solar at home, one of the first questions is:

How many kW of solar do I need?

The answer starts with your EB bill.

Your bill shows how much electricity your home has actually used. By looking at your recent consumption, you can estimate the solar capacity needed to offset a large part of that usage.

The calculation is simple, but there is one thing you need to check first:

Is your electricity bill monthly or bi-monthly?

Here's how to work it out.

The short version

Look for “Consumption in Units” on your EB bill. This is the number used for solar sizing.

If your bill is bi-monthly:

Average of latest 6 bills ÷ 270 = approximate solar requirement in kW

If your bill is monthly:

Average of latest 12 bills ÷ 135 = approximate solar requirement in kW

Once you have the number, your roof and electrical connection can be assessed to arrive at the final system design.

Which number on your EB bill matters?

An EB bill contains several figures, including your consumer number, tariff, sanctioned load, previous reading, current reading, units consumed and bill amount.

For solar sizing, the most important figure is:

Consumption in Units

Electricity consumption is measured in units or kWh.

For example, if your bill shows 850 units, your property has consumed 850 units during that billing period.

Don't use the total ₹ amount of the bill to calculate your solar requirement. Your bill amount can include fixed charges and other components that aren't directly related to the amount of electricity consumed.

First, check your billing cycle

Before calculating anything, find out how frequently your connection is billed.

A bill covering approximately two months is bi-monthly.

A bill covering approximately one month is monthly.

This distinction is important.

For example, 900 units on a bi-monthly bill means roughly 450 units per month. The same 900 units on a monthly bill represents twice the monthly consumption.

So the calculation needs to match your billing cycle.

How to calculate solar requirement from a bi-monthly EB bill

For a bi-monthly connection, use your latest six consumption readings.

Step 1: Add the latest six bills

Let's take an example:

Step 2: Find the average

Divide the total by six:

5,400 ÷ 6 = 900 units

Your average consumption is therefore 900 units every two months.

Step 3: Calculate the solar capacity

Now use the WES planning formula:

900 ÷ 270 = 3.3 kW

So the estimated requirement is approximately:

3.3 kW solar

The 270 figure uses an average planning assumption of around 4.5 units of generation per kW per day over a 60-day billing period.

Actual generation will vary with the site, weather, shading, panel orientation and system design.

Quick reference: bi-monthly consumption

Formula: Average bi-monthly units ÷ 270

How to calculate solar requirement from a monthly EB bill

If your connection is billed monthly, use your latest twelve consumption readings.

Suppose the total consumption across twelve months is:

16,200 units

Calculate the average:

16,200 ÷ 12 = 1,350 units per month

Then:

1,350 ÷ 135 = 10 kW

Your estimated solar requirement is therefore:

10 kW

Quick reference: monthly consumption

Formula: Average monthly units ÷ 135

Why use six or twelve bills?

Electricity consumption changes throughout the year.

In Chennai, for example, air-conditioning can significantly increase consumption during hotter months. Your usage may also change because of holidays, changes in occupancy or new appliances.

Using several billing periods gives you a better picture of your normal usage than relying on one unusually high or low bill.

For that reason:

Bi-monthly connection → latest 6 readings

Monthly connection → latest 12 readings

A real example: from EB bill to solar size

Let's put the calculation together.

A Chennai home receives a bi-monthly bill. Its latest six consumption readings are:

950 + 880 + 920 + 860 + 900 + 890 = 5,400 units

Average consumption:

5,400 ÷ 6 = 900 units

Estimated solar requirement:

900 ÷ 270 = 3.3 kW

So the initial calculation is:

That's enough to give you a useful starting point when discussing solar with an installer.

How much roof space will you need?

After estimating the system size, check whether your roof can accommodate it.

For practical planning, WES recommends allowing around 100 sq. ft. of usable, shadow-free roof area per 1 kW of solar.

The actual layout can vary depending on panel dimensions, roof shape, shading and access requirements.

A terrace may look large but still have less usable area because of water tanks, staircases, parapet walls, trees or neighbouring buildings.

What else should be checked before finalising the system?

Your EB bill gives you the consumption figure, but a final solar design needs a few more pieces of information.

Sanctioned load

The proposed solar capacity needs to work with your existing electrical connection and applicable requirements.

Roof condition

The installer should check the available area, structure, shading and panel layout.

Future consumption

If you're planning an EV, additional air conditioners or other high-load appliances, your future electricity use may be higher than what your current bills show.

Solar system type

An on-grid system is primarily designed to reduce electricity bills. If you also want battery backup during power cuts, a hybrid system may be more appropriate.

What if the calculated size doesn't fit your roof?

That's not unusual.

For example, your electricity consumption may indicate a requirement of 8 kW, but your property may not have enough suitable roof space for the entire system.

In that situation, the design can be reviewed based on your priorities, available roof area and budget.

You may also decide to install a smaller system initially and expand later, where the electrical connection, equipment and roof allow it.

The calculation tells you how much energy you use.

The site assessment determines what can practically be installed.

Common mistakes when calculating solar requirement

Using the bill amount instead of units

Solar capacity is based on electricity consumption, so use units consumed, not the total rupee amount.

Using only the latest bill

A single billing period may not represent your normal consumption. Use the required number of recent readings.

Mixing up monthly and bi-monthly bills

This can significantly change the calculated system size.

Ignoring future loads

An EV or additional AC can increase your electricity consumption considerably. Consider planned additions before finalising the system.

Assuming the whole terrace is usable

The roof needs to accommodate the panels while allowing for shading, obstacles and access.

What should you send to a solar installer?

If you want a proper solar assessment, start by sharing your recent EB bills.

For a quick assessment, provide:

• Latest six bi-monthly bills or twelve monthly bills
• Sanctioned load
• Roof availability
• Major appliances
• Any planned future loads
• Whether you need battery backup

The installer can then assess the consumption history alongside the property and recommend a suitable system.

At WES Energy, the objective is not simply to put a standard-sized solar system on your roof. The system should make sense for how your property actually consumes electricity.

The calculation is simple. The design is not.

You can estimate your solar requirement from your EB bill in just a few steps.

Bi-monthly bill

Average of latest 6 consumption readings ÷ 270

Monthly bill

Average of latest 12 consumption readings ÷ 135

That gives you an approximate solar capacity in kW.

From there, the roof, shading, sanctioned load, future consumption and system type need to be assessed before finalising the installation.

So if you're wondering “How much solar do I need?”, don't start by counting panels.

Start with your EB bill.

Frequently asked questions

How do I calculate my solar requirement from my EB bill?

For a bi-monthly bill, average your latest six consumption readings and divide by 270. For a monthly bill, average your latest twelve readings and divide by 135.

Which number on my EB bill is used for solar calculation?

Use the consumption in units. The total bill amount is not the correct starting point for calculating solar capacity.

How much solar do I need for 1,000 units every two months?

Using the WES planning formula:

1,000 ÷ 270 = approximately 3.7 kW

The final capacity should be confirmed after assessing the property.

How much solar do I need for 500 units per month?

Using the monthly formula:

500 ÷ 135 = approximately 3.7 kW

How many EB bills should I use?

Use the latest six bills for a bi-monthly connection and the latest twelve bills for a monthly connection.

How much roof space is needed for 5 kW solar?

As a practical planning figure, allow approximately 500 sq. ft. of usable, shadow-free roof area.

Can I calculate my solar requirement using only my EB bill?

You can calculate an initial system size from your bill. The final design also needs to consider your roof, electrical connection, shading, future consumption and system requirements.

What if my latest electricity bill is unusually high?

Don't automatically size your solar system around one unusually high bill. Use multiple billing periods to establish your normal consumption. If the increase is permanent, such as from a new AC or EV, include that future usage in the assessment.