How to Choose the Right Solar System Size for Your Home in Bangalore

The short answer

The 1.2 is headroom for 25 years of ageing and losses. The 110 is the average monthly generation per kW across about 30 TerraEnerG installations in Bengaluru. Use the calculator below for your home.

Solar size calculator

Enter your bill or units, add anything you plan to add, and compare the two nearest sizes by the present value of their future savings.

The method, step by step

1. Start with current usage. Average your last 6–12 bills, because summer and winter differ. If you only know the bill, divide by about ₹7 per unit.
2. Add what you plan to add. A new AC or an EV changes your usage for the next 25 years, so size for the home you will have. See the table below.
3. Add about 20% headroom. Panels lose a fraction of a percent of output each year, and inverters, dust and heat also take a share. The headroom keeps the system matched to your needs in its later years.
4. Divide by 110. Our installations in Bengaluru average about 110 units per kW per month. That gives the kW you need.
5. Pick the nearest standard size. We install 3K, 5K, 8K and 10K systems. When your result falls between two, compare them, as in the example further down.

Worked examples

Typical loads you may add

The AC and EV rows follow our sizing method. The other rows use typical ratings and one hour of daily use; check your appliance label and adjust the hours.

Bill to units

Your exact bill depends on slabs and fixed charges. If your bill shows units consumed, use those.

Bangalore sunshine and what we measure

• Theoretical: Global Solar Atlas puts Bengaluru at about 5.28 kWh per square metre per day (1,928 a year), with about 5.46 peak sun hours at the best tilt, which works out to a theoretical yield of about 1,578 kWh per kW per year.
• Seasonal: March is the sunniest month at about 6.85 kWh/m²/day and July the dullest at about 4.05, so the monsoon months generate about 40% less.
• Measured: across about 30 TerraEnerG installations, we see about 110 units per kW per month, or about 1,320 a year. That is roughly 84% of the theoretical figure, which allows for real roofs, heat, dust and shading.

Because BESCOM no longer banks units, a sunny month cannot offset a dull one. That is why sizing for the right amount matters more than sizing for the most.

Source for the theoretical figures: Global Solar Atlas (World Bank Group / ESMAP), as reported by Qbits Energy. The 110 figure is TerraEnerG's own average.

Is bigger always better?

No, and the reason is the export rate. A unit you use at home saves about ₹7. A unit you export earns ₹3.8 on a non-subsidy system (8K, 10K) or ₹2.2 on a subsidy system (3K, 5K). Past the size that covers your usage, each extra kW earns far less. The 8K and 10K systems also don't qualify for the PM Surya Ghar subsidy, which adds up to ₹78,000 on 3K and 5K.

When your result sits between two sizes, compare them by the present value of their future savings:

Take the second home above: 520 units a month needs about 5.7 kW, which sits between TerraEnerG 5K and TerraEnerG 8K. Using the calculator's default assumptions (₹7 per unit, 8% discount rate, no tariff growth, 0.5% yearly panel degradation, 25 years):

Moving up to TerraEnerG 8K costs ₹1.78 lakh more, loses the subsidy, and adds about ₹1.59 lakh of present-value savings, so on these assumptions it ends ₹0.19 lakh behind of TerraEnerG 5K. Most of the extra 3 kW is exported at ₹3.8 a unit rather than used at home. Change the assumptions in the calculator to test your own case.

When the roof is too small

Plan on roughly 65–70 sq ft of shadow-free roof per kW, so 5 kW needs about 325–350 sq ft. If your roof is short of the area you need, you have three options: a smaller system, a layout that uses another part of the roof, or getting more from each panel.

On that last option, our measured comparison found a raised structure over a white-painted concrete surface generated about 15% more than a flush mount on a puff sheet roof, with bifacial panels in the same villa community. About 10% of that was before the flush plant was shaded in the late afternoon. The result combines height, tilt, surface and shading, and covers one month, so read the full case study before relying on it. In a separate comparison, a plant above a white reflective roof coating generated about 12% more than one above an uncoated roof; see the reflective coating case study.

We check your roof, shade, sanctioned load and BESCOM approval requirements during the free site survey. If you have frequent power cuts, ask about a hybrid system with battery backup, which is sized differently.

If you pick a size that's too small or too large

• Too small: you keep paying BESCOM for the units your system doesn't cover, and adding an AC or EV later pushes more of your usage back to the grid.
• Too large: you pay for capacity whose output is mostly exported at ₹2.2–₹3.8 a unit, instead of saving ₹7, and you may lose the subsidy by crossing from 5K to 8K.
• About right: most of your generation is used at home, and the headroom keeps you covered as the system ages or you add loads.

Next steps