Solar Plus Storage for Indian Homes: How to Size a System That Pays Back

Almost every badly sized home system starts with a vendor asking “how many kilowatts do you want?” The right first question is what your electricity bill actually says.

Pull twelve months of bills and note the units consumed each month, and your tariff per unit including any slab effects. Everything else follows from those two numbers.

Divide your monthly units by 30 to get daily consumption. Then estimate how much a kilowatt of panels generates per day where you live — this varies with location, orientation, shading and season, so ask your installer for a figure specific to your roof rather than a national average.

Array size is roughly your target daily generation divided by that per-kW figure. Size generously enough that summer noon produces surplus; that surplus is what makes storage worthwhile.

This is the step almost everyone skips. A battery is only useful for the units you consume when the sun is not shining.

Read your meter at sunset and again at sunrise for a week. The difference is your night consumption. Do it in both a summer and a winter week if you can — air conditioning and geysers move this number a great deal.

• •Take your average night consumption in units (kWh)
• •Decide what share you want covered — covering 100% of the worst night is expensive; most people target a typical night
• •Divide by the round-trip efficiency of the system, since some energy is lost going in and out
• •Add margin for depth of discharge — you do not want to run the pack flat daily

The result is your target usable capacity in kWh. Note the word usable: ask vendors whether the capacity they quote is nameplate or usable, because the difference can be substantial.

Capacity in kWh tells you how much energy is stored. It says nothing about how fast you can draw it. If your evening peak includes an air conditioner, a pump and a geyser, the inverter and battery must supply that instantaneous power in kW, or things will trip regardless of how many kWh are sitting in the pack.

Our guide to kW versus kWh explains the distinction in more depth; it was written for commercial systems but the principle is identical at home.

Annual saving is the units you shift from grid to stored solar, multiplied by the gap between your import tariff and whatever you would have been paid to export them. Divide the installed cost by that annual saving for a simple payback in years.

Then sanity-check it against the warranty period. If simple payback is longer than the battery’s warranted life, the system is being justified on backup value rather than savings — which is a perfectly good reason to buy it, but it should be said out loud.

• •Sizing the battery from total daily consumption instead of night-time consumption
• •Forgetting that a standard grid-tied inverter cannot charge a battery
• •Comparing nameplate capacity against usable capacity between two quotes
• •Ignoring where the battery will physically live — ventilation and heat matter to its life
• •Assuming today’s net metering terms will hold for the whole payback period