How to Choose the Right Solar System Size

Almost every avoidable disappointment with rooftop solar traces back to sizing. A system that is too small barely dents the bill; one that is too large produces electricity you receive limited value for. Getting this right is mostly arithmetic and honesty about your own consumption.

Step 1: Use a full year of consumption, not one bill

Take twelve months of electricity bills and calculate your average monthly units. Using a peak summer bill will oversize the system; using a mild winter bill will undersize it. If your consumption is genuinely rising year on year, use the most recent twelve months.

Step 2: Convert units into indicative capacity

In most parts of India, 1 kW of rooftop solar generates roughly 120 units a month (about 4 units a day, or 1,400–1,500 units a year). So:

Indicative kW = average monthly units ÷ 120

A home averaging 480 units a month lands at about 4 kW. Treat this as a starting point, not a conclusion.

Step 3: Check it against your roof

Allow roughly 100 sq ft of shadow-free area per kW for a residential system. Then walk the roof — ideally in the morning and again in the afternoon — and be honest about shadows from water tanks, parapet walls, staircase rooms, neighbouring buildings and trees. Winter shadows are longer than summer ones, so a roof that looks clear in May may not be in December.

Step 4: Check your sanctioned load

Most DISCOMs limit permitted rooftop capacity in relation to your sanctioned load. If your calculated requirement exceeds it, you may need to apply for a load enhancement — which is straightforward but adds time, and should be identified before installation rather than after.

Step 5: Understand what happens to surplus

Under net metering, exported units are credited against imports, typically with limits on how much can be banked and how it is settled. Under net billing or gross metering — which some states apply — the treatment differs and the value of surplus can be lower. This is a state-by-state matter and it directly affects whether oversizing makes sense.

Step 6: Factor in what is coming

If an electric vehicle, additional air conditioning or a home extension is likely within a few years, it is usually cheaper to size for it now than to add capacity later — subject to what your sanctioned load and DISCOM permit.

A note on subsidy and sizing

For eligible residential consumers, central assistance is capped at up to ₹78,000 for systems of 3 kW and above. Do not let that ceiling drive the design. Sizing to consumption produces a better outcome than sizing to a subsidy band.