Sizing a rooftop system starts with units, not rupees. Take your last twelve electricity bills and note the units consumed each month, then work out an average. Twelve months matter because air-conditioning makes summer bills two or three times higher than winter bills.
Next, divide the average monthly units by 30 to get your daily consumption. In Gujarat, one kilowatt of well-installed rooftop solar produces about 4 to 4.5 units per day averaged across the year. Divide your daily units by 4 and you have an approximate system size in kilowatts.
For example, an average bill of 600 units per month works out to 20 units a day, which needs roughly a 5 kW system. Our online calculator does exactly this arithmetic for you and also estimates panel count and roof area.
Two adjustments matter. If you plan to add an air conditioner or an electric vehicle in the next two years, size up by 15-20% now — expanding later means a second round of DISCOM approvals. If your roof is partially shaded, size down rather than crowding modules into shadow.
Finally, remember the sanctioned load limit: most DISCOMs will not approve a rooftop plant larger than your sanctioned load without an enhancement application. A good installer checks this before quoting.