Work out how many panels, how big a battery and which inverter your home needs, using your own electricity bill and four simple formulas.
Sizing a home system follows the same order every time: how much you use, how big the solar array must be, how big the battery must be, and how big the inverter must be. Our worked example uses a home that uses 300 units a month.
Your electricity bill shows monthly units, and one unit is one kilowatt-hour. Divide by 30 to get daily use. See our power guide for how to estimate individual appliances.
300 units / 30 days = 10 kWh per day
Solar output depends on how many peak sun hours your location gets, which is the equivalent number of hours of full-strength sunshine per day. Many places in India see roughly 4 to 6, but check a local figure. Real systems lose some energy to heat, dust, wiring and the inverter, so use an efficiency factor of about 0.75.
10 kWh a day, 5 peak sun hours:
kWp = 10 / (5 × 0.75) ≈ 2.67 kWp
Using 550 W panels:
Panels = 2,670 / 550 ≈ 4.9, so round up to 5 or 6 panels
With 6 panels (3.3 kWp), expected daily output is:
3.3 × 5 × 0.75 ≈ 12.4 kWh per day
Decide how much energy you want from the battery. This is usually your evening and night use, or the loads you want to keep during a power cut. Batteries should not be fully drained, and the inverter wastes a little energy, so divide by both factors.
5 kWh of evening use, 90% usable depth of discharge, 92% inverter efficiency:
Battery = 5 / (0.90 × 0.92) ≈ 6.0 kWh
So look for a battery of about 6–7 kWh. Learn more about these ratings in our BESS guide.
A 7 kWh battery feeding a 500 W load (lights, fans, Wi-Fi, fridge):
Usable energy = 7 × 0.90 × 0.92 ≈ 5.8 kWh
Runtime = 5.8 / 0.5 ≈ 11.6 hours
Our Battery Life calculator does the same arithmetic for any load.
The inverter must cope with everything running at once, plus the brief surge that motors draw when they start. List the loads you expect to run together and add a margin of about 25% to 50%.
A 1.5 kW air conditioner, a 0.2 kW fridge and 0.3 kW of fans and lights:
Running load = 1.5 + 0.2 + 0.3 = 2.0 kW
Inverter ≈ 2.0 × 1.5 = 3 kW
Low-voltage battery systems carry large currents. On a 48 V battery bank, a 3 kW load draws:
I = P / V = 3,000 / 48 ≈ 62.5 A
Thick short cables and correctly rated fuses are essential on the DC side. Use our Wire Gauge calculator to check voltage drop. Panels and batteries produce DC and the home uses AC, which our AC vs. DC guide explains.
| Type | Battery | Works in a power cut |
|---|---|---|
| Grid-tied | No | No, it shuts down for safety |
| Hybrid | Yes | Yes, for the loads you connect |
| Off-grid | Yes | Yes, it is the only supply |
If solar covers 300 units a month, at an example tariff of ₹7 per unit:
300 × 7 = ₹2,100 per month
Tariffs, net-metering rules and subsidies differ by state and utility, so check your own before deciding.
Try each one before you open the answer.
Daily use = 450 / 30 = 15 kWh. kWp = 15 / (5 × 0.75) = 4 kWp.
4,000 / 540 ≈ 7.4, so 8 panels.
8 / (0.90 × 0.92) ≈ 9.7 kWh.
Divide your daily energy use in kWh by the peak sun hours and an efficiency factor of about 0.75 to get the array size in kWp, then divide by the wattage of each panel.
kWp is the rated peak output of the solar panels under standard test conditions. kW is a general power unit. Real output is usually lower than kWp because of heat, dust, wiring and inverter losses.
Size it for the energy you want to use after sunset or during an outage, then divide by the usable depth of discharge and the inverter efficiency.
A grid-tied system works without a battery but shuts down during a power cut for safety. To keep power during outages you need a hybrid inverter with a battery.
Estimate how long a battery will run your loads.
Open the Battery Life Calculator