Rooftop Solar and the Family Electricity Bill
A household in Thrikkakara, Ernakulam, has the following appliances: a refrigerator whose compressor runs 8 hours a day at 180 W; three ceiling fans of 75 W each used 10 hours a day; six LED lamps of 9 W each used 5 hours a day; a television of 100 W used 4 hours a day; and a 750 W water pump used 30 minutes a day. The family is billed for 30 days. The tariff is ₹4 per unit for the first 100 units, ₹6 per unit above 100, plus a fixed charge of ₹50. A solar vendor offers a 1 kW rooftop system that generates an average of 4 units a day, with net metering. The installed cost is ₹60,000. The vendor claims the system will pay for itself in about four years. The family also wants to know whether the pump should be run during the day.
1. Calculate the household's daily and monthly electricity consumption and the present monthly bill.
Read the worked answer
Convert watts to kilowatts and multiply by hours:
- Refrigerator: 0.18 kW × 8 h = 1.44 units/day
- Fans: 3 × 0.075 kW × 10 h = 2.25 units/day
- LED lamps: 6 × 0.009 kW × 5 h = 0.27 units/day
- Television: 0.10 kW × 4 h = 0.40 units/day
- Pump: 0.75 kW × 0.5 h = 0.375 units/day
Daily total = 1.44 + 2.25 + 0.27 + 0.40 + 0.375 = 4.735 units.
Monthly total = 4.735 × 30 = 142.05 units.
Bill = first 100 units at ₹4 = ₹400; remaining 42.05 units at ₹6 = ₹252.30; fixed charge ₹50. Total = ₹702.30.
2. Calculate the bill after installing the solar system and the actual payback period.
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Solar generation = 4 units/day × 30 = 120 units/month. With net metering, the net import is 142.05 − 120 = 22.05 units.
New bill = 22.05 × ₹4 + ₹50 = ₹88.20 + ₹50 = ₹138.20.
Monthly saving = ₹702.30 − ₹138.20 = ₹564.10.
Payback = ₹60,000 ÷ ₹564.10 = 106.4 months, or about 8.9 years. The saving is real but much smaller than the vendor suggests, because most generation merely replaces units that were billed at the lower slab or offset the fixed charge only partly.
3. Evaluate the vendor's four-year payback claim and advise on running the pump during the day.
Read the worked answer
A four-year payback would require a monthly saving of ₹60,000 ÷ 48 = ₹1,250. The calculated saving is only about ₹564, so the claim is overstated by more than double. The vendor may be ignoring the fixed charge, assuming a higher tariff, or assuming that all generated units are paid for at ₹6.
Running the pump during sunny hours is sensible because the pump then consumes solar output directly instead of importing grid power. With net metering, the financial difference is small, but daytime use reduces simultaneous import and export and is useful if net metering is delayed. The family should also clean the panels and check that the inverter is sized correctly before expecting even the 8.9-year payback.