How Much Does It Cost to Run a Home UPS or Inverter?

A UPS, usually called an inverter in India, charges a battery from the mains and turns its stored power back into AC during a power cut. Every step loses some energy, and you pay for those losses when the battery recharges.

Budget units lose a lot. In a 2016 test of five 1 kVA home UPS units sold in Pakistan, researchers at LUMS found they gave back only 44–54% of the electricity used to charge them, about 51% on average. With a budget UPS and a lead-acid battery, every unit (kWh) used in a power cut takes about two units to put back.

Our typical figure assumes 3 hours of power cuts a day with 150 W of fans and lights on the UPS. They take 0.45 kWh from the battery, and about the same again is lost recharging it. Standby draw of roughly 12 W over the other 21 hours adds about 0.25 kWh.

That's about 0.7 kWh a day, or 22 kWh a month: about Rs 724 in Pakistan at Rs 33.10/kWh. The fans and lights themselves cost the same as they would on mains power, so they aren't counted here.

With the mains on, a UPS still uses power. Its electronics and cooling fan run, and the charger keeps the battery topped up.

Quality inverter-chargers publish this no-load figure. Victron rates its 1,200 VA MultiPlus at 8–10 W with no load, and its 3 kVA and 5 kVA models at about 20–35 W.

Once the battery is full, the top-up charge is small. The SF Powersmart Sine Pro AL manual specifies under 1 A at 13.7 V for a 12 V battery, which is under about 14 W.

India's BEE star label rates home inverters on DC-to-AC efficiency, from 83% for 1 star to over 91% for 5 stars. A 5-star inverter wastes less of the battery's energy during a power cut, but the label doesn't include charging losses.

A bigger battery gives longer backup but doesn't change the losses much. They depend mainly on how much energy you take out of the battery and put back each day.

A 12 V, 150 Ah battery, a common size for a 900 VA home UPS, stores about 1.8 kWh when new (12 V × 150 Ah).

Through an 85%-efficient inverter, that's roughly 1.5 kWh of AC power. In theory it runs 150 W of fans and lights for about 10 hours, or a single 60 W ceiling fan for about 25 hours.

Real backup time is shorter. Lead-acid batteries deliver less at high loads and as they age, and the UPS cuts off at a low-battery voltage (10.5 V in the SF manual) before the battery is completely flat.

Efficient loads stretch the battery. A BLDC ceiling fan of about 30 W runs roughly twice as long as a 60 W fan, and LED bulbs use a fraction of the power of older bulbs.

Most home UPS systems in Pakistan and India use flooded or tubular lead-acid batteries. They're cheap, but they turn a large share of each charge into heat. The LUMS researchers put the low efficiency of budget units down to inefficient inverters and car-type lead-acid batteries.

Lithium (LiFePO4) batteries lose less energy in each charge and discharge and can be used more deeply. They cost more to buy.

Charging the battery from solar panels instead of the grid takes the charging cost off your bill. The LUMS study recommended solar charging to cut the load that UPS charging puts on Pakistan's grid.

In Pakistan, India, South Africa and Nigeria, where power cuts and load-shedding are common, every extra hour of daily cuts cycles more energy through the battery and adds to the losses. On slab tariffs, as in Pakistan, those extra units can also push a bill into a more expensive slab.

Ratings are from the makers' spec sheets and manuals. To see your own UPS's overhead, compare your electricity meter over a few days with the UPS in use and with it bypassed, then enter the difference in the calculator.

• Put only essentials on the UPS circuit, such as fans, lights and the Wi-Fi router. Irons, kettles and heaters drain the battery fast and waste the most.
• Switch to BLDC fans and LED lights. Halving the load halves both the drain on the battery and the energy lost recharging it.
• Choose an efficient inverter: look for a high BEE star rating in India, or the efficiency and no-load figures on the spec sheet.
• Keep the battery cool and well ventilated. ENERGY STAR notes that battery life can halve with every 15°F (about 8°C) rise in temperature.
• Top up flooded batteries with distilled water as the manual says, and keep the terminals clean and tight.
• If you have solar panels, charge the battery from them instead of the grid.

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