Battery backup runtime:
how long an inverter battery will carry your load

From a battery label to hours of backup

A battery label gives you capacity, not time. To find how long an inverter will keep your fan, lights and router running, you convert the capacity to energy, take off what cannot be used, and divide by how fast your appliances draw energy. The answer is only as good as the load you feed in.

A 12 V battery rated 100 Ah stores 12 × 100 = 1,200 Wh nominally. Multiply by an inverter efficiency of 0.90 and divide by a 150 W load, and the runtime is 1,200 × 0.9 ÷ 150 = 7.2 hours, or 7 hours 12 minutes.

Adding up what you will actually run

The load is the sum of the power drawn by whatever is switched on at the same time. Take wattage from each appliance's rating plate, and use the running value rather than the start-up peak.

Pumps, refrigerators and air conditioners draw several times their running power for a moment when they start. The inverter must be rated for that peak, even though it has little effect on the hours. Never size backup from the rating of a 1,500 W heater unless you intend to run it; heating loads empty a battery very quickly.

Usable energy: depth of discharge matters

Batteries are not meant to be drained to zero. Lead-acid and tubular inverter batteries are normally kept above roughly 50% to protect their life, so only half of the nominal energy counts. Lithium iron phosphate batteries can typically be used to 80% or 90%. Check the maker's datasheet, since the permitted depth of discharge varies.

A lithium battery of the same 1,200 Wh nominal capacity used to 80% would give 1,200 × 0.8 × 0.9 ÷ 150 = 5.76 hours, with the same load and inverter. The difference is less about chemistry magic and more about how much of the label you can safely draw.

Losses that shorten the real-world figure

• Inverter efficiency: the conversion from DC to AC wastes energy, often 8 to 15%, and is poorest at very light loads.
• Discharge rate: batteries deliver less total energy when drained quickly, an effect known as the Peukert effect in lead-acid chemistry.
• Age: a battery loses capacity each year, and one at 80% of its original rating gives 80% of the hours.
• Temperature: cold reduces available capacity, and heat shortens lifespan.
• Wiring: thin or long cables waste energy as heat, especially at 12 V, where currents are high.

A cautious planning method is to apply an extra margin of 10 to 20% to the final hours. Compare the estimate against what the battery actually delivers during a power cut and keep a note, since the real behaviour of your own set-up is worth more than any formula.

Working backward: what battery do you need?

Rearrange the formula to size a battery for a target. If you want 6 hours for a 150 W load at 90% inverter efficiency, the usable energy needed is 150 × 6 ÷ 0.9 = 1,000 Wh. With 50% usable, that means a nominal 2,000 Wh battery, or 2,000 ÷ 12 = 167 Ah at 12 V. In practice, you would choose the nearest standard size: two 100 Ah batteries on a 24 V system, for example.

Be careful with systems running at 24 V. Two 12 V 100 Ah batteries in series make 24 V, 100 Ah, which is still 2,400 Wh. Energy adds up; amp-hours do not unless the batteries are in parallel.

Volt-amps versus watts on the inverter label

Inverters and UPS units are usually sold in VA, volt-amperes, which is not the same as watts. Real power is VA multiplied by the power factor, commonly in the range 0.6 to 0.8 for budget units. A 1,000 VA inverter with a power factor of 0.8 can supply 800 W, and no more, so the rating cap sets the maximum load, not the runtime.

Push that inverter to its limit and the hours collapse. With the same lead-acid battery, 600 Wh usable and 90% efficiency, an 800 W load lasts 600 × 0.9 ÷ 800 = 0.675 hours, about 40 minutes. The earlier 150 W load gave 3.6 hours from the same battery, which shows how steeply runtime falls as load rises: a load more than five times larger cuts the time to under a fifth, and a heavy draw usually costs a little extra through discharge-rate losses.

Verifying the estimate in a real power cut

The formula is only a prediction, and a fair check costs nothing. Note the time when mains power fails, along with the battery voltage if your inverter shows it, and note the time it shuts down or the voltage reaches the cut-off. A plug-in energy meter or the inverter's own display can show the real load in watts, which is often higher than guessed because chargers, standby devices and a forgotten second fan are easy to overlook.

If measured hours fall well short of the calculation, work through the usual suspects in order: load higher than listed, battery not fully charged at the start, ageing, and a low depth-of-discharge setting. Each one moves the answer in a predictable direction, and fixing the biggest one first usually gets you most of the missing time back.

Reading the number sensibly

Treat the result as an estimate. Load changes through the night, appliances cycle, and inverters have idle consumption of their own. A runtime calculator gives you the right order of magnitude and shows which appliance to remove when you need another hour. In the example, taking the router and bulbs off leaves 100 W and stretches the 3.6 hours to 5.4.