Battery Backup Calculator

Think of a battery as a water tank. Capacity in amp-hours tells you the tank volume; voltage tells you the water pressure pushing it through the pipes. The product of the two — watt-hours — is the actual energy available, the same way tank volume times pressure gives you the usable water force. Your devices drain that tank at a fixed rate determined by their watt draw, and the time until empty is simply volume divided by flow rate.

The efficiency factor is where most people lose track of their estimates. Between the battery's DC output and your household devices, an inverter performs a conversion that is never perfect. Every inverter turns some electrical energy into heat. A high-quality pure sine wave unit wastes less; a budget modified sine wave unit wastes more. Skipping this factor entirely — treating efficiency as 100% — consistently overestimates runtime by anywhere from a few minutes on small systems to over an hour on large ones.

The calculation uses continuous steady-state power, not peak demand. Many devices draw a surge of current at startup — a refrigerator compressor, a power tool motor, or an air conditioner — that can briefly be several times the running wattage. That surge lasts only seconds and does not dominate the long-run energy math, but it does determine whether your inverter can handle the load at all. Size your inverter for peak surge; size your battery for continuous runtime.

This calculator fits any situation where you know your battery's rated specifications and your devices' power draw: sizing a UPS for a network rack, checking whether a portable power station will run your camping appliances through the night, or validating that a solar storage bank can carry a cabin load without recharging. It is particularly useful when comparing battery options — holding load and efficiency constant while changing capacity and voltage quickly shows which configuration gives you the runtime you need.

The tool is less appropriate for lead-acid battery systems that will be cycled regularly, because it does not account for depth-of-discharge limits or capacity degradation over time. A battery bank that delivers this runtime when new may deliver significantly less after a year of regular cycling. For long-term planning with lead-acid chemistry, apply a 50% depth-of-discharge adjustment by halving your capacity input before calculating.

Do not use this calculator for safety-critical load sizing without verifying results against manufacturer data sheets and an electrician's assessment. The calculation treats battery capacity as fixed and temperature as neutral — neither assumption holds in cold environments or with older batteries. When a medical device or life-safety system depends on the runtime estimate, treat this result as a starting point for a more rigorous analysis, not a final answer.

Using the nameplate wattage instead of measured draw. Device labels show maximum rated power, not typical operating power. A 500W-rated device running at partial load may draw far less continuously. Plug-in power meters cost very little and remove the guesswork entirely. Overestimating the load shrinks your calculated runtime — the real system outlasts the estimate, which is a safe error. Underestimating it inflates the estimate — the real system fails before predicted, which is not safe.

Ignoring depth of discharge limits. Most battery chemistries should not be fully discharged. Lead-acid batteries typically last longest when discharged to no more than 50% of rated capacity; lithium iron phosphate allows deeper discharge but still carries its own threshold. This calculator discharges the battery completely by design, so the real usable runtime for any battery you plan to maintain long-term is shorter than the result shown here. Apply your battery manufacturer's recommended depth of discharge as a manual adjustment to your capacity input.

Treating efficiency as a minor rounding issue. On a large system, the difference between 80% and 95% efficiency is not trivial. For the same battery and load, the higher efficiency system delivers noticeably more runtime — the gap compounds with battery size. Treating efficiency as approximately 100% because it is close to a round number consistently produces optimistic estimates that fail when you actually need the runtime.

The core formula converts battery specifications into runtime using two steps. First, raw energy in watt-hours equals battery capacity multiplied by battery voltage: for the example inputs of 100Ah and 12V, raw energy is 1,200 Wh total battery energy. Second, usable energy accounts for inverter losses by multiplying raw energy by the efficiency decimal: at 85%, that gives 1,020 Wh usable reaching your devices, with 180 Wh lost to inefficiency lost to heat.

Runtime in hours is usable energy divided by continuous load: 1,020 Wh usable divided by 500W yields 2h 2m for the example. The calculation is a direct application of the definition of a watt — one joule per second — expressed in the more practical unit of watt-hours. One watt-hour sustains exactly one watt for exactly one hour, so the division is unambiguous.

Efficiency enters as a multiplier on the numerator rather than a divisor on the denominator, but both forms are mathematically identical. Writing it as a numerator multiplier makes the energy bookkeeping visible: you can see exactly how many watt-hours are lost to the inverter versus how many reach the load. For the example, the efficiency loss of 180 Wh lost to inefficiency is the direct cost of converting DC to AC — not a rounding artifact.

The formula assumes efficiency is constant across the full discharge curve, which no real inverter achieves. Most inverters hit peak efficiency at 50-80% of rated load; both light and heavy loads push efficiency lower. A system sized for 500W but running 50W operates well below peak efficiency, meaning the runtime formula overestimates delivered energy. For precise analysis, use efficiency curves from the inverter datasheet at the actual operating load point rather than a single rated figure.

Battery internal resistance rises as state of charge drops, which increases heat dissipation in the battery itself — a loss the formula does not capture. At low state of charge, terminal voltage sags under load, which can cause inverters to shut down for under-voltage protection before the nameplate capacity is exhausted. The practical cutoff voltage of the system, not zero state of charge, is the real end-of-backup condition in most installations.