Battery Backup Runtime Calculator

How long will my battery last? Understanding runtime

Battery runtime is how long your stored energy can power a given load before it's depleted to its safe limit. It sounds like simple division — capacity divided by load — but two real-world factors mean the honest answer is always shorter than the naive one: you can't use all the battery's rated capacity, and the inverter wastes some energy converting DC to AC. Account for both and you get a runtime you can actually rely on during an outage, rather than an optimistic figure that leaves you in the dark early.

Runtime is the question behind almost every backup decision: "if the power goes out, how long can I keep the fridge, the lights and my phone alive?" It’s also the number most often quoted too optimistically, because it’s tempting to just divide the battery’s headline capacity by the load and call it done. Real batteries don’t work that way — you protect their lifespan by not fully draining them, the inverter skims off a slice converting DC to AC, and cold weather and heavy loads shave off more. The result is that honest runtime is typically 20–40% below the back-of-envelope figure. Knowing the realistic number in advance is what separates a backup plan that carries you through an outage from one that leaves you scrambling when the battery quits hours earlier than expected.

The runtime formula

For example, a 100 Ah 12V battery holds 1,200 Wh nominal. On LiFePO4 at 90% DoD and 90% inverter efficiency, that's about 972 Wh usable. Running a 100 W load, you'd get roughly 9.7 hours — not the 12 hours the raw 1,200 Wh ÷ 100 W would suggest. The gap between those two numbers is exactly the usable-capacity and conversion losses people forget, and it's why backup plans built on nameplate capacity come up short.

Why chemistry changes the answer

The same nameplate capacity gives very different real runtime depending on chemistry. A 100 Ah lead-acid battery should only be drawn to ~50%, giving ~600 Wh usable, while a 100 Ah LiFePO4 gives ~900–950 Wh — over 50% more usable energy from the "same" size. Lead-acid also suffers more at high discharge rates (its effective capacity shrinks when you pull hard), so for heavy loads the lithium advantage is even larger than the DoD difference alone suggests.

Sizing for backup vs everyday cycling

If you're sizing for outage backup, work from the loads you truly need to keep running — fridge, a few lights, internet, phone charging — not your whole home, and decide how many hours you must cover. If you're cycling daily (an off-grid or solar-shifting setup), runtime per cycle matters less than cycle life and depth of discharge, because you'll refill the battery each day. The same battery is "big" for backup and "small" for daily off-grid use, so be clear which job you're sizing for.

How discharge rate quietly shortens runtime

There's a subtler factor that catches people out: the rate at which you draw power affects how much energy a battery actually delivers. Pull gently and you get close to the rated capacity; pull hard and you get noticeably less. Lead-acid suffers from this badly — running a heavy load can cut effective capacity by a third or more through what's known as the Peukert effect — while lithium chemistries are far more tolerant and hold their capacity better under heavy draw. This is why two batteries with identical nameplate ratings can give very different real runtimes powering the same big appliance, and why high-surge loads like pumps and air-conditioners are especially demanding. When you plan backup for heavy loads, lean toward lithium and don't assume the rated capacity will all be available at high current. For light loads (lights, routers, phone charging) the effect is small and the simple formula above is accurate; for heavy loads, treat the estimate as optimistic and add margin.

Frequently asked questions

Take the usable energy — roughly 900 Wh for LiFePO4 or 600 Wh for lead-acid on a 12V 100 Ah battery after DoD and inverter losses — and divide by the appliance's wattage. A 60 W load runs about 15 hours on LiFePO4; a 600 W load only about 1.5 hours.

Because nameplate capacity isn't all usable (depth of discharge), the inverter loses some energy converting to AC, and cold or high-current loads reduce effective capacity. The formula above includes these so the estimate matches reality.

The inverter's own idle draw matters for small loads, but mainly runtime depends on the load you actually run, not the inverter's rating. A large inverter running a small load wastes a little in standby; the bigger effect is simply how many watts your appliances draw.

Reduce the load (efficient appliances, switch off non-essentials), use a higher depth-of-discharge chemistry like LiFePO4, keep the battery from getting cold, and size the bank to your real needs with some margin. For backups, prioritise only essential loads.

Add up the wattage of everything running at once to get the total load, then divide your usable energy by that total. Remember that appliances cycling on and off (like fridges) only draw power part of the time, so their average draw over an hour is lower than their running wattage — which extends real runtime beyond the worst-case estimate.