Battery Backup Runtime Calculator: How Long Will Your Battery Last?
"How long will this battery actually run my house?" is the first question every energy-storage customer asks — and the honest answer is always the same formula, not a marketing number. This guide gives you the runtime formula, a ready-made chart of battery size versus load, the depth-of-discharge rules that separate lithium from lead-acid, and worked examples you can rerun with your own numbers.
The runtime formula
Runtime (hours) = Battery capacity (kWh) × Depth of discharge × Inverter efficiency ÷ Load (kW)
Three multipliers do all the work:
- Depth of discharge (DoD): LiFePO4 batteries safely deliver 90–100% of nameplate capacity. AGM and flooded lead-acid should only be drawn to 50% if you want them to live past a few hundred cycles. A "10 kWh" lead-acid bank is really a 5 kWh bank.
- Inverter efficiency: quality hybrid inverters run 92–97% efficient; use 0.95 for planning.
- Load: the *average* draw in kilowatts — not the nameplate surge. A refrigerator's compressor cycles; its duty average is typically 100–200W.
Battery backup runtime chart (LiFePO4, 95% DoD, 95% inverter efficiency)
Usable energy assumed at 90% of nameplate after efficiency losses:
| Battery size | 200W load | 500W load | 1,000W load | 2,000W load | 3,000W load |
|---|---|---|---|---|---|
| 2.4 kWh (one US2000-class module) | 10.8 hrs | 4.3 hrs | 2.2 hrs | 1.1 hrs | 0.7 hrs |
| 5 kWh | 22.5 hrs | 9.0 hrs | 4.5 hrs | 2.3 hrs | 1.5 hrs |
| 10 kWh | 45 hrs | 18 hrs | 9 hrs | 4.5 hrs | 3 hrs |
| 13.5 kWh (Powerwall-class) | 61 hrs | 24 hrs | 12 hrs | 6 hrs | 4 hrs |
| 14.3 kWh (280Ah wall-mount) | 64 hrs | 26 hrs | 13 hrs | 6.4 hrs | 4.3 hrs |
| 20 kWh | 90 hrs | 36 hrs | 18 hrs | 9 hrs | 6 hrs |
| 30 kWh | 135 hrs | 54 hrs | 27 hrs | 13.5 hrs | 9 hrs |
For AGM/lead-acid, cut every number in half (50% DoD). For quick mental math: at a 500W load — enough for a fridge, lights, internet, and device charging — you get roughly 2 hours of runtime per usable kWh.
What does a typical load actually look like?
| Load | Average watts | Notes |
|---|---|---|
| Refrigerator (modern, Energy Star) | 100–200 | Cycles; surge 600–1,200W at start |
| Chest freezer | 80–150 | Cycles |
| LED lighting, whole house | 100–400 | Depends how many rooms |
| Internet router + modem | 15–30 | The load nobody remembers |
| Furnace blower (1/2 HP) | 500–800 | Gas heat still needs electricity |
| Well pump (1 HP) | 1,000 avg when running | Surge 2,500–3,000W — check inverter surge rating |
| Window AC (8,000 BTU) | 600–900 | Central AC usually exceeds battery inverter output |
| Electric water heater | 4,500 | Generally not battery-backup material |
Worked example: overnight essentials on a 14.3 kWh battery
Essentials: refrigerator (150W avg), furnace blower (600W, 8 cold hours), lights and internet (200W), device charging (50W). Average over the night: the blower runs maybe 50% duty, so call it 150 + 300 + 200 + 50 = 700W average. Runtime = 14.3 × 0.95 × 0.95 ÷ 0.70 = 18.4 hours. One wall-mount battery carries a typical home's essentials through a full night with margin — exactly why the 14–15 kWh class is the residential sweet spot.
Worked example: how long to charge a 100Ah battery with a 200W solar panel
A 12V 100Ah battery holds 1.28 kWh. A 200W panel in good sun delivers roughly 160W after losses: 1,280Wh × 0.5 (AGM usable) ÷ 160W ≈ 4 peak-sun hours to recharge the usable half — about one good solar day. For LiFePO4 (90% usable): 1,280 × 0.9 ÷ 160 ≈ 7.2 peak-sun hours, or roughly 1.5 average days. Double the panels, halve the time.
Battery backup vs generator — the honest comparison
Batteries win on silence, instant switchover, indoor safety, and pairing with solar for indefinite off-grid runtime. Generators win on cost per kWh of long-duration outage and on brute-force loads like central AC. The setups we sell most are hybrids: battery for the first hours and overnight, generator for multi-day events. See our generator fuel consumption chart for the other half of that math.
Stacking and expanding: plan for the second battery now
Almost nobody's storage needs shrink over time — the EV arrives, the home office becomes permanent, the heat pump replaces the furnace. Three decisions made today decide how painful expansion is later:
- Buy a stackable platform. Modular systems (wall-mount 48V batteries in parallel, stackable rack modules) let you add 5–15 kWh at a time with a communication cable, not a redesign. Proprietary closed systems can strand you at the original capacity.
- Oversize the inverter, not just the battery. Runtime scales with kWh, but *what you can run* scales with inverter watts. A 12kW hybrid inverter with 10 kWh of battery beats a 6kW inverter with 20 kWh for most homes — you can always add modules; swapping an inverter means rewiring.
- Match battery chemistry and firmware when mixing. Adding new LiFePO4 modules to an old bank works fine on platforms designed for it; mixing brands or chemistries on one bus does not. Keep the bank on one manufacturer's ecosystem and keep firmware current.
A final sizing sanity check from hundreds of these conversations: customers who size for "essentials only" call back within two years to double capacity. If the budget is close, buy the inverter headroom now and the second battery next quarter — that's the cheapest path to where you'll end up anyway.
Frequently asked questions
How long will a 10 kWh battery run a house?
At a disciplined 500W essentials load, about 18 hours. At a realistic whole-home average of 1,500–2,000W without load management, 4.5–6 hours. Central AC changes everything — usually outside battery range.
Is LiFePO4 really twice the usable capacity of AGM?
Yes for practical purposes: 90–100% usable DoD versus 50%, plus 3,000–6,000 cycles versus 300–500. Cost per *usable* kWh-cycle is several times lower despite the higher sticker.
What size battery do I need for 24 hours of backup?
Tally your average load in watts, multiply by 24, divide by 0.9. A 700W essentials load needs roughly 18–19 kWh of LiFePO4 nameplate capacity.
Can a battery start my well pump or AC?
Only if the inverter's surge rating covers the motor's starting watts (often 2–3× running). Check the inverter spec, not just the battery kWh.
Does cold weather cut runtime?
Yes — lithium capacity and charge acceptance drop below freezing. Batteries with built-in heaters (or indoor/insulated installs) preserve winter runtime.
Can I mix battery brands on one system?
Not on the same DC bus. Expansion works within one manufacturer's ecosystem — same chemistry, same battery management system, current firmware. Mixing brands creates charge-profile conflicts that shorten the life of both banks.
How accurate is the runtime chart for a refrigerator?
Within about 20% in the field — compressor duty cycle swings with room temperature, door openings, and defrost cycles. When the number matters (medical devices, sump pumps), measure your actual load with a plug-in watt meter for 48 hours before sizing.
Build your backup
Portlandia Electric Supply stocks the batteries this chart is built on: the EG4 PowerPro 14.3kWh wall-mount battery, modular Pylontech US5000 4.8kWh 48V modules, and Victron LiFePO4 smart batteries. Browse the full Battery Storage collection and Energy Storage Systems. Sizing a battery bank for solar? Our battery bank sizing guide and solar system size calculator complete the picture — or just call the counter.