Solar Battery Backup Runtime Calculator: Hours by Load & Bank Size
Reading time: ~9 min read
📋 Key Takeaways
- Runtime = usable kWh ÷ daily load kWh. Everything else — inverter losses, depth of discharge, duty cycles — just refines those two numbers.
- "Usable" is not nameplate: lithium (LiFePO4) banks deliver ~90–100% of rated kWh; lead-acid should only be drawn to 50%. A "10 kWh" AGM bank is a 5 kWh bank.
- A typical critical-loads panel (fridge, lights, internet, furnace blower, chargers) draws 3–6 kWh/day — one 10 kWh lithium battery runs it roughly 1.5–3 days without sun.
- Inverter efficiency costs 5–10% on every watt-hour that passes through it, and idle draw (20–60W) burns 0.5–1.4 kWh/day even with everything off.
- The furnace blower and well pump are the runtime killers in most homes — not because of watts, but because of hours. Duty cycle is the number people guess wrong.
Every "how long will my battery last" question is division: usable stored energy divided by how fast the house drinks it. The calculator below builds the daily consumption side honestly — a critical-loads checklist where each item carries watts and hours per day, because a 600W furnace blower running 6 hours eats more than a 1,500W microwave used for 10 minutes. Enter your usable bank capacity in kWh, and it returns runtime in hours and days, with inverter losses already applied. Two presets cover the classic outage scenarios: essentials-only versus a heavier whole-home-lite profile.
Sizing the storage side of a project? Our batteries & storage collection and inverters collection cover LiFePO4 banks, hybrid inverters, and the balance-of-system gear to tie them together.
Battery Backup Runtime Calculator
Usable, not nameplate: LiFePO4 ≈ 95% of rating, lead-acid ≈ 50%.
Applied to all AC energy drawn from the bank.
Typical 20–60 W; burns this 24/7 during the outage.
Daily consumption: —
Runtime: —
Runtime = usable kWh ÷ (load Wh/day ÷ efficiency + idle Wh/day). Assumes no solar recharge during the outage — if the array produces, add daily PV harvest to the bank side. Watt and hour defaults are planning averages; nameplate ratings and measured duty cycles beat defaults every time.
Why "Usable kWh" Is the Only Number That Matters
Battery marketing quotes nameplate energy; outages consume usable energy. The gap between them is where runtime estimates die. A 10 kWh lithium iron phosphate (LiFePO4) wall battery typically allows 90–100% depth of discharge, so it behaves like a 9.5–10 kWh source. A 10 kWh lead-acid bank — AGM or flooded — should be drawn only to 50% if you want it to survive more than a few hundred cycles, making it a 5 kWh source for planning purposes. Two banks with identical labels differ by 2× in real autonomy. When you spec storage for a customer, quote usable kWh and name the chemistry's DoD assumption in the proposal; it prevents the "you said it would last two days" phone call.
Temperature is the second discount. Lithium capacity and charge acceptance sag below freezing — many BMS units block charging entirely under 32°F (0°C), which matters for batteries installed in garages and outbuildings during the exact winter storms that cause outages. Garaged lithium needs either a heated enclosure or a chemistry conversation.
The Loads That Actually Decide Runtime
After hundreds of these conversations, the pattern is consistent. The refrigerator is the anchor load — 100–200W cycling to roughly 1–1.5 kWh/day — and it's non-negotiable, so it's in every profile. Lighting and internet are nearly free on modern LED and fiber gear. The runtime swing comes from three loads:
- Furnace blower: a PSC blower motor draws 600–900W; an ECM variable-speed draws 200–400W. Six hours of PSC blower is 4+ kWh — more than the fridge, lights, and internet combined. ECM retrofits are a runtime upgrade as much as an efficiency upgrade.
- Well pump: 1–1.5 hp means 1,500–2,500W when running. It runs little (30–60 min/day in most homes), but it also sets your inverter's surge requirement — LRA on a pump motor is 3–5× running current, and that's an inverter spec problem, not an energy problem.
- Anything with a heating element: space heaters (1,500W continuous = 36 kWh/day), electric water heaters, and ranges are simply not battery loads at residential bank sizes. Generators exist for a reason — see our generators collection for the hybrid backup approach.
Inverter Efficiency and Idle Draw: the Silent Tax
Two losses ride along with every watt-hour. Conversion efficiency — 90–97% depending on the inverter and its loading — shaves the AC output side; a lightly loaded inverter runs at the bottom of its efficiency curve, which is why oversizing the inverter "for headroom" can cost runtime. Idle draw is worse: the inverter burns 20–60W just being awake, 24 hours a day, whether anything is plugged in or not. On a 10 kWh bank that's 0.5–1.4 kWh/day — up to 14% of the bank gone before the fridge cycles once. Features like load-sensing sleep modes and search-mode thresholds exist specifically to claw this back; on small banks they're worth real hours.
The calculator applies both: loads are grossed up by the efficiency factor and idle draw is added as a 24-hour load. If your measured runtime beats the estimate, the inverter's sleep mode is doing its job.
Adding Solar: Runtime Becomes Recharge Rate
The moment PV enters the picture, the question changes from "how long does the bank last" to "does the array refill the bank faster than the house drains it." A 3 kW array on a decent winter day harvests 9–12 kWh; the essentials preset draws roughly 2.5–3.5 kWh/day. That's net positive — the battery only bridges night and storms. This is why modern hybrid systems spec the array for the load profile and the battery for overnight autonomy, not the other way around. If the project includes panels, our solar panels collection and inverters collection cover both sides of that equation.
A Note on Measuring Instead of Guessing
The defaults in the calculator are planning averages, and houses routinely surprise you. A $30 plug-in energy monitor on the refrigerator for 48 hours settles its real kWh/day better than any table; smart-panel data or a clamp meter on the furnace circuit does the same for the blower. If this is a paying customer's proposal, an evening of measurement is the cheapest accuracy you can buy — and it frequently reveals that the "essential" chest freezer from 1998 draws more than the new refrigerator it sits next to.
Frequently Asked Questions
How long will a 10 kWh battery run a house during an outage?
It depends entirely on the load profile. Running critical loads only (refrigerator, LED lights, internet, chargers — roughly 2.5–3.5 kWh/day), a 10 kWh lithium battery with ~95% usable capacity lasts about 2.5–3.5 days. Add a furnace blower and TV (5–7 kWh/day) and it drops to roughly 1.5 days. Attempt whole-home loads with electric heat or cooking and you're measuring in hours, not days.
How many batteries do I need to run a refrigerator?
A modern refrigerator consumes 1–1.5 kWh per day. A single 5 kWh LiFePO4 battery (about 4.75 kWh usable) runs a fridge alone for roughly 3 days, longer with solar recharge. In practice you never size for one appliance — build the critical-loads list first, because the fridge plus lights, internet, and a furnace blower triples the daily draw even though each load sounds small.
What is the difference between nameplate and usable battery capacity?
Nameplate capacity is the total stored energy; usable capacity is what you're allowed to take. LiFePO4 batteries typically deliver 90–100% of nameplate; lead-acid (AGM/flooded) should be limited to 50% depth of discharge for reasonable cycle life. A 10 kWh AGM bank therefore plans like a 5 kWh bank. Cold temperatures discount lithium further — many BMS units block charging below freezing.
Does inverter efficiency really affect backup runtime?
Yes, twice over. Conversion efficiency (90–97%) shaves every AC watt-hour passing through the inverter — at 90% a 3 kWh/day load actually pulls 3.3 kWh from the bank. Separately, idle draw of 20–60W consumes 0.5–1.4 kWh/day just keeping the inverter awake. On small banks, idle draw plus low-load inefficiency can eat 15–25% of total autonomy. Load-sensing sleep modes recover much of it.
Can solar panels recharge the battery during an outage?
Yes, if the system is designed for it — a hybrid inverter or AC-coupled setup that can island and form a microgrid. Grid-tied-only inverters shut down in an outage (anti-islanding) regardless of sunshine. With an islanding-capable system, even a modest 2–4 kW array usually harvests more daily energy than an essentials load profile consumes, making runtime effectively unlimited in decent weather and the battery purely a night bridge.
Sizing a Battery Backup System?
PES Supply stocks LiFePO4 battery banks, hybrid inverters, and critical-loads panels from 169 authorized brands. Send us the load list and autonomy target — we'll quote the complete storage package.
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