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Battery Backup Runtime

When the power goes out, the question isn't "do I have a battery?" — it's "how long will the battery last with the stuff I actually need to run?" A Tesla Powerwall (13.5 kWh) powering just a fridge and some LED lights might last 4-5 days. The same Powerwall trying to run central AC might last 8 hours. This calculator takes your usable battery capacity in kWh and your total continuous load in watts and returns runtime in hours, days at the same load, total Wh of storage, and a 10%-reserve version for the more realistic case where you don't want to fully drain the battery.

Hours runtime

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Days at this load
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Wh of storage
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10% reserve hrs
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The math — and what "usable" means

Runtime (hours) = (battery capacity in Wh) ÷ (load in watts). 10 kWh = 10,000 Wh; at 500 watts of continuous load that's 20 hours.

Always use usable capacity, not nameplate. Lithium home batteries (Tesla Powerwall, LG Chem RESU, Enphase IQ) have software-limited depth of discharge to extend battery life. A Powerwall 2 has 13.5 kWh nameplate and 13.5 kWh usable (Tesla rates it edge-to-edge). Many other batteries rate 90-95% usable. Lead-acid batteries should only discharge to 50% to preserve cycle life — a 200 Ah × 12V = 2.4 kWh lead-acid bank has only 1.2 kWh usable.

Inverter efficiency also matters: a typical home battery inverter runs 90-95% efficient. The calculator math assumes 100% efficient conversion; in real life expect 5-10% less runtime than calculated.

Worked example: a 10 kWh usable battery running a 300 W refrigerator (cycles on/off, averages ~150 W), LED lights (50 W), modem/router (15 W), and phone chargers (10 W) = ~225 W continuous. Runtime = 10,000 ÷ 225 = 44 hours ≈ 1.8 days. Plenty of room for a 24-hour outage with reserve.

Sizing essential loads

Typical continuous load (averaged, not peak):

  • Refrigerator: 100-200 W average (cycles on/off)
  • Chest freezer: 50-100 W average
  • LED lights (whole house, modest use): 30-80 W
  • Modem + router + cell booster: 15-30 W
  • Furnace blower (gas heat): 400-700 W when running
  • Sump pump: 300-1,000 W when running
  • Window AC (10K BTU): 800-1,200 W
  • Central AC (3 ton): 3,000-5,000 W — large drain
  • Electric range: 2,000-5,000 W per burner. Skip on battery.
  • Electric water heater: 4,500 W. Skip on battery.
  • Microwave: 1,000-1,500 W when running, short-duration OK
  • EV charging: 1,400 W (Level 1) to 11,500 W (Level 2). Skip on battery.

How to use this calculator

  1. Find usable kWh: check your battery spec sheet. Tesla Powerwall 2 = 13.5 kWh, Powerwall 3 = 13.5 kWh, Enphase IQ Battery 10 = 10.08 kWh, LG Chem RESU10H = 9.3 kWh, EcoFlow Delta Pro = 3.6 kWh.
  2. Sum your essential loads in watts (continuous averages, not peaks).
  3. Output: runtime in hours, days at that load, total Wh of storage, and runtime with 10% reserve buffer.
  4. For variable loads (HVAC, sump pump), use a duty-cycle estimate: if your sump runs 5 minutes every hour, average = 5/60 × peak watts.

Common scenarios

One Powerwall (13.5 kWh), critical loads only. Fridge + freezer + lights + electronics + furnace blower (intermittent) averages around 400 W. Runtime: 33 hours ≈ 1.4 days. Adequate for a typical 24-hour storm outage with margin.

Two Powerwalls (27 kWh), critical loads + central AC. Same base loads (~400 W) plus AC averaging 2,500 W when running = ~2,900 W. Runtime: ~9 hours. Without AC, the same setup runs ~67 hours. Lesson: AC is the biggest battery killer.

Jackery 2000 Pro (2 kWh) for emergency essentials. Fridge (150 W avg) + phones + small lamp + modem = 200 W. Runtime: 10 hours. Enough to bridge a short outage; bring out the gas generator for anything longer.

FAQ

What loads should I prioritize on backup? +
In order: refrigerator/freezer (food safety), well pump or sump pump if applicable (water), heat source (furnace blower on gas heat, or space heaters in freezing weather), CPAP / medical devices, lights, internet/phones, then everything else. Skip electric heat, AC, electric range, electric water heater, and EV charging — they consume more than typical home batteries can sustain.
How is "continuous load" different from "peak"? +
Continuous is what the device draws over time — a refrigerator averages 150 W even though its compressor pulls 600 W when running. Peak (or surge) is the brief draw at startup. Battery inverters have a peak rating (often 2x continuous) for compressor startups. Check both the continuous AND surge ratings of your battery inverter.
Does charging the battery during the outage extend runtime? +
If you have solar paired with the battery, yes — sunny days can recharge the battery while it powers loads, effectively giving you indefinite runtime as long as solar matches average load. Most modern systems (Powerwall + solar, Enphase IQ + IQ8) support this "island mode." Solar without batteries shuts off during grid outages for safety; you need the battery for grid-form capability.
How do I find my actual continuous load? +
Two ways: (1) sum up known device wattages from spec labels and use rough duty cycles; (2) install a whole-home energy monitor (Sense, Emporia Vue, Span Panel) that records circuit-level real-time draw. The energy monitor is more accurate and shows you exactly which loads are the biggest drain.
What's the lifespan of a home battery? +
Lithium-iron-phosphate batteries (most current home batteries): 10-15 years or 5,000-10,000 cycles. NMC lithium (older Powerwalls, some EV-derived batteries): 10 years or 3,000-5,000 cycles. Lead-acid: 5-10 years and 500-1,500 cycles. Cycle life depends heavily on how deeply you discharge — shallow cycles (10-30% DoD) last much longer than deep cycles.
Can I add more batteries later? +
Most modern systems support stacking. Powerwall: up to 10 units in parallel. Enphase IQ: up to 4 batteries per Encharge system. EcoFlow Delta Pro: expansion batteries available. Check your specific system's stacking limits. Note that adding batteries doesn't increase peak power output — just runtime.
What's the typical cost per kWh of storage? +
Installed cost in 2025: $800-1,200 per kWh for whole-home battery systems with installation. Portable power stations: $300-600 per kWh including the unit. Battery prices have dropped 80% over the last decade and continue to fall.
How does battery compare to a backup generator? +
Generators: cheaper upfront ($2-8K for a whole-home gas/propane), unlimited runtime as long as fuel lasts, but noisy, require fuel storage, exhaust outside, and need annual maintenance. Batteries: silent, instant transfer, zero maintenance, work indoors, but limited runtime and 3-5x the upfront cost. Most resilient: battery for short outages + generator for week-long events.