How long will a 13.5 kWh Powerwall last?

At a 1,000 W continuous load (fridge + lights + small electronics) a 13.5 kWh Powerwall lasts about 12.4 hours accounting for 92% inverter efficiency and 100% usable depth of discharge. At 2,000 W (adding some cooking or a well pump) it lasts about 6.2 hours. At 5,000 W (near the 5 kW continuous max) it lasts only 2.5 hours. Runtime always equals usable energy divided by continuous load.

What is depth of discharge?

Depth of discharge (DoD) is the percentage of a battery's nameplate capacity that can be safely used. LiFePO4 (LFP) batteries like the EG4 and Fortress Power lines allow 100% DoD. Tesla Powerwall allows 100% in backup mode. Older Li-NMC batteries are limited to 80-90% to preserve cycle life. Lead-acid batteries should only be discharged to 50% — a 10 kWh lead-acid bank only gives 5 kWh of usable energy.

Why does inverter efficiency matter?

The inverter converts DC battery power to AC household power, losing 5-15% as heat during conversion. A modern hybrid inverter (SolarEdge, Enphase IQ, Sol-Ark, Fortress) runs at 92-96% round-trip efficiency. Older string inverters with batteries added on run 85-90%. Older pure off-grid inverters may drop to 80%. Lower efficiency means less usable runtime — a 10 kWh battery at 90% efficiency delivers only 9 kWh of AC power to your loads.

What's the average home load during a power outage?

With essentials only (fridge, lights, Wi-Fi, phone charger) most homes draw 300-800 W continuous, averaging around 500 W. Add a furnace blower or window AC and you are at 1,000-1,500 W. Running a well pump intermittently adds 1,200 W during its ~1 minute cycles. A typical outage load averages 750-1,500 W depending on what you choose to keep running, making a 10 kWh battery last 6-13 hours.

How many Powerwalls do I need for whole-house backup?

One Powerwall 3 (13.5 kWh, 11.5 kW continuous, 15.4 kW peak) covers most 1,500-2,500 sq ft homes with essentials for 8-16 hours. Add a second for a full day of autonomy or to run central AC/heat pump continuously. Three Powerwalls give 40+ kWh and easily cover 1-2 days of typical consumption during a storm. Whole-house off-grid typically needs 30-60 kWh of storage sized to handle 2-3 cloudy days.

Battery Backup Runtime Calculator - How Long Will It

Battery Backup Runtime Calculator

How long will your home battery last? Enter the nameplate kWh, your continuous load, and depth of discharge + inverter efficiency to get runtime in hours and minutes.

Battery capacity (kWh)

Tesla Powerwall 3 = 13.5 kWh. EcoFlow Delta Pro = 3.6 kWh. EG4 PowerPro = 14.3 kWh.

Continuous load (W)

Essentials only: 300-800 W. With fridge and furnace blower: 1,000-1,500 W.

Depth of discharge (%)

LiFePO4: 100%. Tesla Powerwall: 100% backup. Lead-acid: 50%.

Inverter efficiency (%)

Modern hybrid: 92-96%. Older string + battery: 85-90%.

Results

Usable energy —

Runtime (hours) —

Time —

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Battery Runtime Formula

Runtime (hours) = battery kWh × 1000 × depth of discharge × inverter efficiency ÷ load watts. A 13.5 kWh Tesla Powerwall at 1,000 W load, 100% usable DoD, 92% inverter efficiency gives 13.5 × 1000 × 1.0 × 0.92 / 1000 = 12.42 hours. The key insight: nameplate kWh is not what you can use. Real-world runtime depends on usable fraction and inverter losses.

Typical Battery Capacities

Battery	Nameplate kWh	Usable DoD	Max continuous W
Tesla Powerwall 3	13.5	100%	11,500 W
Tesla Powerwall 2	13.5	100%	5,000 W
Enphase IQ Battery 10	10.5	96%	3,840 W
LG Chem RESU 10H	9.8	95%	5,000 W
EG4 PowerPro WallMount	14.3	100%	~6,000 W
EcoFlow Delta Pro	3.6	100%	3,600 W
Typical lead-acid bank (24 V 400 Ah)	9.6	50%	~3,000 W (inverter limit)

Typical Outage Loads

Essentials only (fridge cycling, LED lights, Wi-Fi, phone chargers): 300-500 W average. A 13.5 kWh battery lasts 25-40 hours.
Comfort (adds furnace blower, TV, computer): 800-1,200 W. Battery lasts 10-16 hours.
AC on low (window unit or 1-ton mini-split): 1,500-2,500 W. Battery lasts 5-9 hours.
Everything running (central AC or heat pump + essentials): 3,000-5,000 W. Battery lasts 2.5-4.5 hours.
Whole-house with dryer or EV charging: 6,000+ W. Battery lasts under 2 hours and hits the continuous inverter limit on many models.

Frequently Asked Questions

How do I calculate my home's load?

Add up the running watts of everything you want to run. Use the Generator Sizing Calculator for a quick appliance-by-appliance breakdown.

Does runtime change with load?

Yes — lower load = longer runtime, proportionally. Halving the load doubles the hours.

Should I add multiple Powerwalls?

Yes for whole-house backup longer than ~10 hours or for larger loads. Each Powerwall adds 13.5 kWh and 11.5 kW continuous.

What is round-trip efficiency?

Energy out divided by energy in over a full charge-discharge cycle. LFP batteries + modern inverters are 85-90% round-trip. This calculator shows one-way discharge efficiency only.

How does temperature affect runtime?

Cold temperatures reduce both capacity and max continuous output. LFP batteries lose 20-30% capacity below 0 °C. Install batteries in a conditioned space when possible.

Disclaimer: This calculator is for informational and educational purposes only. Results are estimates and should not be considered professional engineering or construction advice. Consult a qualified professional before making decisions based on these calculations. See our full Disclaimer.

Battery Backup Runtime Calculator

Results

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Battery Runtime Formula

Typical Battery Capacities

Typical Outage Loads

Frequently Asked Questions

How do I calculate my home's load?

Does runtime change with load?

Should I add multiple Powerwalls?

What is round-trip efficiency?

How does temperature affect runtime?

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