The Question Every Battery Buyer Asks
How long a home battery system lasts during a power outage is the most practical question you can ask — and the answer is more nuanced than any single number. Runtime depends on your battery\’s usable capacity, the power draw of your home during the outage, and whether solar panels are recharging the battery while it is discharging. This guide walks through the math so you can calculate a realistic estimate for your specific situation.
The Basic Formula
Battery runtime comes down to one simple relationship:
Runtime (hours) = Usable Capacity (kWh) ÷ Average Load (kW)
A 13.5 kWh battery (Tesla Powerwall 3) powering a home drawing an average of 1.5 kW will last approximately 9 hours. The same battery powering a home drawing 3 kW lasts about 4.5 hours. Your load — what you choose to run during the outage — is the variable you control.
Typical Home Loads During an Outage
Most homeowners do not run everything in the house during a battery outage — they prioritize essentials. Here are realistic load scenarios:
Minimal Essential Load — 500 to 800 watts average
Refrigerator running on cycle, LED lighting in key rooms, phone and laptop charging, and a CPAP machine. This is the minimum comfort level most households target.
- 10 kWh battery: 12–20 hours
- 13.5 kWh battery: 17–27 hours
- 20 kWh battery: 25–40 hours
Moderate Load — 1,500 to 2,500 watts average
Refrigerator, lighting, electronics, and a window AC unit or electric space heater running intermittently. This covers most suburban households running essential comfort loads.
- 10 kWh battery: 4–7 hours
- 13.5 kWh battery: 5–9 hours
- 20 kWh battery: 8–13 hours
Heavy Load — 3,500 to 5,000 watts average
Central air conditioning running, multiple major appliances, EV charging, or well pump cycling frequently. Heavy loads drain battery systems quickly.
- 10 kWh battery: 2–3 hours
- 13.5 kWh battery: 2.5–4 hours
- 20 kWh battery: 4–6 hours
How Solar Changes Everything
A battery without solar recharging is a finite resource — once it is depleted, it is depleted until grid power returns. A battery paired with solar panels is a fundamentally different proposition during a daytime outage.
On a sunny day, a 6 kW solar array produces approximately 4 to 5 kW of power during peak production hours. If your home load during the outage is 1.5 kW, the solar system is producing 3 to 3.5 kW of surplus that goes directly into recharging the battery. A home that drains its 13.5 kWh battery overnight can fully recharge it by mid-morning on a sunny day and have power indefinitely — as long as the sun keeps shining.
This is why solar plus battery storage is so much more capable than battery alone for extended outages. The battery covers the night and cloudy periods; solar covers the day and keeps the battery charged.
Appliance Power Draw Reference
Use these figures to estimate your outage load:
- Refrigerator (full size): 100–200W average (cycles on and off)
- Chest freezer: 80–150W average
- LED lighting (10 bulbs): 60–100W
- Phone charging: 5–20W per phone
- Laptop: 30–80W
- CPAP machine: 30–60W
- Television (50\”): 80–120W
- Window AC (10,000 BTU): 900–1,200W
- Central AC (2.5 ton): 2,500–4,000W
- Electric space heater: 750–1,500W
- Well pump (1/2 hp): 750–1,000W average
- Sump pump (1/3 hp): 300–500W average
- EV charger (Level 2): 3,800–7,200W
How Many Batteries Do You Actually Need?
For a home that wants to run essential loads (refrigerator, lights, phone charging, CPAP) through a typical overnight outage of 8 to 12 hours, a single 13.5 kWh battery is usually sufficient if loads are managed carefully — avoiding AC, space heaters, and EV charging.
For homeowners who want to run AC during summer outages, power a well pump, or have medical equipment with significant draw, two batteries providing 27 kWh of capacity is a more comfortable target. At that capacity, most homes can run moderate loads for 24 hours without solar recharging, and indefinitely with a typical solar array producing on a sunny day.
For whole-home backup with no load management during extended outages, three to four batteries or a larger whole-home battery system is the right architecture.
Battery Capacity Degradation Over Time
Battery capacity declines gradually over years of cycling. Most residential battery warranties guarantee 70 to 80 percent capacity retention at the end of the warranty period — typically 10 years. A battery that holds 13.5 kWh today will hold approximately 9.5 to 10.8 kWh at year 10, reducing runtime proportionally. This is worth factoring into long-term planning, particularly for homeowners counting on specific runtime durations.
Lithium iron phosphate (LFP) chemistry — used in Tesla Powerwall, Franklin aPower, and many other residential batteries — degrades more slowly than older lithium-ion chemistries and handles deep cycling better, which is why it has become the dominant chemistry for residential storage.
Bottom Line
A single 13.5 kWh home battery running essential loads of 500 to 800 watts will last 17 to 27 hours during an outage — enough to cover most overnight events. Running moderate loads including a window AC unit brings that to 5 to 9 hours. Pairing the battery with solar panels transforms a finite backup device into a system that can sustain your home indefinitely through extended outages as long as daylight is available. Size your battery to the loads you actually plan to run, not to power everything in your house.
