A blackout in the middle of a heat wave used to mean sitting in the dark until the utility crews showed up. Today one wall-mounted box can carry your fridge, internet, and a few lights through an outage and recharge itself the moment power returns. Here is what the hardware actually does, what it can and cannot run, and where the real limits sit.
What A Home Backup Battery Stores And Delivers
A home backup battery is a rechargeable energy-storage system that saves electricity for later use, and its capacity is measured in kilowatt-hours. The Tesla Powerwall 3, the unit most US homeowners compare against, carries 13.5 kWh of energy capacity. That is roughly enough to run a typical refrigerator, Wi-Fi router, and a handful of LED lights for well over a day, or an entire modest home for several hours.
Storage is half the story. Output is the other half, and it decides which appliances can actually start.
- Continuous backup power: the Powerwall 3 supplies 11.5 kW continuously, which covers most household loads running at once.
- Motor start capability: up to 185 A LRA, the surge figure that lets air conditioners and well pumps spin up without tripping the system.
- Grid interface: 120/240 V split-phase output at 60 Hz, matching North American household wiring.
- Expansion: additional units can be added at any time, up to 40.5 kWh per unit.
On the older Powerwall 2, Tesla lists 13.5 kWh usable energy out of 14 kWh total, 5 kW continuous charge and discharge, 7 kW peak for backup, and 90% round-trip efficiency. That gap between total and usable energy matters: no battery hands back every electron you put in.
How A Home Backup Battery Works During An Outage
Two habits keep a battery useful: self-consumption, where you store cheap overnight or solar energy and use it during peak-rate hours, and backup reserve, where you wall off a percentage of capacity purely for outages.
Solar changes the math.
| Spec | Powerwall 2 | Powerwall 3 |
|---|---|---|
| Usable energy | 13.5 kWh | 13.5 kWh capacity |
| Continuous power | 5 kW | 11.5 kW backup |
| Peak output | 7 kW | Up to 185 A LRA motor start |
| Efficiency | 90% round trip | 97.5% solar to grid |
| Grid standard | 120/240 V, 60 Hz | 120/240 V, 60 Hz |
| Mounting | Floor or wall, indoor or outdoor | Wall mount |
| Temperature range | −20°C to 50°C | Verified on North America specs |
| Warranty | 10 years | 10 years |
Worth checking before you commit: our tested roundup of backup batteries for outages compares the units homeowners actually install. Tesla’s own Powerwall support documentation lays out the storage and backup behavior in full.
What A Home Backup Battery Cannot Do
A backup battery cannot run every load in your house indefinitely, because its continuous and peak power limits are finite. Central AC, an electric range, and a well pump starting at the same moment can exceed what a single unit delivers, even if the energy is technically there.
The most common mistakes:
- Confusing total with usable energy. The Powerwall 2 holds 14 kWh but delivers 13.5 kWh, and every battery loses some in conversion.
- Assuming every unit matches US wiring. Tesla’s North America and Australia datasheets differ: 120/240 V at 60 Hz versus 230 V at 50 Hz. The wrong model will not work here.
- Ignoring installation conditions. Tesla notes backup power varies depending on local conditions, and the Powerwall 2 needs a 30 A overcurrent protection device plus a recommended operating range of 0°C to 30°C for best performance.
- Expecting a fixed price. Tesla does not publish a list price, and quoted figures from installers vary widely by region and configuration.
FAQs
How long will a home battery power my house?
It depends entirely on what you run. A 13.5 kWh battery can carry a fridge, router, and lights for a day or more, but running central air conditioning will drain it in a few hours. Most owners trim their loads during an outage and stretch backup well past the first evening.
Can a home battery power my air conditioner?
Usually yes, if the unit’s startup surge stays under the battery’s motor-start rating. A soft-start kit on the compressor lowers that surge and widens the list of compatible units.
Do I need solar panels for a backup battery to work?
No. A battery charges fine from the grid alone and will still back up your home during an outage. Solar simply adds the ability to refill the battery while the grid is down, which extends how long you can ride out a long outage.
References & Sources
- Tesla. “How Powerwall Works.” Supports how the system stores energy and switches to backup power.
- Tesla. “Powerwall 3 Datasheet.” Supports capacity, continuous power, and expansion figures.
- Tesla. “Powerwall 2 AC Datasheet, North America.” Supports usable energy, efficiency, and warranty details.
- Kitchprep. “Best Backup Battery for Power Outages.” Product comparison roundup for homeowners.
