A solar battery backup stores electricity from your panels in a home battery and feeds it to selected circuits or the whole house when the grid goes down.
Your panels make power all day, most of it vanishing into the grid at sunset. A solar battery catches that energy and returns it at night, during peak rates, or during an outage.
What Actually Happens Inside A Solar Battery Backup?
Panels make DC electricity; an inverter converts it to AC. Surplus charges the battery instead of exporting. During normal operation, panels power the house first, the battery charges second, and the grid absorbs the rest if your interconnection agreement allows export. When the grid drops, the system islands itself and the battery becomes the source; when demand rises, the battery discharges back through the inverter.
Batteries with a built-in solar inverter, like the Tesla Powerwall 3, can reduce the need for a separate inverter in some configurations. Older battery-only designs are AC-coupled and typically need external inverter equipment. That difference decides whether your existing solar gear can be reused or replaced.
What Do The Specs And Costs Look Like Right Now?
Capacity is measured in kilowatt-hours (energy), output in kilowatts (power), and both decide how long your house runs. A battery with big kWh but small kW will run a few lights for many hours but can’t start a well pump.
| Spec | Tesla Powerwall 3 | Tesla Powerwall 2 |
|---|---|---|
| Usable capacity | 13.5 kWh | 13.5 kWh |
| Continuous power | 11.5 kW | 5 kW |
| Round-trip efficiency | Not published on Tesla’s AC datasheet | 90% |
| Solar inverter | Built in | Not included (AC-coupled) |
| Solar connections | Six MPPTs, up to 20 kW input | External inverter required |
| Chemistry | LFP | Not stated on the datasheet |
| Warranty | 10 years | 10 years |
| Mounting | Wall or floor | Floor or wall (per datasheet) |
Those are market estimates, not a manufacturer list price.
How Do You Size A Battery For Your House?
List which loads you want running during an outage, then work out how many hours you need them. Solar United Neighbors’ battery guide recommends exactly that order: name the loads before picking a capacity.
- List critical loads — fridge, internet, a few lights, medical equipment.
- Estimate runtime: add up their wattage and multiply by the hours you want covered.
- Confirm whether the battery is AC- or DC-coupled, which determines what existing equipment survives.
- Verify inverter and electrical-panel compatibility with a licensed installer, since panel capacity and utility rules have the final say.
Whole-home backup needs a system designed for it from the start; most installs power a subpanel of chosen circuits. For real-world comparisons of what a full system costs and delivers, our tested battery backup solar picks lay out the numbers side by side.
Two spec details trip people up. Published ratings vary by region and version — Tesla’s Powerwall 2 AC datasheet is written for Australia at 230 V / 50 Hz and must not be read as a US electrical spec. And some multi-unit claims, like four units totaling 54 kWh, need checking against your regional Tesla datasheet before you plan on them. Battery work involves high-voltage DC and grid interconnection, so Solar United Neighbors’ battery storage guide is a sound starting point before you talk to an installer.
What Do People Get Wrong Most Often?
- Assuming one battery powers the whole house. Backup covers selected loads unless the system is purpose-built for whole-home coverage.
- Confusing kWh with kW. Capacity decides how long; power decides what starts and runs. You need both.
- Treating quoted prices as official MSRP. They shift with installer, region, and extra electrical work.
Ignoring inverter and panel compatibility is the fourth, and the most expensive — it can force upgrades or stop the battery from working as intended. Your final check before signing anything: know your critical loads, confirm coupling type, and get a licensed installer to sign off on panel and inverter fit.
FAQs
Can a battery keep my air conditioner running in an outage?
Sometimes, but central AC draws far more starting power than a single battery’s continuous rating allows. Smaller mini-split units are more realistic on one battery. Whole-home coverage including AC requires a larger system designed for it, and the installer should confirm the starting surge your unit needs.
How long does stored solar energy last overnight?
It depends entirely on the loads you run. A 13.5 kWh battery powering a fridge, router, and a few LED lights can stretch through the night and into the morning. Add a heater, well pump, or AC and that same charge disappears in a few hours.
Does adding a battery make my solar work during an outage?
Without a battery, most grid-tied solar shuts off automatically during an outage for safety. Adding a battery with backup capability lets the system island itself and keep selected circuits powered. This requires correct inverter and panel compatibility, so an installer should verify your equipment first.
References & Sources
- Solar United Neighbors. “Battery Storage Guide.” Supports the critical-load sizing workflow for battery systems.
- Tesla. “Powerwall 2 AC Datasheet.” Source for Powerwall 2 capacity, power, efficiency, and mounting specs.
- CNET. “Thinking About a Tesla Powerwall? Here’s What It’ll Actually Cost You.” Source for installed price ranges reported in 2026.
- Tesla. “Powerwall.” Official product page for Powerwall 3 capacity, power, and warranty details.
