What makes it a system rather than a battery
The marketing uses "home battery backup system" for everything from a 500Wh box upwards. One specification actually divides the category.
120V only. The unit has household outlets. Whatever you want backed up must have a plug and must be within reach of a cord. That covers a refrigerator, a freezer, the network and lighting, and it covers nothing that is hardwired.
120V/240V split-phase. The unit can supply both legs of a residential service, which means it can feed a transfer switch and therefore reach the well pump, the water heater, the furnace and the sump pump.
Everything else — capacity, app, chemistry, brand — sits downstream of that one distinction, and the price step across it is large.
| Tier | Reaches | Electrical work |
|---|
| 2,000Wh, 120V | Fridge, freezer, network, lighting, devices | None — extension cords |
|---|
| 3,800-4,100Wh, 240V split-phase | All of the above plus well pump, furnace, sump pump | Transfer switch and an inlet, installed |
|---|
| Expanded, 8-27kWh | Multi-day critical loads | The same, plus space and weight planning |
|---|
| Installed home battery | Whole panel, automatic changeover | A quoted installation project |
|---|
How long these actually last
Using 85% inverter efficiency, the figure this site uses throughout and documents on the methodology page.
| Load | Average draw | From 3,840 Wh |
|---|
| Fridge, freezer, network, lighting | 300 W | About 10.9 hours |
|---|
| The same, plus a furnace fan cycling | 450 W average | About 7.3 hours |
|---|
| Critical loads only, aggressively shed | 150 W | About 21.8 hours |
|---|
| Well pump duty on a normal household | Intermittent, high surge | Adds little energy, needs the surge headroom |
|---|
Rows are capacity x 0.85 / load. The well pump row is deliberately not a number: pumps draw heavily and briefly, so they cost surge capability rather than watt-hours.
The pattern worth noticing is that a day is achievable and a weekend is not, unless there is solar behind the battery or the outage has restoration windows in it.
Battery or standby generator?
This is the real decision and the answer depends on how long your outages run rather than on which technology you prefer.
A battery wins when outages are hours, when the house is in a place where a running engine is a problem, when there is solar to refill it, and when silence and zero maintenance are worth paying for.
A standby generator wins when outages run for days, because a fuel line has no capacity limit and a battery does. It also starts itself while you are away, which no plug-in battery does.
The direct comparison works through the whole trade, and the whole-house generator page covers what to ask before taking quotes on the other side of it.
What the installation actually involves
For a 240V unit feeding critical circuits, three things, in this order.
1. A transfer switch. Sized to the battery's output, wired to the circuits you chose. This is what makes backfeeding mechanically impossible.
2. An inlet. A weatherproof connection point so the battery is not tethered through a window. The inlet box explainer covers the ratings that matter.
3. A permit and an inspection in most jurisdictions. This is not optional and it is not the place to save money.
Budget for that work as part of the purchase rather than as a surprise afterwards. The installation cost page sets out the ranges.
The short version
- 240V split-phase or it is not a system. That one line divides the category.
- Budget the switch with the battery. A 240V unit with no way into the panel is a 120V unit.
- Plan for a day, not a weekend. Unless solar is part of the plan.
- Expandable beats big if you are unsure, because capacity is the specification people misjudge.
