Home Battery Backup Without Solar: How Long It Lasts

Most home battery pages assume panels on the roof. Plenty of people want the battery without them: an apartment or a rental where panels are not an option, a house whose roof faces the wrong way, or simply a budget that covers one of the two this year.

That works. It also changes what you are buying, and the change is worth understanding before you size anything.

A battery without solar is a reservoir, not a source. It fills from the grid while power is on, runs your house while power is off, and refills when power returns. Nothing tops it up during the outage. With panels, a bank that is flat by Tuesday morning is part-full again by Tuesday evening. Without them, what you started the outage with is what you have.

This page is about the stationary, prefabricated kind: a case holding the battery and the inverter, standing where you put it. If you want the portable kind you carry to a campsite, that is portable power stations. If you want to build a bank from cells and choose your own inverter, that is off-grid and RV batteries.

Two numbers decide it, and most listings lead with the wrong one

Kilowatts decide what can run at once. This is the inverter’s rating, and it is a wall: draw more and the system shuts off rather than browning out.

Kilowatt-hours decide how long it runs. This is the battery, and it drains at whatever rate your loads set.

A system can be strong on one and weak on the other. Sizing by capacity alone gets people a unit that runs the fridge for two days and trips the moment the microwave starts. Sizing by output alone gets a unit that starts everything and lasts an evening.

Work the loads out first with the appliance power budget, then ask how long a candidate system carries them with the outage runtime calculator.

Expect about three quarters of the rating to reach your outlets

Rated capacity is cell capacity. Two things sit between it and your appliances: the reserve the battery management system holds back, and the losses in converting DC to AC.

One manufacturer’s own published runtimes, worked against the rated capacity of the units they describe, land at 75 to 78% of the number on the box across loads from 600 W to 1,800 W. On a smaller unit running a 10 W light, the same arithmetic gives barely half, because the system’s own electronics draw a constant amount whether or not anything useful is plugged in.

So a 2.7 kWh system is a 2 kWh system for planning purposes, and a small load punished by standby draw makes that worse.

What empties it fastest is heat

Every resistive load, which means anything that makes heat, is measured in kilowatts rather than watts. A space heater is 1,500 W. A kettle is much the same. An electric range element, a hair dryer and a clothes dryer are all in that class.

Against a 2 kWh usable reservoir, 1,500 W of heat is about eighty minutes. The fridge, the freezer, the router, the lights and the phones that people actually need during an outage are a tiny fraction of that, which is why a modest system covers the essentials comfortably and cannot pretend to cover comfort.

Stacking, and what it locks you into

Systems in this class expand by adding the manufacturer’s own battery packs, which is both the appeal and the catch.

The appeal: you can start at one unit and add capacity later without redesigning anything, and the expansion usually raises what the system can accept on charge as well as what it can store.

The catch: the packs are proprietary. A custom bank built from cells can be extended with any battery meeting its specification, from whichever supplier is sensible at the time. A stacked system can only be extended with parts from one manufacturer, at whatever they charge when you need them, for as long as they keep making them.

That is a real trade, not a reason to avoid the class, and it is worth pricing into a decision that is meant to last a decade.

The one system whose figures are published

Disclosure: This page contains affiliate links. If you buy through them I may earn a commission, at no extra cost to you. This never affects which products I recommend or what I say about them.

Everything here was read on BLUETTI’s own specification tab, plus figures the manufacturer supplied directly. It is one system because it is the one whose numbers have been read at the source, and checkable is not the same as best.

Three things in that table decide most purchases.

It does 240V, through the 14-50R, which the portable class does not. That is what puts a well pump or a deep water pump within reach, subject to the surge caveat below.

The charging window cuts out at both ends. No charging below 32°F or above 104°F. These units are heavy enough that they live where they are put, and an uninsulated garage or an outbuilding crosses both limits in an ordinary year.

Adding a battery raises the charge rate, from 3,840 W to 6,240 W, so a stacked system refills faster than a single one when the grid returns.

What they do not publish, and you should ask

A surge rating. The specification tab gives 3,840 W continuous and 7,680 W of “Lifting Power”, and BLUETTI defines Power Lifting as a mode for pure resistive loads, kettles and heaters, rather than for motors. Starting a well pump or a compressor is a motor question, and no motor rating and no duration appears anywhere for this system. Ask before you buy if a motor is the reason you are buying.

A switchover time in the manual. BLUETTI advertises a 0 ms switchover for this system. Their portable units publish 10 to 15 ms in their manuals, and the manual figure for this one has not been read here.

The depth behind the expansion battery’s cycle rating. The base unit’s 6,000 cycles are stated at 95% depth of discharge, on BLUETTI’s word. The expansion pack’s 6,000 carries no depth at all, and a cycle count without one is not comparable.

Connecting it to the house is a separate question

Everything above is about the box: what it holds, what it supplies, and how long it lasts with things plugged into it.

The moment you want it to feed the house through its own wiring instead of extension cords, that is a different subject with different rules, different hardware and, in most places, a permit and an inspection. It starts at choosing your backup power type, which is where this site keeps the topology question and the safety separation that goes with it.

Scope, and what this guide does not do

It names one system. That is not a claim that nothing else is worth owning. It is the one whose governing figures have been read at the manufacturer and not off a listing, which is my rule for naming anything.

It does not cover grid-interactive systems. Selling power back, running in parallel with the utility, and anything involving an interconnection agreement is a different class of equipment with its own standards, and it is out of scope here.