How long will a home battery last in a blackout?

Quick answer: divide the energy your battery can give by your average load. Take the usable capacity, subtract any reserve the system holds back, and divide by the load in kilowatts. A full 24 kWh battery holding back 20% has about 19 kWh to give: roughly 32 hours at 600 W of essential loads, or about 6 hours at a 3 kW whole-home evening. Solar during the day can stretch that further.

How long will a battery last in a blackout? It comes down to three things: how much energy is in the battery when the grid fails, how much of it the system will use, and how fast your home draws it down. None of them is fixed, so treat any single figure, including ours, as an estimate rather than a promise. The method below lets you work it out for your own home.

How long will a battery last in a blackout? The method

1. Start with usable capacity, in kWh. Use the usable figure, not the nominal one: a GoodWe GW8.3 module is 8.32 kWh rated and 8.0 kWh usable, while a FOXESS CQ7 module is 6.96 kWh, all of it usable.
2. Allow for how full it is when the grid fails. A blackout at 5 pm on a sunny day finds the battery full. One at 6 am, after a night of use, may find it near its lowest setting.
3. Subtract the reserve, the share the system won’t use. Settings vary between systems and installations, so check yours.
4. Divide by your average load in kW: 600 W is 0.6 kW.
5. Allow a few per cent for conversion losses. The datasheets put battery-to-AC efficiency at 97.0% on the FOXESS H1 G2 and 97.8% on the GoodWe ESA single phase.

In one line: hours of backup = usable kWh × share available ÷ average load in kW. For a full 16 kWh battery holding back 20%, that is 16 × 0.8 = 12.8 kWh, and 12.8 ÷ 0.6 = about 21 hours at 600 W.

Worked examples for 16 to 48 kWh

The table uses our package sizes and is illustrative. It assumes the battery is full when the outage starts, 20% is held back, the load is steady and no solar comes in. The loads are assumptions too: 300 to 600 W for essential circuits such as the fridge, lights and internet, and 1.5 to 3 kW for a whole home running through the evening.

Two things stand out. On essential circuits, even the smallest battery here covers most of a day, and the larger ones run for several days. A whole home running hard in the evening gets through the same battery in hours. The 32 and 40 kWh GoodWe sizes sit between the rows shown.

What does a typical home draw?

The Essential Services Commission uses annual usage of 4,000 kWh as its example household when it sets the Victorian Default Offer. That is about 11 kWh a day, an average draw of roughly 460 W across 24 hours. Averages hide the peaks: overnight use is low, while the early evening, with cooking, heating or cooling and hot water, can run at several times the average. That is why the table separates essential loads from a whole-home evening.

Seasons matter as well. A winter blackout comes with short days, little solar to top the battery up and heating loads, including the fan on gas ducted heating. Summer brings more solar but also air conditioning, which is easy to leave off in an outage and hard to carry if you don’t.

Power limit vs energy limit

The battery’s capacity in kWh sets how long it can run your loads. The inverter’s backup output in kVA sets how much can run at once, and a bigger battery doesn’t change it. The FOXESS H1 G2 supplies 5 kVA of backup (6 kVA for 60 seconds) whether it sits in front of 27.84 kWh or 41.76 kWh, and the KH8 gives 8 kVA. The GoodWe ESA single phase gives 10 kVA (20 kVA for 10 seconds) whether it has 16 kWh or 48 kWh behind it. Ask for more than the backup output allows and the inverter’s protection steps in, so a long-lasting battery still needs loads that fit. Choosing what goes on backup is covered in whole-home vs essential-circuit backup.

What makes a battery last longer in a blackout?

• Solar during the outage. FOXESS’s H1 G2 and KH manuals say the system supplies emergency power from the panels or the battery when the grid is off, and GoodWe’s three-phase ESA manual says the panels supply the loads first during the day and the excess charges the battery. On a sunny day that can carry you from one evening to the next.
• A charged battery when it counts. GoodWe’s three-phase ESA manual describes a backup mode that keeps the battery charged to a set backup level while the grid is on, using solar or grid power. Where your system lets you set a reserve, raising it before forecast storms trades a little daily saving for more hours in an outage.
• Fewer and smaller loads. Essential-circuit backup does this automatically. With whole-home backup, switch off the heating, cooling, cooking and hot water you can do without.
• Timing. Run the dishwasher or washing machine in the middle of the day, when the panels can cover it.

Brand details are in GoodWe ESA backup in a blackout and how FOXESS backup works.

Sizing a battery with blackouts in mind

Size the battery for your evening and overnight use first, because that is where it earns its keep every day; our guide to what size home battery you need covers it. Then check the outage case: will the backup output carry the loads you want, and will the battery cover them for the length of outage you care about? The GoodWe ESA 9.99 kW single-phase system with 16 kWh gives whole-home backup at 10 kVA, and the FOXESS H1 G2 system with 41.76 kWh pairs a large battery with 5 kVA of essential-circuit backup. Build Your System asks whether you want backup and sizes the system around your answers.

Frequently asked questions

How long will a 10 kWh battery last in a blackout?

With 10 kWh usable and 20% held back, about 8 kWh is available. That runs 300 W of essential loads for about 27 hours, 600 W for about 13 hours, a 1.5 kW evening for about 5 hours and a 3 kW evening for under 3 hours. These figures are illustrative, for a full battery with no solar coming in; your own loads and settings will change them.

Can a home battery run a house for days?

It can if the loads are small and the sun helps. At essential-circuit loads of a few hundred watts, a large battery holds out for several days on its own, and solar during the day extends that further on systems that use it in backup. Running the whole house, including heating, cooling and cooking, the same battery may last only one evening.

Will solar recharge my battery during a blackout?

On systems built for backup, yes, during daylight. FOXESS’s H1 G2 and KH manuals say the system supplies emergency power from solar or the battery when the grid is off, and GoodWe’s three-phase ESA manual says the panels supply the loads first and the excess charges the battery. A solar system without a backup function shuts down in a blackout and can’t recharge anything.

Why did my battery run out sooner than the maths said?

Usually for one of four reasons: the battery wasn’t full when the outage started, the reserve held back more than you assumed, a big appliance ran longer than you thought, or the evening load was higher than the average you used. Conversion losses take a few per cent as well. The history in your monitoring app shows what was drawing power.