How Long Will a 10kWh Battery Last? Overnight and in Years

"How long will it last?" is the question we get asked about batteries more than any other, and it's really two questions wearing the same coat. Some people mean "will it get me through the night?" Others mean "how many years before I have to replace it?" Both are fair. Here are both answers.

How long will a 10kWh battery last?

Overnight, a 10 kWh battery lasts as long as its usable capacity divided by your average load. At an average draw of 1 kW, that's roughly 10 hours; at 2 kW, roughly 5 hours. In years, a modern LiFePO4 home battery is typically expected to last 10 to 15 years, and most manufacturer warranties run 10 years, commonly guaranteeing around 70 per cent of the original usable capacity at the end of the term.

For most Sunshine Coast family homes, a properly sized 10 kWh battery covers a normal evening through to morning. Whether it does at your place depends on what you run after dark.

Part 1: How long a 10kWh battery lasts overnight

The simple formula

Hours of runtime = usable battery capacity (kWh) ÷ average load (kW)

That's it. Kilowatt-hours are the size of the tank. Kilowatts are how fast you're drawing from it. Divide one by the other and you get hours.

A few things change the real-world answer:

• Usable vs nominal capacity. Some batteries quote a total capacity that's a bit higher than what you can actually use. Always work from the usable figure on the datasheet.
• Backup reserve. Many people set the battery to keep some charge in reserve for a blackout. If you hold back 20 per cent, a 10 kWh battery gives you about 8 kWh for everyday use.
• Conversion losses. A small amount of energy is lost converting stored DC back to AC for the house.
• Your load isn't flat. The kettle, oven and air conditioner draw a lot for a while; overnight, the house might only be running the fridge and standby gear.

Worked examples (illustrative loads only)

These loads are made-up round numbers to show the method, not measurements from any particular home. Your real numbers come from your own usage data.

Look at the jump between rows. The battery doesn't change; what you run does. Air conditioning, pool pumps, electric hot water boosting and EV charging are the big drains.

How to work out your own number

The easy way is to look at your usage, not guess it:

1. Get your interval data. Your retailer or your existing inverter app can show half-hourly or hourly use.
2. Add up your use from roughly 4pm to 7am. That's the window a battery has to cover once the panels stop producing.
3. Compare it to the battery's usable capacity, minus any backup reserve.

For most Sunshine Coast homes, that 4pm to 7am window lands somewhere between 8 and 16 kWh. That's why 10 kWh and 15 kWh are the two sizes we quote most often. A 10 kWh battery covers a normal evening through to morning for a typical family home: cooking, some air con, a hot water boost, no big surprises.

What about a 20kWh battery?

Same formula, double the tank. At a 1 kW average load, 20 kWh usable lasts about 20 hours; at 2 kW, about 10 hours.

But bigger isn't automatically better. A 20 kWh battery only earns its keep if:

• your evening and overnight use is genuinely that high (EV charging, a home business, a big house running air con), and
• your solar array produces enough surplus to fill it on an ordinary day.

A battery that never fully cycles earns nothing on the unused capacity. There's no point buying 20 kWh of storage if a 6.6 kW array only has 10 kWh of surplus to put into it. We go through that trade-off in more solar panels or a bigger battery.

"How long will a 10kW battery last?"

This one's usually a typo for kWh, but it's worth clearing up. kW is power, kWh is energy. A "10 kW" figure on a battery system normally describes how much power it can deliver at once, not how much it stores. A battery with 10 kWh of storage and 5 kW of output can run a 5 kW load for about two hours, or a 1 kW load for about ten.

Both numbers matter. Capacity (kWh) decides how long you last. Power (kW) decides what you can run at the same time, and whether a big motor will start.

How long a battery lasts in a blackout

In a blackout, runtime depends on two things: what's backed up, and whether the sun comes out.

• Essential circuits only (fridge, lights, some power points, internet) can run for many hours, and with solar recharging the battery during the day, often a couple of days.
• Whole-home backup with air conditioning drains the battery far faster.

Once the battery has islanded from the grid, your solar keeps running and recharges it through the day, which turns a few hours of backup into multi-day resilience. That only works if the system was specified and wired for backup in the first place, which not every installation is. Our guide to solar battery backup during a power outage covers what to set up before storm season.

Part 2: How many years does a solar battery last?

Expected service life: 10 to 15 years

A modern lithium iron phosphate (LiFePO4) home battery is typically expected to give 10 to 15 years of service. LiFePO4 is the chemistry behind nearly every battery we install, because it's thermally stable and tolerates daily cycling well.

Batteries don't usually fail with a bang. They fade. Each year the battery holds a bit less than it did when new, so a 10 kWh battery in year eight might give you noticeably less than 10 kWh. It still works; your runtime just shrinks.

What the warranty actually guarantees

Manufacturer warranties on the brands we install are typically 10 years, written as a capacity guarantee: commonly around 70 per cent of the original usable capacity at the end of the term, or a stated cycle count or energy throughput, whichever comes first.

That "whichever comes first" matters. A battery that's cycled hard every day, or used for heavy EV charging, can reach its throughput limit before the ten years are up. Sigenergy's published warranty is a good example of how these are written: it guarantees 70 per cent of usable energy for ten years or a minimum throughput figure for each module size. Read the warranty for the exact model on your quote, not a summary on a website.

On top of the manufacturer warranty, every Keystones install carries our own 10-year workmanship warranty, and we handle claims for you.

What shortens battery life (and what helps)

• Heat. Queensland summers are hard on any battery. A location out of direct afternoon sun, with clearance for airflow, helps. Garages and shaded south walls are common choices.
• Poor installation and commissioning. Wrong settings, wrong firmware or a mismatched inverter and battery cause more early problems than the cells themselves.
• Oversizing or undersizing. A battery that's too small gets worked hard every cycle; one that's too big never earns its keep.
• Neglect. A quick look at the app every month or two will catch a fault before it costs you a season of savings.

So what does that mean for payback?

Battery payback on the Sunshine Coast typically models out at around 7 to 12 years, or 6 to 8 years for households with heavy evening loads. With a 10 to 15 year expected life, a well-sized battery has time to pay for itself and keep going. An oversized one may not.

Is 10 kWh the right size for you?

It's the right size for a lot of homes, which is why it's our most-quoted size. Roughly:

• 5 to 6.5 kWh: units, townhouses, couples out at work all day
• 10 kWh: the standard Sunshine Coast family home
• 15 kWh: larger homes with a pool pump or ducted air conditioning
• 20 kWh and up: EV charging, home business, rural properties, whole-home backup

Most systems stack in modules of around 4 to 5 kWh, so you can often start at 10 and add capacity later. If you want us to size it from your actual usage rather than a rule of thumb, our battery installation team will run the numbers before quoting anything.

Frequently asked questions

How long will a 10kW battery last?

If you mean a 10 kWh battery, divide its usable capacity by your average load: about 10 hours at 1 kW, about 5 hours at 2 kW. If you genuinely mean 10 kW, that's a power rating (how much it can deliver at once), not storage. Check the kWh figure on the datasheet to work out runtime.

How long will a 20kWh battery last?

Using the same formula, 20 kWh of usable storage lasts about 20 hours at a 1 kW average load or about 10 hours at 2 kW. It only makes financial sense if your evening and overnight use is high enough, and your solar array big enough, to fill and empty it most days.

How many years does a solar battery last?

A modern LiFePO4 home battery is typically expected to last 10 to 15 years. Manufacturer warranties are usually 10 years, commonly guaranteeing around 70 per cent of original usable capacity at the end of the term or a stated cycle count or throughput, whichever comes first.

Will a 10kWh battery run my house overnight?

For a typical Sunshine Coast family home without big overnight loads, usually yes. Most homes use 8 to 16 kWh between about 4pm and 7am. Heavy air conditioning, pool pumps or EV charging overnight can push you past what 10 kWh covers.

Does a battery lose capacity over time?

Yes, gradually. Each year a battery holds slightly less than it did new. Warranties are written around this, typically guaranteeing around 70 per cent of original usable capacity after 10 years. Keeping the battery out of direct heat and having it commissioned properly helps it age slowly.