The night the lights went out
You know this story. A storm rolls in, the power cuts at 9 pm, and your phone torch becomes the brightest thing in the house. The fridge hums down into silence. The router blinks once and dies. And somewhere in the dark you start doing the maths you wish you'd done in daylight: what did you actually need to keep running, and for how long?
That question is the whole sizing problem. Most battery quotes start from the other end, with a big round number on a brochure. Start from your night instead.
Pick the job before you pick the size
A battery does different jobs, and each one wants a different size. Evening self-use means storing daytime solar to use after sunset. Essentials backup means keeping the fridge, lights, router and heating controls alive through a cut. Whole-home backup means carrying on almost as normal, air conditioning and oven included. Off-grid means no grid at all to fall back on.
Those aren't small differences. They're the difference between a 5 kWh battery and a 30 kWh one. So write down your job first, in one sentence: keep the fridge, lights, router and boiler controls going for one full day if the grid drops.
How many kWh solar battery do you need? The maths
Here's the formula, with nothing hidden in it:
Rated battery size = usable energy you need ÷ depth of discharge ÷ system efficiency
Usable energy is what your chosen loads actually eat over the backup period, in kWh. Depth of discharge is the share of the battery you're allowed to use without damaging it. System efficiency covers inverter and wiring losses. The free solar battery calculator runs this same sum, and it lets you build the load list appliance by appliance instead of guessing a daily total.
Nominal capacity is not usable energy
A battery labelled 10 kWh doesn't hand you 10 kWh. A lithium iron phosphate battery (LiFePO4) typically lets you use around 80% of its nominal capacity. A flooded lead-acid battery lets you use about 50% before you start shortening its life. That alone means a lead-acid bank needs roughly 60% more nominal capacity than a lithium one for the same usable energy.
Then the inverter takes its cut. The calculator on this site defaults to 85% system efficiency, and that's a sensible planning figure unless your installer gives you a real one. Skip both adjustments and you'll buy a battery that looks right on paper and runs dry hours early in a real outage.
Days of backup multiply everything
Every extra day of autonomy multiplies the bank. One day of essentials is a modest battery. Three days of the same loads is three times the usable energy, before you've even touched depth of discharge or efficiency. Off-grid homes usually plan for two to three days. Grid-connected homes backing up essentials usually need far less, because the grid is the backup for the backup.
A worked example, with real-ish numbers
Say your essentials list looks like this for one day without power:
- Fridge, cycling on and off: 140 watts for 9 effective hours = 1.26 kWh
- Router and phone charging: 12 watts for 24 hours, plus a bit for phones = 0.35 kWh
- Lights, six LEDs: 9 watts each for 5 hours = 0.27 kWh
- Boiler controls and a fan: 180 watts for 4 hours = 0.72 kWh
- TV or laptop in the evening: 90 watts for 3 hours = 0.27 kWh
Add them up and you get 2.87 kWh of usable energy for the day. Now run the formula for a lithium battery at 80% depth of discharge and 85% efficiency: 2.87 ÷ 0.80 ÷ 0.85 = 4.22 kWh rated. A 5 kWh nominal battery covers that day, with a little slack. The same loads on lead-acid at 50% depth of discharge come out at 6.75 kWh rated, so you'd be shopping in a different size bracket entirely for the same night in the dark.
Stretch the lithium example to two days and the usable energy doubles to 5.74 kWh, which needs 8.44 kWh rated. Round up, because batteries come in set sizes. That lands you near a 10 kWh battery for two days of essentials. Notice what happened: nothing about your house changed, only the number of days, and the answer doubled.
For a bigger household, say your home uses 23.7 kWh a day and you want to back up 40% of it for a single day. That's 9.48 kWh usable, or 13.94 kWh rated on lithium. A 14 to 15 kWh battery. The answer follows your loads, not someone else's average.
kWh is not kW, and the mix-up will trip you
Capacity in kWh says how long a battery lasts. Power in kW says how much it can run at once. A battery can hold plenty of energy and still trip if you switch on too much together, because its inverter has a maximum output, often around 5 to 10 kW for a home system.
During a cut, run the big items one at a time, and check the inverter rating on any quote, not just the kWh figure in the headline.
Can your panels refill it?
A battery you can't recharge is a one-night gadget. Multiply your panel array in kW by your peak sun hours, then knock off about 20% for heat, dirt, wiring and controller losses. A 4.2 kW array at 3.8 peak sun hours gives roughly 12.8 kWh a day, which refills a 10 kWh bank on a decent day. In winter the same array gives far less, so size for your worst month, not a July average. If you haven't sized the panels yet, read our guide to sizing your panels first.
So what size should you buy?
For most grid-connected homes, the honest answer lands in one of three bands. A small, efficient home keeping only fridge, lights and router alive overnight will often manage on around 5 kWh. A typical family home backing up essentials for a full day, or shifting evening use onto stored solar, usually lands around 10 to 13.5 kWh. Whole-home backup, air conditioning included, starts around 20 kWh and climbs fast from there.
My verdict: buy for essentials first. A battery sized to keep the important things running through the outages you actually get beats a whole-home battery you had to stretch to afford. It gets used properly, it recharges fully on a normal day, and it leaves budget for the panels that refill it. You can add a module later if outages get longer. You can't un-buy an oversized bank that spends its life half-charged. And before you spend anything, check whether solar pays back in your state or region.
This is planning guidance, not installation advice. Battery installations carry real electrical and fire risk, and they should be fitted by a qualified installer to your local rules.
Questions people actually ask
Is a 5 kWh battery enough for a house?
For a small, efficient home that only needs the fridge, lights and router through a short outage, yes, it can be. It won't run heating, cooling or the whole home overnight.
How long will a 10 kWh battery last in a power cut?
It depends entirely on what you run. On essentials drawing around 500 watts, a 10 kWh lithium battery gives roughly a day once you account for usable capacity and inverter losses. Add a 3.5 kW air conditioner and you're looking at a few hours instead.
Does cold weather shrink a solar battery?
Yes, and lead-acid suffers most: it can lose around a third of its usable capacity near freezing. Most lithium chemistries must not be charged below 0°C without a heater or a battery management system that prevents it, so cold-climate homes should size up and check the chemistry limits before buying.
Should I buy a bigger battery just in case?
Usually no. An oversized lead-acid bank that never reaches full charge degrades early, and every battery ages on the calendar whether you cycle it or not.
Do the load list tonight, while the lights are on. It takes ten minutes and a look at the labels on the back of the fridge and the boiler. If the answer surprises you, good. That's cheaper than finding out during the next storm.