What Size Solar Battery Do I Need? Sizing by Daily Use

The core formula

Battery sizing comes down to three numbers: how much energy you want to store per day, how many days you want it to last without recharging, and how deeply the battery can be drained safely. Multiply the first two, divide by the third, and you have the nominal capacity in kWh. Everything else is refinement.

Depth of discharge matters

You can't use every kWh a battery holds. Depth of discharge (DoD) is the share you can safely draw before harming the battery's lifespan. Modern lithium batteries often allow around 90%, while older lead-acid types are typically limited to about 50%. That's why a 10 kWh lithium pack delivers far more usable energy than a 10 kWh lead-acid one.

Power rating vs energy capacity

Two specs describe a battery, and both matter. Energy capacity in kWh is how much it holds — that sets how long it runs. Power rating in kW is how fast it can deliver — that sets how many appliances it can run at once. A battery with plenty of kWh but a low kW rating may store your evening but stumble when the oven, kettle, and heat pump all switch on together.

Frequently asked questions

Do I need a battery to store one full day?

Usually not. If you stay connected to the grid, many households only store the evening and overnight portion, using the grid as backup for cloudy stretches.

Can I add more batteries later?

Many modern systems are modular, so you can start smaller and expand. Check that your chosen system supports stacking before you buy.

Does an oversized battery pay off?

Not usually. A battery you rarely fill or empty sits idle much of the time, so its cost per useful kWh rises. Size it to your real overnight need.

How long will a solar battery last?

Lifespan is usually rated in charge cycles or years — often around 10 years or several thousand cycles for lithium. Staying within the recommended depth of discharge helps it reach that.