Battery sizing calculator
Choosing the right capacity for your home battery is essential. If the battery is too small, you’ll still end up buying expensive electricity from the grid. If it’s too large, the extra cost of the battery may never be paid back through energy savings.
What you’ll need
To estimate the ideal battery size for your property, you should have the following information ready:
- Annual Solar Generation: How much electricity do your panels produce? (Estimated in kWh).
- Daily Energy Usage: How much electricity does your household use on average per day?
- Evening Usage: Roughly what percentage of your energy is used after the sun goes down?
- Major Loads: Do you have a heat pump or an electric vehicle? These significantly increase your storage needs.
Sizing methodology
As a general rule of thumb for UK households:
- Low Usage (2,000 kWh/year): A 2kWh to 5kWh battery is usually sufficient to cover evening lighting and basic appliances.
- Average Usage (3,500 kWh/year): A 5kWh to 10kWh battery is the most common choice, covering most evening and overnight needs.
- High Usage (5,000+ kWh/year): A 10kWh to 15kWh (or multiple modular batteries) is recommended, especially if you have a heat pump.
Key considerations
Solar vs Grid charging
If you have a large solar array, you want a battery large enough to “soak up” all the excess power on a sunny day. If you don’t have solar but are using a Time-of-Use tariff, you want a battery large enough to store all the cheap overnight power you’ll need for the following day.
Depth of Discharge (DoD)
When looking at battery sizes, check the Usable Capacity. For example, a battery might be marketed as 5kWh, but if it has a 90% DoD, you can only safely use 4.5kWh.
Pro Tip: Many modern battery systems are modular. This means you can start with a smaller battery (e.g., 5kWh) and add another module later if you find you need more capacity.