The size of battery storage you need depends on how much energy you consume daily and how many days of autonomy you want. A typical UK household uses 8-10kWh per day, so a home battery of 10-15kWh provides roughly one to two days of backup. For off-grid use, calculate your daily load, multiply by your desired autonomy days, then add 20% headroom.
Battery sizing is one of the most common points of confusion when planning a solar or off-grid system. Too small and you run out of power at the wrong moment; too large and you have spent money on capacity you never use. Here is how to work it out properly.
Step 1: Calculate Your Daily Energy Use
List every device you plan to power, its wattage, and the number of hours per day you use it. Multiply wattage by hours to get watt-hours (Wh), then total everything up.
Example for a basic off-grid cabin:
- LED lighting (50W x 4h) = 200Wh
- 12V compressor fridge (40W average x 24h) = 960Wh
- Laptop (65W x 5h) = 325Wh
- Phone charging (10W x 2h) = 20Wh
- Water pump (150W x 0.5h) = 75Wh
- Total: approximately 1,580Wh per day
Step 2: Decide on Autonomy Days
Autonomy days are the number of consecutive cloudy or low-production days your battery should cover without solar input. For UK use:
- Grid-tied home battery with solar: 1 day autonomy is typically sufficient
- Off-grid cabin with a generator backup: 2-3 days
- Fully off-grid with no generator: 3-5 days during winter
Using the cabin example above with 3 days autonomy: 1,580Wh x 3 = 4,740Wh needed.
Step 3: Account for Depth of Discharge
Lithium batteries (LiFePO4) can typically be discharged to 80-90% of their rated capacity without significant cycle life impact. Lead-acid batteries should only be discharged to 50%. Using LiFePO4 and a 90% depth of discharge:
4,740Wh / 0.9 = approximately 5.3kWh of rated capacity needed.
Add a 20% safety margin: 5.3 x 1.2 = 6.4kWh minimum. Rounding up, a 10kWh battery bank provides good headroom.
Home Battery Storage: Typical Sizes
For UK homes pairing battery storage with solar panels:
- 5kWh: suits a smaller home or flat, primarily to shift self-generated solar surplus to evening use
- 10kWh: the sweet spot for a 3-4 bedroom home with moderate consumption
- 15-20kWh: larger homes, EV charging, or those wanting full overnight coverage
EcoFlow's DELTA Pro Ultra system, for instance, scales from 6kWh to 90kWh using modular expansion batteries, making it suitable for everything from a flat to a small commercial property.
Portable Power Stations: Sizing for Specific Uses
For portable power stations rather than fixed home batteries:
- Camping weekend (2 people): 500-1000Wh covers phones, a light, and a small fan
- Van life (daily use): 1-2kWh per day is typical; a 1-2kWh station with solar top-up works well
- Home emergency backup: 2kWh minimum to keep a fridge and some lights running overnight
Do Not Forget Inverter Efficiency
When running AC devices from a battery via an inverter, expect 5-10% efficiency losses. If your daily AC load totals 1,000Wh at the wall, the battery will actually need to deliver around 1,100Wh to account for this. Build this into your calculations.
A Note on Battery Chemistry
LiFePO4 is the recommended chemistry for both home and off-grid storage in 2026. It offers the best combination of cycle life (2,000-4,000+ cycles), thermal safety, and cold-weather performance. Lead-acid batteries are still used in some budget systems but require more maintenance and occupy roughly twice the space per usable kWh.
