Bigger is not automatically better.
A useful first-pass size is driven by the solar energy you can store and the electricity you are likely to use after solar production falls.
Two numbers to find
1. Daily solar exports
Look at your electricity bill, solar inverter or monitoring app. This tells you roughly how much energy is leaving the house that might instead be stored.
2. Grid usage after solar hours
This is the load the battery could potentially replace. Households with high evening use, electric heating/cooling, pools or EV charging can have very different profiles.
A simple first-pass rule
A battery generally cannot create value by shifting more solar energy than you have available to store, and it cannot replace more evening grid consumption than you actually use. That is why Battery Simple's calculator takes the smaller of available export, after-solar demand and battery daily throughput.
Reasons you might deliberately size differently
- Backup power requirements.
- Future EV purchase or electrification.
- Time-of-use or dynamic tariffs.
- VPP participation.
- Seasonal solar and load differences.
- Inverter and connection limits.
Official program guidance specifically notes that a battery that is too large for the solar system and/or inverter can limit potential benefits.
Estimate a useful size