1. What you want from the battery
A battery can increase self-consumption, provide energy in the evening or act as a reserve during a power cut. Sometimes it does all three, but each goal leads to a different configuration. If the main aim is backup, what matters is the power available and the circuits it supplies; if it is self-consumption, what matters is the energy you use in the evening.
2. How much you use between evening and morning
Look at your consumption after sunset: lighting, the fridge, the television, appliances on standby and, perhaps, electric heating. That figure, in kWh, is a starting point for the capacity. You can find it out fairly precisely through consumption monitoring.
3. Usable capacity, not just nominal
A battery with a nominal 10 kWh does not necessarily provide 10 kWh of usable energy. Compare the usable values, as declared by the manufacturer, and pay attention to the warranty conditions.
4. Power, not just capacity
The battery’s power, in kW, decides which appliances you can run at the same time. An electric oven or a heat pump can exceed the power of a small battery, even if the stored energy would be enough.
5. Compatibility with the inverter
The battery has to match the inverter: voltage, communication, current limits. Not every combination is possible or accepted by the manufacturers. See also hybrid inverters.
6. Warranty, safety and installation
Ask for information about the warranty (years and cycles), about the product’s certifications and about the installation conditions: temperature, ventilation, space. A good battery installed in an unsuitable place can have a shorter life.
A simple example
If you normally use 6 kWh between evening and morning, and you want to cover only that consumption, a usable capacity of 6–8 kWh is a reasonable starting point. If you also want a reserve for a few hours without the grid, add the consumption for those hours. The result remains indicative until the full analysis.
For technical details, see the page on battery storage, and for the complete architecture, solar systems with batteries.