The answer
Would a bigger battery save you more money?
Not necessarily. An extra kWh of battery capacity saves nothing if you rarely fill and empty it. Compare the spare solar electricity you actually have, the power you use later in the day and any off-peak grid-charging plan. Ask for two or three sizes modelled across different seasons, including storage losses and the battery's usable capacity rather than its headline capacity.
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Storage capacity is not supply power
Capacity in kWh is how much energy the battery can hold; output in kW is how much it can provide at one moment. A battery may hold enough energy for evening use yet be unable to run several high-power appliances together. The manufacturer may also reserve some capacity, so ask for usable rather than nameplate kWh.
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The useful size can change with the seasons
Solar surplus often fills storage more readily in summer than winter. A future heat pump or EV could raise demand, but that plan should be shown as a separate scenario. Energy Saving Trust advises sizing around current use, any renewable generation and intended tariff charging. Model a typical summer day, a winter day and a shoulder-season day to see whether the larger option earns its added cost.
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Savings and backup are different jobs
Storing solar to avoid an evening import has a financial value after charging losses and the export payment you give up. Backup during a power cut needs compatible inverter and isolation arrangements; it is not automatic with grid-connected storage. If resilience is important, specify the circuits, maximum power and likely duration. Energy Saving Trust notes battery life is generally shorter than panel life, so include likely replacement in the comparison.
How can you tell if extra capacity earns its keep?
Give the installer your half-hourly electricity data if it is available, or at least a realistic account of when the home is occupied and which large loads run after sunset. Put estimated solar output against those hours and count how often there would be enough spare energy to fill each battery option. A battery that looks fully used on a bright July day may have much less to store through a Scottish winter.
Ask for a side-by-side comparison of perhaps a smaller and a larger usable capacity. Show the extra kWh discharged over a year, the electricity price avoided, the export income forgone, round-trip losses and the extra installed cost. If the model assumes cheap overnight charging, state the tariff and test what happens if that deal ends. The larger option should win on useful energy or resilience you actually need, not on a higher capacity figure alone.
Finally, read the warranty and backup specification separately. A long panel life does not mean the battery lasts as long, and a battery that can shift energy into the evening may still shut down during an outage without backup hardware. Those are questions the survey can settle before you commit to a particular brand or size.
What to check next
Your next steps
Where we checked the facts
Check the original information.
These official pages back up the rules and technical details above. They can change, so check them again before spending money or starting work.
Apply this to your own property
Could solar work on your roof?
A free survey checks your roof and electricity use to see whether solar panels, a battery or both may fit your property.