The useful starting point
The best solar battery is one that fits your generation, demand and existing equipment. A capacity figure alone cannot establish that fit.
Separate a new installation from a retrofit
When solar and storage are being installed together, the designer can consider the panels, inverter and battery as one system. With an existing array, first identify the inverter, its remaining warranty, its supported equipment and any existing export arrangements.
Ask for two clearly scoped options where appropriate: retaining the existing inverter with compatible storage, or replacing it as part of a new combined design. Include lost warranty, installation disruption and additional hardware in the comparison.
Solar energy has an alternative value
Storing surplus solar means giving up any export payment that energy could earn. The value of a battery is therefore not simply every discharged kWh multiplied by the full import rate.
For example, delivering 5 kWh from storage at 90% round-trip efficiency needs about 5.56 kWh of solar charging. At hypothetical rates of 27p import and 12p export, the avoided import is £1.35 and the foregone export is about 67p. The difference is around 68p before other costs. If no export payment would have been available for that energy, the alternative value can differ.
Compare products without losing the installation detail
A Powerwall 3 proposal and a SigenStor proposal may describe different capacities, controllers and backup equipment. Match them against a common brief before comparing totals. Ask for usable storage, commissioned charge/discharge output, supported PV arrangement and the equipment needed for a grid outage.
Our Tesla guide and Sigenergy guide explain those product-specific questions. They are based on documents, not claimed performance tests.
Design for a year, not one summer evening
A useful illustration should account for periods when the battery cannot fill from solar and periods when household demand is too low to empty it. Cheap-rate grid charging may help, but tariff compatibility, control settings and charging windows become part of the design.
- Ask for monthly generation and consumption assumptions.
- Check whether the same EV charging load has been counted twice.
- Keep backup reserve separate from capacity used in a savings example.
- Confirm whether the battery can accept surplus at the rate the array produces it.
- Ask what happens to automation if the internet connection fails.
Make your shortlist measurable
Write a short brief with your existing equipment, annual demand, normal evening use and any essential backup circuits. Use the battery tool to compare scenarios at the same energy rates. Then ask installers to explain any difference between their predicted result and your assumptions.
Do not treat an equipment warranty as a guarantee of a particular annual saving. Your usage, tariff and system settings remain important.
Published information supports your research. Your installer should confirm the exact product, suitability, scope and terms for your property.
Suggest a correction ↗Your next useful step
Put the information to work.
Take the questions that matter to your project, explore the numbers and decide what to ask next.
