Edit the numbers as your quotes come in

Other things worth knowing about battery sizing

• Usable vs. nameplate capacity. The headline kWh figure on a spec sheet is sometimes the gross/nameplate capacity rather than what you can actually draw on — ask installers specifically for the usable capacity after depth-of-discharge limits, since that's what this tool assumes throughout.
• Power rating matters as much as capacity. A battery's kW output (not just its kWh capacity) determines whether it can cover a spike in demand — an oven, a kettle and an EV charger running together can exceed what a small inverter can deliver, even with plenty of charge still stored.
• Typical UK household sizes. Most homes land around 5–10 kWh usable; households with an EV, a heat pump, or a larger family often go for 10–20 kWh, sometimes as stacked/modular units from the same manufacturer.
• Diminishing returns are real. Sizing for a typical day rather than your single heaviest day is usually the better financial call — the last few kWh of a "cover every worst case" battery often sit unused for most of the year, as the chart above shows for your own usage.
• Chemistry. Most modern home batteries use LFP (lithium iron phosphate) cells — more thermally stable than older lithium chemistries, typically rated for 6,000+ cycles and backed by 10-year warranties.
• Degradation. Usable capacity fades a little each year — commonly a few percent per year over a decade — so a bit of headroom above your calculated minimum helps the system still meet your needs later in its warranty life.
• Expandable systems. Many batteries are modular — it's often cheaper to start slightly smaller and add a second unit later if you need it than to oversize from day one and pay for capacity you rarely draw on.