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Where sunshine becomes a Tuesday evening
A battery stores energy chemically: charging pushes ions one way between two electrodes, discharging lets them flow back, releasing electrons through your circuit on the way. Three numbers define one: capacity (Ah at a voltage — multiply them for Wh of energy), usable depth of discharge (lead-acid ~50% before damage; LiFePO4 80–90%), and cycle life (full charge-discharge counts before fading — 300–500 for lead, 3,000–6,000 for LiFePO4).
Modern lithium batteries include a BMS (battery management system) — a circuit board guardian that balances cells, blocks over-charge/over-discharge, and in good batteries blocks charging below 0°C, which damages lithium chemistry. For an unheated UK shed in winter, low-temperature charge protection (or a self-heating battery) is a spec worth seeking out, not a luxury.
Sold by Ah at a system voltage. The 2026 UK landscape: 12V 50Ah LiFePO4 ~£110–180 (starter), 12V 100Ah LiFePO4 ~£200–450 (the default building block — the spread reflects BMS quality, cell grade and brand), 12V 200–300Ah ~£400–900, 24V and rack-mount 48V units for bigger builds. Lead-acid "leisure" batteries still sell at ~£90–130 per 100Ah, but remember only half is usable.
Buying signals that matter more than price: a named cell manufacturer (EVE, CATL), a real BMS spec sheet, low-temperature protection, and a warranty someone will honour. The no-name bargain listing is where this market hides its disappointments.
The battery is the heart of an off-grid system and usually its biggest single cost — everything else exists to fill it or empty it politely. It sets your autonomy (how many grey days you survive), and its chemistry dictates the charge profile, the fusing, and even where you can put it. Size it from daily Wh × autonomy days ÷ usable fraction — and fuse it like you mean it.
From EcoPowerful — plain-English DIY solar & wind guidance for UK homes, with a free instant system plan builder. Guidance is general; 230V fixed wiring always needs a qualified electrician. Last updated 2026-06-13.