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Why the cheap battery is usually the expensive one.
Lead-acid (including AGM and "leisure" batteries) is the traditional choice: cheap upfront (~£90–130 for 100Ah). But you can only use about 50% of the rated capacity without wrecking it, it manages 300–500 charge cycles, hates being left part-charged, and flooded types vent hydrogen (needs ventilation).
LiFePO4 (lithium iron phosphate) costs more upfront (~£200–450 for 100Ah) but you can use 80–90% of the capacity, it lasts 3,000–6,000 cycles, charges faster, weighs a third as much, and doesn't mind sitting part-charged through a grey fortnight.
Do the maths per usable kWh over the battery's life and LiFePO4 wins by a wide margin — typically 4–6× cheaper per kWh delivered. Lead-acid still makes sense for genuinely tiny budgets or rarely-used setups.
Whichever you choose: fit a correctly rated fuse within 30cm of the positive terminal. A shorted battery can deliver hundreds of amps.
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.