Battery Options for Shed Solar Systems

The number that matters: usable capacity

Battery labels quote total amp-hours; sheds run on usable amp-hours, and the gap between the two is where money disappears. Discharge a flooded lead-acid battery past about 50% routinely and its plates sulphate; a few months of winter abuse turns a 100Ah battery into a 40Ah one. AGM tolerates a little more. Lithium iron phosphate (LiFePO4) cells happily cycle to 80–90% depth of discharge for thousands of cycles — so a 100Ah lithium battery delivers roughly twice the usable energy of a 100Ah lead-acid, weighs a third as much, and charges faster from a winter-weakened panel.

Run the cost per usable kilowatt-hour over the battery's cycle life and the conclusion is blunt. A £90 leisure battery offering ~0.6kWh usable for perhaps 300 cycles costs around 50p per stored kWh delivered. A £280 LiFePO4 battery offering ~1.1kWh usable for 3,000+ cycles costs under 9p. Lead-acid is not cheaper; it is cheaper-looking.

When lead-acid still earns a place

Three honest exceptions. First, genuinely light duty: a shed visited a dozen weekends a year, where any battery will age from time rather than cycling, and £90 beats £280. Second, sub-zero charging: LiFePO4 cells must not be charged below 0°C without a low-temperature cut-off or internal heating — most decent 2026 BMS units include this, but in an unheated highland shed a cheap AGM dodges the issue entirely. Third, very high surge loads on a tight budget, where a big lead bank's sheer plate area covers a winch or compressor start that would need a pricier lithium BMS rating.

Sizing the bank for your shed

Take your daily watt-hours (the method is on the homepage and worked through in the sizing post), multiply by the days of autonomy you want — two days suits most lighting systems, three for a garden office that cannot fail on a grey Tuesday — and divide by usable fraction and system voltage. A 400Wh/day workshop wanting two days of autonomy on lithium needs 800Wh ÷ 0.85 ≈ 950Wh, which at 12V is an 80Ah battery: buy the standard 100Ah and stop optimising. The same maths on AGM demands 160Ah and twice the weight on the shelf bracket.

Match the charge controller to the chemistry: lithium wants a controller with a LiFePO4 profile (absorption around 14.4V on a 12V system, no equalisation), while flooded lead-acid actually benefits from periodic equalisation charges that would harm lithium. Cheap kits ship controllers locked to lead-acid curves — one more reason kit-buyers should read the kit guide before paying.

Second-hand, second-life, and what to avoid

Ex-telecoms AGM batteries and "second-life" EV modules circulate cheaply and tempt every shed owner eventually. Telecoms AGMs are a reasonable gamble at under £1 per amp-hour if recently date-stamped. Bare EV modules are not: they arrive without a BMS, at voltages that do not match shed equipment, with energy densities that turn wiring mistakes into genuinely dangerous events. If a listing mentions "NMC pouch cells" and "great project," scroll on. Fusing and installation practice for whatever you buy is covered on the wiring page, and full system budgets including battery line items are on the costs page.