Light a cellar or shed without mains power

Rechargeable LED bars, a 12V solar kit or a proper circuit: a watt-hour calculation tells you straight away which one survives real use.

Split the space into zones: the entrance (door, stairs), where light must come on instantly; storage shelves, which need steady light to read labels; and the walkway, where a low level is enough.

Work out your need in Wh

Battery capacity is measured in watt-hours (Wh). If only mAh is given: Wh = mAh × voltage ÷ 1000. A 2000 mAh bar at 3.7 V stores 7.4 Wh, about 80% of it usable: 5.9 Wh.

Runtime (days) = usable energy (Wh) ÷ daily use (Wh/day)

Example: 2 trips a day + 1 hour of DIY a week

Zone	Light	On time	Energy
Stairs	2 W bar	2 × 1 min = 0.033 h/day	0.07 Wh/day
Walkway + shelves	2 × 3 W bars	2 × 5 min = 0.17 h/day	1.0 Wh/day
Workbench	12V 15 W bar	1 h a week	2.1 Wh/day on average
Total			≈ 3.2 Wh/day

Stairs and walkway only (1.07 Wh/day): each 5.9 Wh bar lasts about 3 months on the stairs and 12 days in the walkway. Add the workbench (3.2 Wh/day) and bars are no longer enough: a 12V 12 Ah LiFePO4 battery (12.8 V × 12 Ah = 154 Wh, 120 Wh usable) lasts 37 days with no recharge.

Options compared

Option	Energy available	Best for	Limit
Rechargeable sensor bars (×3)	5–10 Wh in total	Short trips	Recharge every 1–2 months
Rechargeable head torch	5–15 Wh	Searching, DIY	Only lights where you look
Portable power station + 12V/USB bar	250–500 Wh	Occasional use, power cuts	Heavy to carry up for charging, costly
12V solar kit (30 W panel + 12 Ah LiFePO4)	≈ 150 Wh	Daily use, several zones	Outdoor panel, cable entry, low winter output
Mains circuit from the consumer unit	unlimited	Heavy use, tools	Electrician’s work

Sizing a 12V solar kit

Winter is the hard part. The safest figure is December output for your exact location, which the European Commission’s free PVGIS tool gives in Wh per watt of panel (it covers the UK, and the US through its NSRDB data). As a prudent figure we use 0.5 Wh per watt per day (northern France in December, losses included); much of the UK will be close to that, most of the US well above it.

Panel = daily use (Wh/day) ÷ 0.5. For 3.2 Wh/day, 6.4 W would do in theory: fit 20–30 W for grey spells.
Battery = daily use × sunless days ÷ 0.8. For 10 days: 40 Wh, i.e. a 12V 6 Ah LiFePO4 (77 Wh).
Charge controller = panel short-circuit current × 1.25: a 30 W panel (about 1.8 A) is fine with a 10 A controller with a lithium profile.

Connect battery first, then panel, then loads; disconnect in reverse. If the space becomes a workshop with power tools (a 600 W drill for 15 minutes = 150 Wh, 45 days of lighting), you need a real circuit installed by an electrician (BS 7671 / Part P in England; NEC and permit in the US).

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