Nigel M brought next idea :
I want to set up a 12v lighting system in a outbuilding, possibly with a
12v battery to be topped up by a solar cell. The system will need to
provide light for a 4m x 4m room, for an hour or so a day.

It really depends upon how much light you might need. You can buy 12v
light fittings as used in caravans and for camping, these are fairly
efficient consuming 8 to 12watts each. Alternatively you might consider
the 12v quartz lights with built in reflector, which are normally run
from a transformer in domestic situation. These can be obtained in
20watt and 50watt versions.

Obviously the higher the wattage used, the larger the battery would
need to be and the solar cell to recharge it. In addition you might
need a control on the charging system. For what an adequate solar cell
and battery would cost, you might find that running a mains supply
would be the cheaper option.

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Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.org