Which is more important to seedlings, heat or light?

[AkionTotocha]

Deep in the South West it looks like I'm getting away with toms in the unheated GH. They've been out there for a month give or take - and in that time night time temps have not often gone below 9 degrees IIRC
Mrs Balders has a limited tolerance for seedlings on windowsills... so they have to be move on sooner than I'd like.
A chum at the allotment picked up a parafin heater off eeebay for £16 which seems to be keeping his GH at about 15 degrees overnight. I might need to look into an electric heater that I can set with a timer as I can't see Mrs B out at night fiddling with a wick when I'm working away. Anyone have any advice / expoerience with a leccy one (ideally I'd like it to be solar or heatsink powered but that aint gonna happen at least until the boffins make some massive inroads into battery efficiency.

You could dig a massive pit under the greenhouse and fill it with pipes, backfill with soil or sand and have a raised fan/vent inlet and outlet connected to the pipes. Just put a low powered fan on it and two solar pannels (or just one) with a splitter to a cheap battery (maybe a car battery? Or if you use a really small fan just a few rechargable house batteries?)

In the day the fan is on, sucking the warm air out of the greenhouse and pumping out cool air from the soil. In the night the fan is on (powered by the battery) sucking the cool air from the greenhouse and pumping the warm air that it collected from the day back into the greenhouse. That way the only thing you have to provide power for is a fan, which needs a lot less power than an electric heater.

Depending on your batteries you could maybe even use very cheap solar pannels (under £10), you'd just need to make sure when you stood on the floor your weight didn't crush the pipes.

[MarkPelican]

Akion, I know there are people who suggest this and I've seen designs of greenhouses that have a bank of soil on the north side which heats up when the sunshines but, personally, I think whilst it extends the time that the temperature is high a little, an electric heater would do the job.

I've run an experiment this year with my Raspberry Pi. I put a barrel of water in the greenhouse in the sun. It heats up in the day and cools down overnight. However, the amount by which it cools overnight is measurable only in a coupl of degrees meaning that the amount of heat given out to the greenhouse overnight isn't very much.