I'll be heating (frost protection) my greenhouse this winter with a 2 Kw
electric heater. The thermostat switches the heater on when the temperature
drops below 5° C. The heater has 2 settings :
Full - 2Kw
Half - 1Kw
Obviously the first setting would use twice as much power as the second but
presumably would be on for less time. As a matter of interest would they
both use the same amount of power over a period?
Approximately, although it may vary according to the thermal characteristics
of the greenhouse and the thermostatic control. The variance will be about
how perfectly the heater spreads its heat and how perfectly the thermostat
measures it and could go either way. This assumes that the weaker power
setting is sufficient to maintain temperature.
I suspect that the higher setting would use a little more.
This is because the typical bimetal thermostat will tend to operate
after the temperature has overshot the set point. There is also
something of a time inertia.
The greater the heat supply, usually the more the overshoot.
Approximately, the heat loss is proportional to temperature, so the
more the temperature overshoot, and the time of that, the greater the
Of course all of this will vary with the particular thermostat and the
Realistically, you would need to measure electricity consumption on
two different nights of the same minimum and profile of temperature to
This is an expensive hobby, though, even with bubblewrap in the
To email, substitute .nospam with .gl
I'm not a gardener and have never had a greenhouse, but
even 1kW strikes me as excessive -- it's only to keep the
frost off, not to get it up to room temperature. I would
also doubt that the plants, when temperature is not far
off zero, will like to have a hot draft blowing round them
every few minutes as the thermostat cycles on and off (if
this is a fan heater).
I grow most of our own vegetables in our small garden and have a small (8' x
Last year I bought a fan heater, designed for greenhouse use, and used it on
1kw on a 4 degrees C setting.
It wasn't warm in there unless the sun shone, it certainly didn't put the
plants under any strain, but it did mean that I could start seeds growing
weeks before I could otherwise as well as keep other crops growing (albeit
slowly) and therefore have an almost continuous production of vegetables.
I recommend it.
When my uncle was living in Canada, he had a greenhouse/conservatory
in which there were a couple of lemon trees, and they used to comment
on how nice it was to stoll out and pick a fresh lemon for their
gin and tonics. One day, they actually worked out the cost, and it
was something like £10 per lemon when you took the power requirements
When I first started keeping bees my first pot of honey had cost about £500.
But there's more to it than that. If we took that argument to its logical
conclusion we'd be talking directly to people instead of using internet,
we'd be walking instead of driving cars, keeping sheep, spinning and weaving
and sewing our own clothes ...
Growing your own vegetables doesn't have to be more expensive than buying
them, especially when quality and freshness are considered. When you
consider the pleasure gained no price is to high to be paid. What do most
people spend on 'entertainment'?
We don't even have a telly.
I'm not advocating the simple life for everyone, far from it, but the cost
of a lemon or a cabbage has to be balanced against many other factors.
Indeed. There is as pharse 'opportunity cost' which applied to e.g.
veggie grow8ng would say something like
'I have a spade, a fork, and can shit in a bucket, and 1/4 acre of land,
am a psnsioner, and couldn't get a job if I tried, that paid more than
£5 an hour 2 days a wek, and I'd pay tax on that anyway'
'So if I simply dig my plot and plant £20 worth of seeds, and shit on
them regularly, I will get a fine crop of veggies that go in teh
supermarket for a few hundred quid every year'
'and my general health and attitude will be improved as well'
Obviosuly employing union labour at £30 an hour and uisng industrial
garered fertiliser, and jumping through EEC hoops would make it totally
uneconomic to supply teh local supermarket, but, its better than sitting
watching 'Trisha' every afternoon innit?
Thanks to everyone for your helpful replies. Looks like I'll be doing a bit
of experimenting. One thing's for certain - I'm not going back to paraffin
heating. I mistakenly bought a 2Kw one of those last year - what a
nightmare - not cheap to run either.
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