How much does it cost to heat a greenhouse?

I'm considering getting a greenhouse for my cacti however one thing I'm trying to work out is a rough estimation of how much it would cost to heat the thing it's for far from from frost hardy cacti so the mimimum I'd be happy at would be around 12°c in winter

 

Try putting

greenhouse heating cost calculator

into Google - quite a number of greenhouse and/or heater companies have Calculators on their websites. You might need to try several to find one that suits your "shape" of greenhouse AND the material it is made from - as not all do everything IME

 

problem is alot of them want dimensions which I can't possibly know until I own and measured one

 

Okay so I've hashed out a rough estimate from this site http://www.hartley-botanic.co.uk/greenhouse-power-calculator.html

and I get an output of 747 watts for a standard 8x6 greenhouse with a maximum temp of 10°C inside and a minimum temperature of 2°C outside now maths not being my strong point how do I get from that to a cost?

 

it may be worthwhile to investigate the solar PV greenhouse panels

http://www.invest-in-solar.co.uk/

there are newer orange coloured panels aswell:

http://www.cernunnos-homes.co.uk/wp-content/uploads/Greenhouse.pdf

 

Heat loss is dependent on surface area (of the "glass") and how thermally efficient it is ... so I'm afraid you need the dimensions

I found these - surface area for various footprints - but it will obviously depend on height to eaves etc:

6' x 4' = 195
6' x 8' = 273
6' x 12' = 352
8' x 12' = 462
8' x 16' = 555

Calculator: http://www.littlegreenhouse.com/area-calc.shtml

Conversion of Watts to BTUs (some calculators still use them ...)

1,250 watt 4.3 Btu
2,000 watt 7.6 Btu
3,000 watt 10.2 Btu
Then you need the average low temperature for the winter months

http://www.weather.com/

use the SEARCH to find your location, then pick MONTHLY in left menu, then at the bottom of the "calendar" choose AVERAGES

and the material (for the "heat loss value") your greenhouse is made from

Then use this Calculator (it has a heat loss value table) (needs temperatures in F)
http://www.littlegreenhouse.com/heat-calc.shtml#cost

Slight problem I have with this is if you say you want to keep the greenhouse Frost Free, you put in minimum temperature of 0C, and your average-minimum-temperature is 0C or above, then it will say zero cost ! and clearly there will be some frosty nights, and the occasional -5C or -10C that will mean the heating is on flat out all night.

 

As an estimate I would say "flipping expensive" it costs to much to just stay a couple of degrees above 0 to keep a constant 10 well it might be more cost effective to heat a room in the house if possible depends on circumstances and value of the cacti,but trying to achieve a hot house in these times

 

Yes, good point; well worth looking at the cost of artificial lighting "indoors" as an alternative. A well insulated building will be streets better than a glass house, and any "heat" that you generate, from the lights, will contribute to the overall house-heating, and reduce, somewhat, your conventional central heating costs.

No idea whether that is a good idea for Cacti?, but I've certainly had fun overwintering stuff under a Metal Halide lamp the last two winters (but BUGS are a major nuisance, so anything coming into the "den" needs to be well sanitised first)

*Dim* personally I think this is a waste of time & money I'm afraid. The amount of "sun power" you get in the Winter (e.g. for Solar Energy generation by PV / solar hot water) is 1/10th what you get in the Summer ... so you'll have energy in the Summer, and not much in the Winter - the time it would be most needed for heating.

The Victorians attacked this in a number of ways: half-brick greenhouses for better insulation, excavated greenhouses (only the roof above-ground) to use warmth from soil below, and minimise surface area exposed to the cold air, and using rotting manure as a heat-source (which is incredibly labour intensive to renew every month or two ...) - plus they had plenty of working horses supplying the "fuel" for free

Best starting point today is to consider the insulation first & foremost - it will be cheaper to reduce the heat requirement, than pay for the heating fuel itself. Air tightness, Bubblewrap lining (or even double glazing, or multi-layer poly-carb - restricts light though) and so on.

For alternative energy a Wind Turbine might work - more wind in Winter than Summer - but you need a clear, unobstructed (by houses & trees) view towards the prevailing wind, absent that the economics don't stack up, the turbulence from trees & buildings wrecks the ability to keep the turbine running relatively continuously and thus efficiently

 

so on to the next problem... how do I get electricity to the greenhouse without shelling out £300 or so to come and get a sparky to do it? seems it's illegal under part p of some law

 

Use a Bottle Gas heater? More expensive running-cost than Electricity, because it is less thermostatically controllable, but avoids all the wiring issues. (Some condensation issues, but less than Paraffin).

Probably a non-starter, but just through I'd mention it

 

Well I was trying to work out how safe it would be to just have an extension lead with a waterproof plug at one end and an RCD at the other for 5 months of the year

 

You need waterproof cable as well as a waterproof plug, my O.H. did it for me and we ran the cable through narrow tubing ,to lessen the risk of damage to cable and fixed it to fence with pipe clips.

 

I don't think that using averages will work, because all the warm temps over the required minimum would negate an equilevant amount of the heating costs for when temperatures were lower than the minimum.

 

thats the old method ... the new solar PV panels work on light (and not sunshine) ... so they do work in winter, but work at their best in summer

obviously, daylight is less in winter, but summer also has very dark cloudy days .... thats how I understand it ...

one of my clients installs these, and from what he told me, the farmers are the guys snapping them up ... so if that is the case, there must be merit in them

so, if that is the case, it may be worthwile investigating further if you are keen to set up a good greenhouse

this explains the winter questions

http://www.solar-energy-panels.info/solar-pv-panels-work-in-winter.php

 

I`ll agree with that I dont see any sense heating the air in a greenhouse if the insulation value is almost nothing. You`re better off trying to maintain a few extra degrees in the soil temps to keep the roots happy.