Not a lot, but depends on the wattage.And whether it has a thermostat.
Just think of it as a light bulb. If it is rated at 100watts, then if it is on for 10 hours constant 1kw of electricity is used. Electricity is priced as KW/hour and depends on who you get it from, but average is 10p.
So a 100 watt propagator at full power costs 10p for 10 hours. A 500 watt one would be 50p, etc,etc.
With a thermostat, it would be less as the propagator wouldn't be on all the time.

Thanks that's a big help :)) I think I'll get myself one !

or maybe a heat mat instead if you have a lot of plants to play around with.
That's all I use. Just fill up your seed trays, put a lid over, place on the mat and that's your propagator.

If you decide to get one go for a thermostatically controlled one. much more versatile. I've had a Stewart one for 10 years now. Never let me down and never noticed a hike in the electricity bill.

Typical retail/domestic versions aren't very high powered - maybe about 40-50 watts at most for one with a variable thermostat. I use mine (8 watts - doesn't have a variable thermostat) with a timer switch on and off for half an hour at a time - or take a bit of time to find out how long it takes to warm up/cool down and then get the optimum switching times.
(8 watts doesn't seem much in context with old 100 watt light bulbs but I've used low energy lights bulbs for 30 years - apart from my kitchen I don't have lights over 6 watts and I wouldn't deliberately leave a light on for days at a time)

I love mine Hywel, I have had different ones for many years, the thermostatic are the best I agree, also look for the plastic covers to have a vent you can open and close, can help stop damping off

Thanks for your answers. I shall take it all into consideration when I buy one :o)

I agree with Pam,Hywel..Thermostatically controlled,mine has 5 settings,which is so useful at different times,and a vented lid is a must..You may still have to wipe the condensation off the inside of the lid,at times,as you don't want it dripping on your seedlings or little plants....I would get the biggest one you can accommodate...Mine is about the same size as a shelf on my Growhouse staging.and fits nicely on the top shelf,in the Conservatory..Plenty big enough for my needs :o)

I don't like those thermostats that have settings.
I prefer the ones that are based on actual temperature. Much more accurate.
As to venting, if you have a propagator with one lid and have seeds set off at different times, that's when these all in one type propagators fall down as opposed to a heat mat whereby you can take individual lids off the seed trays.
Heat mats can be made to any size. I have 2, both 6 foot by 2 foot. And of course, they can be folded and stored away when not in use.

.If I had two heat mats the size of yours,I would need an extension..We don't all have the space ,or a big garden to fill,or room for a Greenhouse.As much as some of us would love more garden,we have to accept our limitations..
I have had my propagator for 14 years,and never had a problem ,either with damping off,or germination..A good idea having heat mats,so it is helpful advice to those
who can ..

My greenhouse is 8 foot square. Not very big, but if you had an 8 foot by 6 foot one that's one heat mat down either side ( I also have a heat bed along the side with cables )

You don't actually need a greenhouse either. One of the best onion growers I know is 75 years old, has just had a heart bypass to go with his hip replacement, and has invested in heat mats and T5 lights to go into his spare room so he can look after them more easily.

That would take up about half of my back garden ! :o)
Nice to hear that gentleman still has the will and the energy,to enjoy his gardening..I don't think our Grandson would take kindly to sharing our spare room,with heat mats,when he sleeps over though :o)

I also have a neighbour that is keen on showing onions. Couple of years ago they put in a downstairs shower in a room 8 foot by 3 foot, with under floor heating. Guess what that shower area is used for now during the winter :)

I'm with you Bloomer - I couldn't fit a greenhouse in my back yard either - I stretched to a polythene collapsible cold frame to protect pot plants last winter. A propagator is my little luxury to get a few seeds started.

My Shower base comes in handy too,SG..for cuttings when we go away in the summer..,a big seed tray,with well watered Capillary matting,and they were just fine after two weeks holiday :o)

We have to manage to find ways and means to compensate for lack of space,Urbanite..We do have an 8x6 shed,so that takes up quite a bit..and I can just about squeeze in a cold frame,and another plastic Growhouse on the path at the side of the Bungalow,so it suits me just fine :o)