What can I use to raise temp by 10-15c in my shed grow room???

So definitely, the cold is stunting my plants, I tried using a washing up bowl of water & an aquarium water heater that I had, thinking warm water would generate some sort of heat n maybe raise the temp a little, It did, about 2 degrees C. but it is now -3 outside! so not much good.

I need some sort of small & cheap heater that can raise the temp by around 15 degrees C.

What sort of things are available? I'd have it on 24/7 for the next few weeks until they finish, heating bills are already high for the house so one that is cost effective would be great!

Thanks guys!

oil filled radiator maybe or a little plug in heater. i've got a cpl of the little heaters range from 500watts to 1000watts. either will work and are fairly cheap unfortuantely power consumption may be an issue. but there are several to choose and may find a few that are fairly cost effective. got a decent selection at the local home improvement stores or check out amazon

HPS EVERYWHERE! ;D
try seal off what you can
turn down ventilation?