Fish tank heater in rain barrels (passive heat)

Hello, I have a 10'x10' greenhouse that I am moving some rain barrels into to help me heat the greenhouse.(Currently there is no heat) I was wondering If I was to put electric fish tank heaters into the barrels, would the cost of electricity be worth the amount of heat produced? I think I will also need a small fan to move the heat around the greenhouse. My goal is to keep the greenhouse above freezing.
Thank you!
DW

Average Fish tank heaters are not usually more than 300w and they operate (in a home environment) in a range of maybe 70-90degF. Not very economical in the long run to operate on a continuous basis as would likely be needed in a barrel full of water in an outdoor environment with temps below freezing. Another option is a water bed heater wrapped around the barrel. I tried using an old 500w water bed heater on my 180gal Aquaponics fish tank in my 40-50deg house garage and it ran all the time to keep it in the high 60's (I didn't think to insulate the tank at the time). If the greenhouse isn't insulated as is typical, to heat it with electricity you would need something along the lines of a modified electric hot water heater element or hot tub heater of at least 1500w. Also when you go with electric heat of large wattage (over 1500w) it's preferable to use a 220v unit instead of 110v. Depending on how close the green house is to the electric outlet (unless you've actually wired it directly) don't try to run any heater on an extension cord unless it's a heavy gauge wire.

That said about electric heat. You could try to use a solar water heater if you ever see enough sun. But that might become more involved if the temps are below freezing often but it's doable. Or maybe a small wood stove (rocket heater or ?) and a heat exchanger (copper coil, radiator etc) using thermosyphon action or a small pump that would only be run occasionally to keep the water warm enough for radiant heat.
I'm considering these options for my greenhouse aquaponics tank heating. My avg temps bottom in mid Jan at 41hi 21low so I'm leaning towards wood fired heat with maybe solar as a secondary system spring and fall.

It all depends on how much money you want to spend and how much unattended operation you want. The possibilities are almost endless.

Here's some more info on electric heat. I did an experiment today with a 1000w inline 110v heater. I connected it to the output hose of the 300GPH pump on the aquaponics fish tank. It was in the lo 50's outside, my tank with about 160gal in it was at 49 deg. After running it for about 3.5hrs it was 57deg.I'm going to run it up to the lo 60's then see how long it takes to cool down. My tank is insulated with blankets and the top is not completely covered. I'm not trying to use it for radiant heat (Although there will be some as the water is pumped in to the grow bed gravel), but to keep fish tank warm. It might give you an idea of how much energy would be used with an electric water heater.

John, are you still using the inline heater?

Just used it once on both tanks. got them up to62 deg. it's been warm but going to get frigid. not sure it will keep up but might have tou use it. quite a draw on the power cord i use to run the pump

I was afraid of that. I'm going to keep fooling around with evacuated solar tubes and those copper heat tubes. I'm hoping that passive solar will work.

I'm having a hard time fabbing the copper tubes though.

here's a link to what I'm NOT having success making...

copper tube