[modproductions]

Ok let do some MATH

Formula used:
1 calorie = 4.18 watt/sec
1 calorie heat 1 litre up 1 deg Celsius
Correction : 1 calorie hean 1ml up 1 deg
1 kilowatt = 1000 watts
1 hrs = 3600secs
1 gal = 3.78 litres (I round up to 4 because of the tank heat inertia)

My guess is that it is not worth it, the heat in 50 gal of water contain not enough energy to make it viable.

50gal = a bit less than 200 Litres

let say that the inside of the garage is at 0 Celsius (37F) and you bring 200 Litres of water at 100 Celsius (212F)
That 200 litres contain (100*200)=20,000 calories of potential energy (heat)

That water tank will contain:
20,000 calories =
83,600 watt/sec; or
83.6 kilowatt/sec; or
0.0232 kilowatt/hrs

We pay electricity in kilowatt/hrs
In US/Canada, the highest residential electric rate is Connecticut at 18.10 cent/kilowatt/hrs, the US average is a around 11.50 cent/kilowatt/hrs and the lowest is Province of Quebec at 6.8 cent.

0.0232kilowatt/hrs * 18.10 cent = 0.4 cent
So in that 50gals water thank that you had to buy and install and carry all the time, you accumulated 0.4cent worth of heat. Or you could start a 1000watt electric heater for 2 minutes and have the same amount of energy.

Correction, 1 calorie heat 1 ml of water, not a liter. Just multiply everything by 1000