I did test it and it appears to cost 1.5% in power. About 25 watts at high speed.
I also took it apart to figure if it really is 5000 va and 22.7 amps constant run.
I estimate it will do 1 unit at 1550 watts 24/7/365 with zero issues.
So for 1 unit in a garage it is okay it should protect the gear and last but it won't last doing the 2 units it is rated for.
Keeping the unit low spec will also minimize the power loss factor. 1.5% is excellent, and the price is right as well.
The Larson transformer I was looking at was 11KVa (TX2402081P11KVA) 240v to 208v (max 53 amps) and costs about $600. I could run about 5 S15 off it with a solid safety margin but it would require extensive rewiring of my circuits. Not worth doing for a couple months of heat at the house.