Update:
The battery blanket/warmer came in the mail yesterday from
Auto Parts, Motorcycle Parts, Car Parts, Truck Accessories & Parts, Auto Accessories - JC Whitney seven days after I ordered it. The battery warmer specs are 50W, 125VAC, 28inch by 5inch with a 32 inch electric cord. But measured power consumption is 43 watts. The circumference of my tractor battery is 28 inches. What a happy coincidence!
The info card that came with the battery warmer/blanket says that:
At 80degree F 100% battery power is available and 100% engine power is required to start
At 32degree F 85% battery power is available and 165% engine power is required to start
At 0degree F 65% battery power is available and 250% engine power is required to start
At-20degree F 45% battery power is available and 350% engine power is required to start
In case you're wondering, I have a new battery that is fully charged (1.28 specific gravity) and the temperatures rarely go down as low as zero degrees in these parts. But I hate to chance going out there late in the winter and being unable to start the tractor.
Installing the blanket/warmer was a five minute job which takes a half hour when you have to go looking in the garage for pliers, snips, cable ties, extension cords. I have the plug fastened to the grill guard with about two inches showing. That'll make it convenient to plug in the extension and minimize damage from stray branches etc. The power extension cord is a little longer than necessary so that I could loop it around the steering wheel. That'll keep me from driving off with it still connected.
I plan to use a temperature controller set to a 40 degrees F trip rather than a timer or leaving it on all the time. More about that when it comes later this week.