Thanks, this is what I needed to see. Does she have electric or gas for heat?I think your numbers are probably pretty close.
My mom's 14kw Generac ran for 9 days straight during the polar vortex icemageddon and IRRC it was around $300'ish. It was the only time it's ran for more than and hour or so, and she's elderly and was undergoing chemo at the time. It was a godsend for her and if it cost $1000 she wouldn't have complained.