Does anyone power their engine block heater with portable power?

The consensus seems to be that three hours is optimal, but no matter. Two would still benefit.

I've thought through a somewhat similar approach after buying a townhouse last year with no easy way to plug in. My idea was to use a battery and inverter to power it at home, and move the power supply inside for charging while I was gone. I concluded this was too expensive and impractical.

Here is the analysis I used, modified slightly to fit your circumstances: Let's assume a 12V battery for the jump starter, 2 hours charge time, and a 400W heater. Let's also assume a complete discharge of the jump starter. You wouldn't want to do that for the sake of longevity, but it makes the math simpler. The math suggests a need for 67 amp-hours at an absolute minimum: 400(W) ÷ 12(V) * 2(hours). Most (all?) jump starters have much lower amp-hour ratings, and their lead-acid batteries likely won't survive long with daily deep cycling. Of course, a less-than-complete discharge would require an even higher amp-hour rating.

Now, I'm not the most educated on things technical, so my analysis is subject to correction.

 

Jimbo, I think your conclusion is correct. In this case it is more economical to use the Prius engine as an engine preheater, or in other words, just get in and drive.

Tom