Looking for "Block Heater at Work" solution

I have a block heater (EBH) installed on my 2007, which helps my morning commute to work. I do not, unfortunately, have access to an outlet at work to warm up the engine block before my return home. I'm searching for off-the-grid solutions; the EBH needs roughly 400 watts of power.

Solar panels do not appear to be an option. While I do park on a roof, making solar worth exploring, the maximum afternoon wattage appears to be in the neighborhood of 25-50 watts..........assuming ideal conditions and quality panels.

A 1000 watt generator is an option, although new generators are expensive. I've found a used 1000 watt for as low as $150, but those suckers run on gas...........and I am unsure whether there would in effect be any total gas saved by substituting one "generator" (my Prius) for another (Honda 1000 watt).

So anyway, I thought I'd post this to comb the combined expertise of the Prius community. Thank you in advance.

What I really want is a battery or "capacitor", which can plug in at home, capture 400 watts, store it, can be plugged in remotely later, and deliver those 400 watts remotely to my EBH. I suspect I may have just invented a toy that does not yet exist . But maybe not.