Why don't you make do with the smaller generator but improve its abilities with some batteries as well? You won't have to add enough batteries for all-electric range to cover your full commute, only maybe 1/4-1/3 of your commute. This will keep your generator running near peak load to top off your batteries but minimize the reduction in acceleration and possibly top speed that a 10hp limit would give you. This is a serial hybrid configuration that folks have taken to calling a "range-extended electric vehicle."
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