Reply #6: Estimates seem to vary, but your 10hp seems optimistic for highway speeds [View All]

[muriel_volestrangler]

eg this:

Cylinder deactivation works because only a small fraction of an engine's peak horsepower is needed to maintain cruising speed. Passenger cars require 25 or so horsepower to cruise at 60 mph while 35 horsepower is enough to drive a large SUV at that speed.

http://findarticles.com/p/articles/mi_m3012/is_5_184/ai_n6077801/

or this, which claims 'about 20'.

There's a calculator here, that you may find useful if you do build something. It suggests some typical values, and Wikipedia gives more. A fairly typical result for a modern small car seems to be about 10hp for 55mph, but more like 20hp for 70mph. It depends on the speed you want on the highway, then.

Remember that if the engine just generates power to keep the battery charged (which if you want to run it at a constant efficient speed it would need to, rather than directly driving some wheels), there will be loses in the generator/charging system, probably more than in a mechanical transmission.