Faulty comparison?

I follow (and link to) a fair number of energy related blogs. One of them is Consumer Energy Report, which aggregates posts from a variety of journalists and energy professionals with various specializations. A recent post was entitled "High Cost Prevents Electric Cars From Penetrating the Market." In this post, Andrew Holland discusses his thinking behind the lack of sales of pure electric cars and the financial troubles befalling some of their manufacturers. However, he's also quite complementary of the Chevy Volt, a vehicle I've considered.

But in the comment section, Buddy says

I’ve also never understood, however, how I can run a 5K Honda generator for a minimum of 2 hours ,and occasionally 4 hrs, on a single gallon of gas, that hybrid vehicles cannot achieve the 100 MPG rate easily.

Is this a valid comparison? After all, I certainly can't drive my hybrid  Lexus CT200h for 2, let alone 4 hours on a single gallon of gas. I think looking at Buddy's comment with the Lexus as the representative is quite reasonable. It's the same drive train as the ubiquitous and iconic Prius hybrid and I've gotten 51.38 miles per gallon total in the 14 months or so that I've driven it.


I wrote a simple Wolfram Mathematica program to determine an estimate for the power required at an input speed and an estimate for the mileage to be expected. The program comes gratifyingly close to my actual mileage at the speeds I've checked so I suspect that the power requirement is likely pretty close, the wild card being the engine efficiency.

In any case, let's go on. Buddy's Honda generator has a maximum rated output of 5 kilowatts, but I'm very doubtful that the two hours to which he refers is at full load. Nevertheless, I'll use it anyway because... who knows? And I suspect the two hour figure is at closer to full load while the four hour figure is at a much lesser load.

5 kilowatts is 6.71 horsepower. I calculated that, at 55 m.p.h., my Lexus requires from its Atkinson cycle internal combustion engine (and, where required, the generator and electric motor) about 13.8 horsepower and achieves about 54 m.p.g. If I assume that, at that cruising speed, the Lexus is capable of 28% efficiency, then I'd be burning fuel to release heat energy at the rate of 13.8/.28=46.4 horsepower. This is 34.6 kilowatts or 34,600 joules per second. Now, upon burning, a gallon of gasoline releases about 120,000,000 joules of heat energy so this gallon should last 3,470 seconds or just shy of 58 minutes. A quick check: at 54 m.p.g. and 55 m.p.h., a gallon ought to last an hour!

OK, so we have that I need 13.8 horsepower, a bit over twice what Buddy's generator supplies at 5 kilowatts or 6.7 horsepower. Multiplying my one hour by that 2 figure, I'd think his generator should run for about 2 hours at 5 kilowatts if it was as efficient as my Lexus. I don't think there's any reason for Buddy to be surprised!