Relatively simple question about building a generator

[freethought202]

Hi, I'm interested in building a gas powered DC generator to charge a small 36 volt battery bank. I think a 500w 36volt generator would do fine, but my question is, what would be a minimum displacement necessary to turn the generator? Basically, the smallest, most efficient, gas engine to charge 3 12v,12Ah batteries. So, I need to charge 432Wh as little fuel as possible. I was looking into it and found that 1 kilowatt converts to about 1.3 horsepower and found an engine with a displacement of 10cc's which had an output of about 1.6 hp. I think my figures must be wrong or there's a factor I'm not considering because this engine is pocket sized I'm not even speaking figuratively. I only need half a kilowatt, but I don't think I can get that out of this engine, It looks to me like I would need more like at least a 25cc, but I want to get an opinion. Thanks in advance.

 

[Baluncore]

You have not specified the time you permit to charge the 36V 12AH batteries.
A power of one watt is one joule per second. You must consider the time it must take.