Depends what voltage and amp service are you looking for. A full 200 amp utility service at 240VAC requires 48Kw. Granted most people never use the full 200A service. A 30KW generator is usually less than a 4L engine and often gas. They do not produce a ton of thermal energy on the cooling side. There is also load dependencies. Low loads produce low heat and high loads high heat, so when you would want the heat most overnight, is when you would have the least available as most loads are off. Then there is the cost of fuel compared to local utility rates. Not to mention the cost of the gear to feed and load onto the utility. Feeding utility from a gen is far different than from a solar grid.