To throw in another underlooked option think diesel. I realize nationwide that home heating oil is fairly rare compared to natural gas but it is quite popular in New England. Many folks do not realize that now that HHO has gone low sulfur that its effectively interchangable with diesel fuel. Therefore many folks have 275 gallons tanks of potential generator fuel in the basement. Diesel is the highest BTU content fuel so more power with less fuel. Diesels also tend to have a lot more torque to ride through load changes so they work better with well pumps. These sudden loads are called step loads and lightweight natural gas or propane generators generally have far less torque and do not respond as quickly to step loads. The military buys a lot of diesel generators and they tend to be heavy but very beefy. They are designed to be repaired and run for long hours but rarely are so they end up in the surplus market. The trade off is diesels are usually loud. The military did produce some fairly quiet inverter generators (MPE831A) but they are 120/240 volt 3KW units so may be too small for some folks. For folks who really need a long run backup generator the surplus military generators are the way to go.
Of course many off gridders have the old low speed Lister Diesels. Many are decades old and so overbuilt that they will outlast the owners. The Brits stopped making them yeas ago but the Indians still make clones (listeroids). The EPA stopped allowing them to be imported but there is a firm in Canada that will sell you an air compressor based on the same parts that reportedly can be readilly be converted to diesel engine.
Of course many off gridders have the old low speed Lister Diesels. Many are decades old and so overbuilt that they will outlast the owners. The Brits stopped making them yeas ago but the Indians still make clones (listeroids). The EPA stopped allowing them to be imported but there is a firm in Canada that will sell you an air compressor based on the same parts that reportedly can be readilly be converted to diesel engine.