Letter Re: Sizing a Retreat AC Power Generator

Jim –
Great blog! I wanted to point out an important calculation everyone missed – internal combustion engines produce less power at higher elevation. Generators are (of course) rated at sea level. It’s important to de-rate generator capacity by 3.5% per 1,000 feet of elevation or your generator will be undersized. (A 5,000 “label watt” generator is [effectively] only a 4,000 watt generator where I live at 6,000 feet.) Density altitude on a warm summer day can easily be 2,000 feet higher than that. My rule of thumb: after sizing for load, size generously for elevation or you’ll be buying twice. Hope this helps everyone…
Other food for thought: You don’t need to run all your big loads simultaneously. If the grid stays down, it’ll be a blessing just to have refrigeration – it doesn’t need to be like today where we run everything at once while blow-drying the dog! There’s no reason you can’t shut off the freezer if you need the well pump. The simplest transfer switch allows you to control power to various loads, and this allows you to use a smaller generator to accomplish everything. My genset is home built using a Listeroid (Lister clone) diesel engine and generator head purchased separately. This generator (significantly oversized to run a MIG welder, lathe, mill or compressor/plasma cutter combo) cost me less than $3,000 including truck freight and welding up a stout steel frame (probably $4,500 now, given the weak dollar, steel prices and current shipping rates). Based on decades of British Empire experience with these beasts in third world countries, I expect it will give 30,000-to-50,000 hours of service with minimal maintenance. It gingerly sips fuel and is easily operated on biodiesel or waste vegetable oil without modification.
Regards, – Fred H.