I'm thinking of getting a propane-powered whole-house backup generator with
the automatic switch on when the power fails. I went through the electric
bills for the last twelve months and found that we used an average of 54.9
kilowatt hours per day or about 2.3 per hour. Quite a bit, but we have a
well with pump and my wife keeps the greenhouse warm all winter with an
I have a little gas-powered generator rated at 6,500 watts that I use to
power the pump, boiler, refrigerators, etc., but it would be nice not to
have to run out in the rain to start and fuel it during an outage.
I've read that an 11,000 watt unit should be enough to power a house, and we
wouldn't be using the electric oven or dryer during outages. That would be
almost 5 times the average usage, which should give coverage for start-up
Does anyone have any official formulas for this sort of thing?
The determination is a bit of a black art but for a good ball park idea
check some of the off grid news groups and blogs. (you can trust but best
to verify independently what you're told)
First identify the mission critical requirements. Then the start and run
requirements. With that you can guesstimate the require size of the gen
set. Rotory gen sets have ~ 30% surge capacity, inverter gen set system
less or not at all.
Also if you're careful and a bit anal retentive you can balance the
start/run loads and do more with a smaller gen set. As example a well pump
can be run and shut off when the reserve tank is full or left off if you
don't need running water except for select times of the day (cooking washing
This can be done with a pencil, paper and a clock or with an automated
system (as a function of how much you want to spend).
Depending on the local circumstances you might benifit from salavging the
waste heat for your wife's green house.
My kids have a 10KW (mil surpluses) with a 3K battery (salvaged from
communications towers) backed dc to ac inverter. Duel fuel NG with a 250 G
LP back up.
They can run, depending on load, the innverter for 3+ hours before a
recharge. This to reduce the need for running the gen set or starving the
neighbors. Works great for the microwave, TV/internet and the fridge.
We have provided the neighbors within safe range of a HD extension cord
power (enough to run, in cycles, the fridge, window unit (this is N. Texas)
TV/internet and a few lights) Key benefit, aside from being neighborly, this
has cut the complaints about the gen set noise to zero.
The neighborhood posse sets up BBQ grills in the street in front of the
house (we provide trouble lights) with tables/benches pulled from the back
yards so we have a community cook out in the evenings. But not a power
outage single event as this happen at various times of the year just for fun