This is turning into a REALLY interesting discussion! I use LESS than a gallon of gasoline a day (maybe 1/2 gallon tops usually) running my generator, maybe a gallon max on high usuage days like washing every textile item in the house or something). Yeah my late mother NEVER ran the A/C for exactly that reason. Got to the point she wouldn't run the (gas) furnace either. This got bad once she was on oxygen...don't want to use the wood-fired fireplace with that going on heaven forbid there should be a leak and you get an oxygen-enriched atmosphere in the house: BOOM!
I have a wood-burning stove and I'd say it's fairly efficient. Turned way up (airflow wise) it goes through a load of wood quickly but it will easily last the night...though if I get up in the middle of the night I jam more in anyway. I'm not sure how much I use each winter but I can tell you it's probably 10-15 cords-ish. I don't even run it from like April-October though since I have a diesel stove heating my hot water which contributes heat to the house as well. And I'm of the "Just throw on a sweater" mentality! I see others running their woodstoves in June and think "WTF?" Here it's probably 59-60 degrees farenheit in here, maybe 50 outside, so not bad. When I do have the woodstove going I keep it at 70-75 minimum LOL.
For me the power hogs are the washer/dryer (not so much even the dryer since it's propane), microwave, toaster oven (those two REALLY rev up a 2Kw generator), and power tools. (Chopsaw, skillsaw, shop-vac, etc. But those aren't used as often.
I know people that have solar but I don't think anyone never has to run a generator, though people can easily go multiple days without running one. It sure helps though! I had/have a nPower 400w 12v wind turbine but it's since died (siezed up). Not huge but it made a difference...although there were never days I did not have to run my generator since another power hog is a chest freezer, my inverter will run it but it kills batteries quickly. Doesn't draw that much but it's a prolonged load...I run things like a chopsaw, toaster, etc. on my batteries/inverter and although they draw in excess of 1500w each on a 2000w inverter they're only on for very short periods.
I have a propane stove.
As far as at the time the place was built, my house has been built at (I counted) TEN, yes TEN different times! (I know, crazy, right?) I have black, white, and yellow romex ranging from 70s-now. I'd probably wire the water heater straight to the grid but plug my inverter/charger into the house so it would kick in in the event of a power outage. Some of it's going to be redone in the next year or so though so there SHOULD be a #4 wire going into my panel! So it will likely be closer to code than it is now.
At the rate I run my generator it's probably about the same as some people's commutes sittng in traffic for 1.5-2hrs each way with the car idling the whole time LOL. (If I start driving I'll turn the car off when stopped in a road construction zone, etc; my mother would always do that). I use power pretty sparingly when not running my generator to make my batteries last longer though...one light on in the whole house, ONLY in the room I'm in, etc.
Oh, I'm very much a CFL user for that very reason! (Gag all you want LOL). If I redid ALL the wiring I'd make as many things 12vDC as possible: lights, radio, etc. and only turn on an inverter when needed. So close to your idea there. I have a 2000 watt inverter and in theory with a nice big battery bank and lots of solar panels and maybe some wind turbines so in theory I could use up to 2000w of 120v in ideal conditions...after that I'd have to start my 10Kw generator. I intend to wire up my water heater element to that generator someday so if it's going it's giving hot water too. I've even contemplated putting it's radiator in the house somewhere.
I wonder if your grandparents' panels still work though?