The cheapest source of power is conservation. However this can be carried to extremes.
Example: A 50 w plugged in laptop vs 150 w plugged in desktop. 100 w difference. That's 2.4 kwhr/day about 1000 kwhr/year. If your off grid power runs 50 c /kwhr that's $500 per year. In typical use 1 laptop lasts 3 years, 1 desktop 6. Two laptops - 1 desktop = pays for lots of electricity.
I've worked hard to reduce my power usage, but I keep bumping into a few unstopable objects:
* Satelite internet. Seems to be about 200 w continuous. Since we use computers 20-30 times a day in this house, turning it on and off is not practical. I suppose I could turn it off at night.
* Network infra-strucutre. 20 watts. 12 for the switch, 8 for the wifi.
* Fridge.
* Freezer
If you are living off grid, it may pay to get propane fridge and freezer. Or create a super insulated chest fridge. (Guy in Oz did that -- rigged up a thermostatically controlled power plug for a freezer to use it as a chest fridge, added lots of insulation other than the radiator side, and has a circulation fan that runs when the compressor runs. Claims some absurdly low power use. AND if you have a lot of thermal mass in it (Bottom layer of water bottles) then you can run it with fairly intermittent power. E.g. If the wind turbine+battery system handles all the constant tiny loads, and you fire the generator for an hour a day to top up the battery, run the table saw, and run the fridge.
Another way to conserve if you are building is to wire your house for both 110 and 12v. By using RV lighting, stereo, etc you can avoid the inverter inefficiencies. (Remember however that you may need BIG wire. 15A at 120 v = 1800 watts = table saw. 15A at 12 v = 180 watts. = lighting one room.)
A final possibility is 'single utility' If natural gas is already in your area, getting connected to it is often cheaper than getting connected to the electrical grid. NG is usually cheaper per energy unit than diesel, and it burns quite cleanly.
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I ran a few numbers using commercial wind turbines grid connected (No battery banks) and assuming that surplus power was bought at 6 c/kwhr.
Generator sizing was $3/watt. Capacity factor was 0.3
It worked out to being about a 5% ROI. So if you have a good site, it may be worthwhile to put up as much as a MW of capacity. (In Alberta there are different rules for under vs over 1 MW.)
The guys on this forum who can build 20' generators on 100 foot towers can probably do better, assuming they count the build time as a hobby.
As others have pointed out, if hooking up powerlines is too expensive, then it can pay for itself quite quickly. One acquaintance claims that small scale IC engine power generators will run you $1/kwhr, 2/3 of that for fuel. I think he is high, but he's in a position to know better than I do. I figure between 25 and 40 c assuming a listeroid engine + generator, and a thermostaticly controlled shed to keep it warm when idling.