Hmmm, I was blogging about this very subject last week.
Where I am in the SW UK we have the most expensive mains power.
10.55 pence per kWh is the cheapest I can buy it.
I have a original working start-o-matic, it will burn a gallon every 4 to 5 hours at full load, eg 2.5 kW
4.5 hours per gallon (imperial) = 1 hour per litre.
so litre price of fuel / 2.5 = cost per kWh in fuel alone.
BTW, I am not going anywhere near the 24/7 availability of grid power vs genny power.
pump diesel is 90 pence per litre, so 90 / 2.5 = 36 pence per kWh
red diesel is 45 p a litre, so brings it down to 18 pence per kWh, in line with most electricity retailers tariff for the first 10 kWh etc
10.55 pence (inc VAT) that I pay per kWh x 2.5 = 26.375, so unless my fuel is less than 25 p per litre, even leaving out genny maintenance, I'm losing money.
If you think you can turn even free chip oil into decent fuel for less than 20p a litre you are lying and not including the cost of collecting it, your labour, consumable, energy used, etc etc etc.
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Wanna know where it really falls down?
This numbers above are based on running the genny at peak efficiency, I use about 13 kWh per day here, so that means running at peak efficiency and 100% output for 6 hours and charging battery banks and inverters which then provide me with 24/7 power.
Minimum £1500 for decent battery bank and inverter and charger, more like £2500 to do it properly so I use house power exactly as I do now, like I can turn on anything I want when I want for as long as I want.
Even if my fuel is totally free, my genny miraculously never needs any maintenance, I still need 5 to 6 years to show an ROI on capital costs of the battery bank and inverter alone, add genny, battery maintenance, etc make it 8 to 10 years.
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Wanna know wny else it ain't even a dream?
I can buy economy 7 power from the same company for 4.5 pence per kWh, and charge my battery bank that way, that leaves 12 pence for every litre of fuel used, to cover fuel, making fuel, maintenance and materials, my time and labour, etc etc etc.
Can't be done.
ECONOMIES OF SCALE, you CAN'T BEAT A BIG SYSTEM on efficiency.
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IF you can get 7.9 pence per kWh from the grid, and you can buy at 4.5 pence per kWh, then you have a margin of 3.4 pence per kWh, call it 1.4 by the time you lose efficiency in the charge / discharge cycle, so you have 1.4 pence per kWh left to repay the capital investment, say £2,500, and show a profit, so you have a max of about 30 kWh per day available to you, so 30 x 1.4 pence = 42 pence per day, 150 pounds a year, 16 and a bit years to show a profit, just about in time to replace all your capital equipment.
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Oh, but I am cogenerating so I get all that HEAT argument comes next.
I'll be generous, for every 10 kWh or usable elecricity you generate, you can reclaim another 10 kWh of useful heat, and I am being really, really, really generous, and not going anywhere near mentioning that the equipment to do this just quadruples the capital cost and ongoing maintenance of your genset.
I use 13 kWh per day of electric, so lets say I can also get 13 kWh per day or useful heat energy.
Like I said, LOTS of capital intensive ancilliary equipment and ongoing maintenance to do that, and at 2.5 pence inc VAT which is what I buy mains gas for I'm saving a whopping 2.5 x 13 = 32.5 pence per day, or 120 quid a year.
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shoot the messenger if you like, but run the math, there is absolutely no way to gen/cogen/trigen and do it for less per kWh than the grid, less capital than the grid, less maintenance than the grid, less time than the grid, less enviornmental impact than the grid.
as long as you have a hole in your ass you will not beat the grid on cost or enviornmental issues, simply because of economies of scale.
if you wanna do it because you live 30 miles from the nearest pylon great
if you wanna do it because you just wanna do it for kicks great