# Costs per kWh

Dear Physics friends!
I would be very grateful if you could help me. I am currently working on my master thesis and I have the following question:
The overall annuitized capital costs of a distribution grid amount to: 900 USD per kw and year.
How do I get the costs per kwh?
Thanks a lot!
Best,
Marianne

Divide by the number of hours in a year.
That's 24 x 365

Thanks a lot for the answer. helped me a lot.

russ_watters
Mentor
$.1/kWh? Seems high. Where'd the data come from? Redbelly98 Staff Emeritus Science Advisor Homework Helper I pay 0.17$/kWh, and I'm in central NJ not too far from you Russ.

russ_watters
Mentor
I pay 0.17 \$/kWh, and I'm in central NJ not too far from you Russ.
That's altogether, though - the OP said the distribution charge. Assuming that's what was really meant, the distribution charge around here is roughly 1/3 of the total cost of the energy.

That said, now that I know the OP is in Africa, my frame of reference is gone!

Redbelly98
Staff Emeritus
Homework Helper
That's altogether, though - the OP said the distribution charge.
You're right, I missed that.

Great! thanks a lot for your help!

how many households can be supplied with diesel generator

Hey,
If I have a diesel generator with a capacity of 300 Watts and households consume on average 50kWh of electricity per months (Kenya), how many households can be supplied with such a generaor?
Thanks a lot.
Best,
Mari

russ_watters
Mentor

Welcome to PF.

watts * hours = watt-hours

So 300 watts for 1 month is 218 kWh. So you could perhaps serve 4 houses with a 300 W generator. But, whether you can really supply 4 houses depends more on the peak usage of each house than the total monthly usage. 300 W is not a lot of power (it won't even run a coffee/tea pot), so odds are that you can't even supply a single house unless the generator is charging a bank of batteries that can then provide a variable load to the houses.

Another way to look at it is that your generator will run five or six small electric light bulbs at the same time. (5 of 60 watt bulbs).

A fridge takes about 30 to 40 watts so you could run maybe 8 of them together. (Not at the same time as the lights!)

A computer takes about 150 to 200 watts - you might be able to power two if you turned off the lights and the fridge. A television is about the same or slightly less.

So really, one or two houses might be supplied if they were careful to share the electricity. (One or two fridges, one television and a couple of lights all on at once would be the limit)

Thanks a lot for your answers. Does is mean that with a 300 Watt generator I can produce 300 Watts every hour? So if the generator is mainly used for lighting between 7pm and 10pm, around 3 households could use around 2 electric bulbs. This means then that the households consumes 360Watthours per day, 10800 (10.8Kwh) Watthours per month.
The generator could then supply 27000 Watthours (27kWh) per month.
Are these calculations correct?
Thanks a lot!

Watts is a measurement of continuous supply or consumption of power.
That doesn't mean 'over a period of time' it means 'all the time'

A 60 watt bulb, uses 60 watts of power while it is on and it needs a 60 watt generator to supply it.

It's proportional to the rate you are burning diesel. (pints or gallons per hour)

The total amount of power supplied over a time period is measured in watt-hours (or kilowatt hours). That is proportional to the total amount of diesel you burn in that time.

So, 3 households, using 2 bulbs each would use (assume 60 watt bulbs) 6*60 = 360 watts.
Say we turn off one of the bulbs - that gives 300 watts - your generator could handle that.

In which case, for 3 hours each day your generator is supplying 300 watts - so that's 300 * 3 = 900 watt-hours per day. Over 30 days that's 27 kilowatt-hours per month.
(The kilowatt hours figure isn't useful for anything - what matters is that 300 watt limit on the generator)

Great, thanks a lot, now I understand it. Do you maybe know approximately how much diesel a generator of that size needs per hour? Is it common to use a battery with diesel generators

Wikipedia says: a modern diesel plant will consume between 0.28 and 0.4 litres of fuel per kilowatt hour

So your 300 watt generator will use about 100ml (0.1 litres) in an hour in theory - I would guess it may be twice that figure in practice.

It isn't easy to use batteries for a couple of reasons.
First, the output voltage of the generator will probably be your local mains voltage (I think that's 240 volts in Kenya?). And it will be AC. (Alternating current)
Batteries (car batteries) are usually 12 volts DC (Direct current).

Although you can charge a battery from your generator, using an ordinary battery charger, the battery won't be able to power your house appliances.

You could set up low voltage lighting using car headlight bulbs running on the batteries - but don't try to connect these type of bulbs directly to the generator - they will blow.

The most efficient use of your generator would be as russ-watters suggested - charge some batteries from your generator and supply those to people to take away and run their lights. You could keep the generator running all the time charging enough batteries for several households to take away and use, bringing them back when empty to change for a fresh one.

Batteries won't let you run many things but they will be ok for lighting.

russ_watters
Mentor

One little quibble:
A fridge takes about 30 to 40 watts so you could run maybe 8 of them together. (Not at the same time as the lights!)

A small fridge may average 30-40 watts, but when it is actually running it will draw upwards of 1500W. A 300W generator will not run a typical fridge without a battery to smooth the power draw.