kWh to KW Made Easy?

I didn't say that!

me neither.

That was never five minutes.

Nope.

The kWh is a unit of energy equal to 3,600,000 Joules. The kW is a rate of doing work equal to 1000 Joules per second.

So to get an average consumption from the bill, take the kWh used and divide them by the number of hours over which they were used, and the answer is in kW, meaning the average rate of consumption of energy over the bill period.

Example:

1000kWh used over 90 days is equivalent to an average continuous consumption of 0.463kW.

SO if I used 1,942 Kwh between 07/06/2010 and 08/04/2010. 29 days divided by 1,942 kWh = 67 KW average per day, continuous consumption? What size solar system or fuel cell would bee neede to power my home, according to the abouve figures? One quote was for a fuel cell that would produce around 3 kW, for $10,000. How many of these units would be required to power my home?

Thank You, you have been most helpful to me in a way that even I, a novice, could understand. Thanks again.

It's important to differentiate between 'quantity' and 'rate'. kw is the rate at which energy is used or generated, whilst kwh is a quantity, 1kwh is the amount of energy used if it is consumed at a rate of 1kw for 1 hour.

No, 1,942 KwH /(29 day * 24 H/day)≈ 2.8 Kw being the average power usage during these days. So a 3 kW power source will only have 200 watts of power in reserve when you are actually using your average power. Likely your peak power usage will easily exceed 3 KW. Now what your actual peak power usage is not known from the data you've given. What added conservation or limitations you could set on your house is also not known that might both reduce your average and peak power usage. Also the average power usage will seasonally change depending on your region.

Sizing a stand alone or supplemental power source is not a simple exercise. Many on site conditions should be considered by a qualified, licensed firm able to examine the site.

the simplified approach would be summing-up all your power consuming components, use a +25% of total load connected for future growth..then SWOSSHH!!!!..like magic you can now decide your power source..you can't use your energy consumption as basis for your calculation but the total connected load itself...this is for instances where all your components will be used simultaenouosly...

Let's look at this from a practical perspective... unless you have large sums of money, in which case practical goes out the window...

While I applaude the notion of going OFF GRID... you are proposing to spend in the neighborhood of $15k to $20k in capital for this project. Assuming you spend $1.5k a year in current electric usage... your ROI (Return on Investment) would be 15+ years (especially if you add the maintenance and installation costs associated).

In 15+ years, who knows where our technology will be... (remember our cell phones 15 years ago?) Doesn't seem like a good gamble to me, perhaps because I don't have large sums of cash to throw at this.

Sorry gents... just the Consultant coming out in me...

You forgot the consider what the grid tie in will cost

in some areas this can far exceed $20k....

Have you estimated the increased cost of electric, natural gas etc. over the next 15 years?

A concern only if the cost/benefit analysis leads to an uncertain result. But you are correct that if this investment return is not expected to happen for X number of years and the decision point happened to be just X-1 then anticipated increased electricity costs may tip the scale. Similarly though maintenance and equipment failure costs could move the return on investment to much later in time than this quick estimation. Remember if the return on investment is 15 years down the road, then all of the parts should be designed to last more than 15 years or the repair and replacement costs should also be included in the estimate.

between 07/06/2010 and 08/04/2010, 29 days

This becomes 30 days!

Your calculation shows an average use of of 3 KW per hour(67KW /24Hr ) you need to size the fuel cell unit bigger than that.

You need to size your fuel cell based on the maximum load in KW you would use at any instant. For example if you use your dryer which probably is around 5KW load then your 3KW unit is not enough to cater your peak demand.

"3kW per hour"?

"67kW/24Hr"?

Wossat?

There is no such reasonable unit as KW per hour.

Think of it this way. The difference between a kilowatt and a kilowatt hour is a lot like the difference between torque and horsepower. In both cases, the first is a measure of instantaneous energy whereas the other has a time factor.

So for example, if your home has a peak electrical energy consumption of 4KW when everything electrical is turned on, then the measure of that consumption would be 4 kilowatts per hour for every hour that everything is turned on. However, it's very seldom that everything in the house it turned on at once.

So, taking the number of KWh on your electrical bill and dividing it by the total number of hours covered by the bill will only give you the average consumption. But, the actual moment-to-moment consumption tends to vary wildly throughout the day.

Basically, the term kilowatt hour simply means that one kilowatt was used continuously for one full hour.

"..torque and horsepower. In both cases, the first is a measure of instantaneous energy whereas the other has a time factor..." We know what you mean, but it's not quite right. Being pedantic (because it may cause confusion), Torque is related to energy by E=ΤØ ffej

GA ER V nice analogy (now stolen)

GA JDG "first in best dressed correct"

Agreed to the whole thing, 34point5.

ER I've added my vote for you to help correct this.

If you go off grid, the energy you use has to come from somewhere: solar, for heat and electricity (and Vitamin D :-), water, wind, propane, diesel, gasoline, methane, etc. My experience has been that hot water should come from solar thermal panels, electricity from photovoltaic panels, cooking from solar oven, or oil-filled solar panels, propane or local methane. Lighting from LEDs, refrigeration from propane, methane or natural gas. Very much a personal opinion. I'd suggest serious study of the many internet sites on going off grid. It isn't easy, it isn't cheap, and it can be very technical. And the slugs eat your tomatoes. The coyotes eat your livestock. Your neighbors drink your liquor and borrow your tractor and turn it over in a ditch.

I live in the middle of the city. I don't like coyotes, I shot plenty of them when I was a kid in Nebrask and I certianly wouldn't put up with neighbors drinking my booze either. I want all the advantages of city living without the worry of what high fuel costs in Arabia might do to my electric bill over the next 30 years or so.

I fail to see where the 4 comes in here. Kwh is KW times the hours used. Thus to find either Kw or Hours, you have to know one unit. Therefor to find KW you have to know the hours. Then divide the kWh by the hours and you will be left with KW. Simply put, if you had a 1 kw kettle boiling for 2 hours youve used 2Kwh of energy.

EDIT.

Never mind all answers taken care of already, with some

As has been mentioned (redfred), calculating your average consumption is a bit of a red-herring when sizing your power supply. You need to work out the possible maximum consumption. My house averages considerably less than 3kw, but peaks often exceed 12kw and could theoretically exceed 15kw, so I would be looking at 16-18kw minimum. Otherwise you might find that you can't use the kettle and toaster at the same time (as I recently experienced in Spain), and everyone knows that toast without tea is less than half as good.

Let me attempt to explain it in a simpler way. Speedometer of car can be compared with KW meter (watt meter) and the odometer with the energy (KWh)meter. Let us say that you want to travel a distance of 2400 miles (equivalent to the total energy you want to consume, KWH) in say 5 days. You may not drive (consume) all 24 hours and may be driving say 8 hour per day (not necessarily continuously) and may not be able to travel (consume) at a constant speed. (speed can be compared with Kilowatt KW). You want to buy a car for the purpose and want to know what should be the maximum speed (KW of source) at which the car should be able to run. Your average speed (KW) over a period of 5 days is 2400/ (5x24)= 20 miles per hour. But actually at times you would have driven at higher speeds and may be at lower speed for some time (traffic problem). Your car should be capable of run at least a speed around 2400/(8X5)= 60 miles per hour (or little more to take in account little lower speed).

Very well said sir. GA

OP must be thoroughly confused by now.

Must be, he disappeared.

2400 miles away, apparently...

Brilliant, sir.

Damn, I should have thought of it.

NO! This is completely wrong. Google for KW and KWh. After you gain an understanding of the terms, come back.

NO! This is completely wrong. Google for KW and KWh. After you gain an understanding of the terms, come back.

You mean like kW & kWh?