malc said:
Well I may have got confused. The definition of a kwh is 1 kw per hour
http://whatis.techtarget.com/definition/0,,sid9_gci797759,00.html what
he may mean is 2000 Ah although this is still quite a lot to store.
No, you've misread the definition (and that page's wording isn't all that
great).
Energy is power *times* time. A kilowatt-hour is a measure of energy. A
kilowatt is a measure of power.
When you say, "...a kwh is 1 kw per hour", you are using the word 'per'.
And mathematically, that means 'divided'. So you are essentially saying
"...a kwh is 1 kw divided over an hour". And that is wrong, it makes no
sense. It would be like saying an amp-hour (AH) is an amp divided over an
hour. Doesn't make sense.
An amp-hour is much like a kilowatt-hour. It is a measurement of a flow
*multiplied* by a time period. An ampere is a measure of the flow of
current, a kilowatt is a measure of power (which is the flow of energy).
When you multiply either by time, you get a measure of the total amount of
what has 'flowed'. In the case of amp-hour, since amperes is a measure of
electric-charges flowing in a given amount of time, when you multiply it by
the length of time it has been flowing, you get a measure of how many
electric charges have gone past a point in the wire (such as into or out of
a battery). In the case of kilowatts, since a kilowatt is a measure of
energy flowing in a given amount of time, when you *multiply* it by the
length of time it has been flowing, you get a measure of how much energy has
gone past a point in a wire.
A usage of 2kwh in one day means the OP uses just 2kwh of energy over a 24
hour period. The fact that he's specified his energy usage over a 24 hour
period, using a unit of measure that has a 'time unit' built into the name
*does* add a lot to the confusion. One could use energy at the rate of 2
kilowatts for one hour. That would be a total energy of 2 kwh. Or one
could use energy at the rate of 100 watts. After twenty hours, they would
have used 2 kwh.
To use 2 kwh in one day, you could use energy at the rate of 4000 watts for
just 1/2 an hour, then turn everything off for the remaining 23.5 hours. Or
you could use energy at the rate of 200 watts for 10 hours and then turn
everything off. Or you could use energy at the rate of 83.3333 watts for a
full 24 hours. All of these will total the same amount of *energy*, 2
kilowatt-hours. But of course the equipment to support those different
power levels, and the losses change with the different power levels.
If the OP gets just 5 hours of good, strong sunlight, then if he can collect
solar energy at the rate of 400 watts, he would collect 2 kwh of energy in
those five hours of sunshine. Of course if he's storing it and using an
inverter, there will be losses in each step of the process. So he may want
to collect solar energy at a higher rate, say 500 watts. Then over the same
5 hours of good strong sunlight, he will have collected 500*5/1000 = 2.5 kwh
of energy. Some of that energy will be 'lost' in inefficiencies in storage
battery so he may get just 2kwh 'delivered' to his loads. If he wants to
collect enough energy in one day to support two days of operation (prepare
for a cloudy day tomorrow), he might have to collect energy at the rate of
1000 watts. Then over the same 5 hours of good strong sunlight, he will
collect 5*1000 / 1009 = 5 kwh. He can then use 2.5 kwh of that energy today
(2 kwh in loads and 0.5 kwh in losses), and the remaining 2.5 kwh of energy
tomorrow.
Hope this clears up some of the confusion...
daestrom