Deleted

jrawle

Deleted_User said:

"It's true that you do not know the power at each point in time overnight, but you can still express an average power."

So you are saying that the formula 

KWh = KW x Hours.

is incorrect?

[...]

What you haven't so far grasped - along with many other people, and I'm really not having a go - is that "power" is actually a rate. It's an instantaneous measure of the rate of energy usage, how many joules of energy you use per second. It only becomes an energy value when you multiply by a time value. The units used should therefore also be multiplied by time.

When dealing with power that varies with time, to find the total energy used, you would integrate over the time period. That means you are getting into the realms of calculus, which I guess many people haven't encountered unless they have done at least A-level maths.

Perhaps the simplest analogy is to go back to driving. If you drive at 40 mi/h for an hour, then 20 mi/h for an hour, how far have you driven?

40 mi/h x 1 h + 20 mi/h x 1 h = 60 mi.  *

You have driven 60 miles, not "60 miles per hour". Your average speed is 30 mi/h. Both speed and average speed are measured in mi/h.

To go from our motoring analogy back to energy, replace:
mi/h with W or kW
mi with Wh or kWh

(* A lot of confusion is because the energy unit kWh includes a multiplication by h, rather than being "per hour", which is why I have written miles per hour as the more technically correct mi/h rather than MPH.)

QrizB

In the calculation of ACCUMULATED POWER IN and ACCUMULATED POWER OUT a black box is all you need. So why pretend you've done some fancy conversion into energy when you haven't?

You can't accumulate power, whether in or out.

You can integrate power over time, and this gives you energy.

If I run a 60 watt lamp for 2 hours, I haven't "accumulated 120 watts". I've used 120 watt-hours (or 432 kJ).

QrizB

If it makes you feel better by adding up a bunch of numbers, go for it. Just don't expect anyone else to agree that the result means anything.

Reed_Richards

Deleted_User said:

When your goal is to calculate the difference between POWER IN and POWER OUT It doesn't matter whether the incoming/outgoing power was generated/drawn in one second, one hour or one year. Time/hours is completely irrelevant. 

You're talking about "POWER" but what you mean is ENERGY.  It's easy to get the two terms confused, a "Power Station" would be much more accurately described as an "Energy Station" and there are many instances in common parlance where "power" is used to mean energy.  But an electrical device runs on energy.  You have started a lot of threads and in several I have read you confuse power and energy and nobody yet has managed to get you to see the difference.  Power isn't drawn in "one second, one hour or one year", that's energy.        

Gerry1

Deleted_User said:

TOTAL KW and KWh are the same thing - they are interchangeable.

Absolutely not.

kW is a measure of power.  A 1kW one bar electric fire is still a 1kW electric whether it's on for an hour or a year.

But the energy used in a year would be 8760kWh, so they're definitely not the same thing, nor are they interchangeable.

Gerry1

I don't like the idea of turning a fridge off overnight because it will get warmer than it otherwise would, hence risks of food poisoning.  You'd have to set it to run colder during the day, which would defeat the object of the exercise.

Far better to have a system that 'pings' the fridge every few minutes to see whether it's calling for power, and then fires up the inverter for a set period.  Rinse and repeat until breakfast time.

It would make a good software project ! 

mjm3346

The fridge temperature (normal use) may have risen to the point where the chiller unit was going to kick in at exactly the same time as the power went off, on those occasions (reverse and all points in-between will also happen) there is the greatest risk of it getting too warm for food safety before the power resumes.

Reed_Richards

Gerry1 said:

Far better to have a system that 'pings' the fridge every few minutes to see whether it's calling for power, and then fires up the inverter for a set period.  

Don't you mean calling for energy, @Gerry1 ?

Gerry1

Reed_Richards said:Gerry1 said:

Far better to have a system that 'pings' the fridge every few minutes to see whether it's calling for power, and then fires up the inverter for a set period.  

Don't you mean calling for energy, @Gerry1 ?

Not really.  The 'ping' would be a snapshot to determine what the power drawn by the fridge would be.  If it's less than a few watts, then the system would do nothing and try again a few minutes later.  If it was say 10W or more then it would switch the supply on for the set period.

victor2

Gerry1 said:

Reed_Richards said:Gerry1 said:

Far better to have a system that 'pings' the fridge every few minutes to see whether it's calling for power, and then fires up the inverter for a set period.  

Don't you mean calling for energy, @Gerry1 ?

Not really.  The 'ping' would be a snapshot to determine what the power drawn by the fridge would be.  If it's less than a few watts, then the system would do nothing and try again a few minutes later.  If it was say 10W or more then it would switch the supply on for the set period.

I don't claim to know how a typical fridge/freezer operates, but would guess that upon receiving power they run the compressor then let the thermostatic controls decide if it is needed, so the "ping" would have to be tailored to deal with an initial demand before deducing whether or not the supply should be left on?