Learning about power losses in the system

[bigsmile]

The watt is still calculated by the electricity meter. As I understand, you can't really measure watt directly, it has to be calculated from some sort of average of current multiply voltage.

As your other post suggests, the digital electricity meter considers energy flows both way to be electricity used. I can't think of another way of realizing this than using the integral over time of the absolute value of the product of the instantaneous current and voltage. If it's done this way, certainly when the CFL starts to push current back into the grid, that will be counted as electricity used. That means the utility company will be billing more than the CFL actually used. Actually, not only it's charged for the extra energy it doesn't use, it's also charged again when it returns this energy back to the grid.

On the other hand, the old style electricity meter takes the integral of the product of instantaneous current and voltage, not the absolute value, so the old meter will be charging the real power used.

No not really. Utilities cannot charge customers for VAR,s only watts. It is commercial and industrial customers who have to pay for VAR's + Watts. Residential PF does not add up to a hill of beans.

[Steve]

I did some fun calculations.
Note that I am not going to actually use these heavy loads, but listed them anyway. I wouldn't drain my battery to 100% DOD either. These are just some 240ah battery running times based on a 90% efficient inverter:

CFL 15W/.63PF/.9 = 26W/12v = 2.2ah = 110 hrs
TV 20" 29W/.57PF/.9= 57W/12v = 4.75ah = 50 hrs
TV Rear Projection 110W/.65PF/.9 = 188W /12v = 15.7ah = 15 hrs
Fridge, kitchen 178W/.99PF/.9= 200W /12v = 17ah = 14 hrs
Air Conditioner 725W/1.0PF/.9= 805W/12v = 67ah = 3 hrs
Space Heater 1266W/.99PF/.9 = 1421W/12v = 118ah = 2 hrs

Note: The Fridge and AC was measured while the compressor was on. It didn't measure starting or fan (only) readings.

I've read that very low current draws may deliver longer AH times.
High current draws will last much shorter than the rating.

[Sunking]

Steve not trying to confuse you any more, but there is also another factor you might are overlooking called the Peukert Law. Simple definition is: As the rate increases, the battery's available capacity decreases.

When you buy a deep cycle battery the amp hour rating is specified at a 20 hour discharge rate.Example for a 200 AH battery, the 20 hour discharge rate = C/20 = 200 / 20 = 10 amps. So in theory if you took a fully charged 200 AH battery, put a 10 amp load on it, the battery should last 20 hours before being completely discharged. With me so far?

But what happens is say I put a 100 amp load on the battery? How long will it supply 100 amps? The formula is H = AH/A = 200/100 = 2 hours right? WRONG, dead wrong because Mr Peukert just robbed you. Your 200 AH battery went from 200 AH down to 100 AH. so the real answer is 100 ah / 100 a = 1 hour, not 2.

Look at this specification for a Surrette EIW 250 It is a popular golf cart battery that can be used in RE applications. It is specified a 6 volt 250 AH battery at 20 hours. Now look and see what the various AH rating at say 100 hours, 10 hours, down to 1 hour. The range is 333 AH down to 90 AH.

The reason I bring this up is because you tried to figure out the time large loads will last like a fridge, AC, and heater. Your calculations would be way off.

[Steve]

Thank you Sunking.

I've done some reading about battery characteristics and have come across the theory you posted. I was unsure about how much the higher loads would actually decrease ah performance. Thanks for the links.

[Sunking]

Thank you Sunking.

I've done some reading about battery characteristics and have come across exactly what you posted. I was unsure about how much the higher loads would actually decrease ah performance. Thanks for the links.

Peukert is most notable in Lead Acid batteries, not so much in other chemistry like Lithium and and NiMh