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PW2 Load Questions (power factor and such)

PW2 Load Questions (power factor and such)

Some of the terms confuse me - hoping for help. The PW2 specs say capacity of 13.2kWh of "AC Energy". It says Power Factor Output Range is +/- 1.0 adjustable and Power Factor (full-rated power) of +/- 0.85. Round Trip Efficiency is given at 89%.

My scenario is pure backup power charged by the grid.

First, I am confused how to account for loads with power factors less than 1.0. For example, I have an old freezer with a power factor of 0.60 and a running amps of 2.0. This is 240VA but only 144W (120v * 2a * 0.6pf). Should I count 144(W) against the 13.2 capacity or should I be counting 240(VA)? Let's ignore duty and defrost cycles of the freezer as I can measure those separately.

Second, how do I account for Round Trip Efficiency of 89%? I don't care about some loss charging the PW2 - that's just a few extra bucks for kWh. I am more concerned of the impact when PW2 is supplying power during grid outage. Is this saying I will be losing a further percentage from 13.2 capacity due to inverter efficiency loss? If so, how much?

Thanks.

sashton | 24. lokakuu 2017

There will be conversion losses on charging and discharging. Static holding losses are minimal with this chemistry. The round trip efficiency will be an aggregate of both of these. There are other factors that significantly affect the capacity of the system such as ambient temperature and duty cycles. From my (limited) experience I have had instances when I have put more than 13.2kWh into the system and have had many times when I have taken 13.2kWh out so, at least with a new battery, I assume that 13.2kWh is the capacity after losses but I stand to be corrected.

sashton | 25. lokakuu 2017

I went on to look at my stats so far. I have had a PW II for 77 days. I have pushed 936kWh into it and got 813kWh out. That equates to slightly worse than the quoted round trip efficiency but 87% isn't too shabby. It is only charged by solar during the day (so commonly doesn't get a full charge) and it is usually fully discharged the following morning.
We have a mixture of resistive and reactive loads (two fridges; one freezer and a number of switch mode power supplies for NAS, laptops, routers etc) but the overall power factor for the house is usually about 0.05. At that level I just worry about the apparent power readings.

mikemcq | 25. lokakuu 2017

Thanks sashton. What is your worry about apparent power readings? And, are you sure you meant a household power factor of 0.05?

sashton | 25. lokakuu 2017

Sorry, I didn't mean to imply that apparent power was a concern, just that it was the metric that I primarily worked from. The power factor reading was taken from an import/export meter (Fronius smart meter + data manager) I had installed before embarking on the whole off-grid trip to get some idea of our demand - to size the PV; micro-hydro; small-wind and storage devices. In fact I underestimated the impact of moving the whole family to EVs so I am in the process of adding an additional 6kWp PV at present.

mikemcq | 25. lokakuu 2017

I think you are mis-interpreting the 0.05 as a household power factor. A blended household value of 0.95 would be more realistic. NB: Real Power in Watts = (Volts * Amps * Power Factor)

In any event, I appreciate the info about efficiency.

Can anyone clarify whether I should be using W or VA for loads against the capacity of the PW2? (per question 1 in op) Normally I would use W against a kWh capacity but the wording about "adjustable power factor" and "full-rated power of 0.85 pf" has me puzzled.

sashton | 25. lokakuu 2017

You are absolutely correct. I was reading the wrong variable from the api. Once corrected I am now getting 0.9999999776.. (the figure went on to 17 decimal places for some reason) on the main phase. The value I had pulled was the PowerFactor_Sum - whatever that is - and it appears to only deviate significantly from zero when the milking parlour chillers are running.