Sizing battery for inverter - power factor?

[recre8]

Hi Guys/Martin

Due to consistent loadshedding in South Africa, I'd like to power my PC and two LCD monitors for the duration of the 2h30m outages. I used a Efergy socket measurement which shows the computer and the two monitors drawing about 145-150W on average. With the screens on, the power factors of my load is a dismal 0.55
By my calculation that means an apparent power of 150W/0.55 = 272VA
Now I'm struggling to understand what exactly that means for me when sizing the required battery. I'd ideally prefer to not discharge the battery below 50%. My calculation using just the real power gives me this:
150W * 2.5 hours =  375Wh
375Wh / 12V battery = 31.25Ah battery
Maximum 50% discharge: 31.25Ah * 2 = 62.5Ah battery

But I'm unsure how the power factor changes this. Using the same calculation but substituting 272VA instead of 150W, it means the battery will need to be 113Ah

Can someone help me understand what will actually be drawn from the battery (ignoring all losses)?

[MJLorton]

Hi Recre8,

Running short on time for a full answer but I think these links will provide all you need:

http://forum.solar-electric.com/forum/solar-electric-power-wind-power-balance-of-system/advanced-solar-electric-technical-forum/23060-power-factor-effect-on-grid-hybrid-inverters
http://www.electroschematics.com/5730/inverter-backup-some-facts/

Basically your inverter needs to cater for the VA required, but base the Watt hour capacity calculations on the real power. A bad power factor could increase heat losses...so add some capacity on top of your normal loss calculation for that.

Hope that helps.

Cheers,
Martin.

[recre8]

Thanks Martin

Yes, I was planning on using a 600W / 1000VA inverter which should probably be sufficient to to drive a 150W/ 275VA load then. I read the links you provided and watched your three-part video on power factor and correction. If I understand everything correctly: a 150W load even with a terrible power factor will still only draw 150W (plus a little bit in losses) from the battery. The apparent power is just the actual power and "extra power" the inverter and cabling will need to handle because of the phase shift between the voltage and current the load is causing?

[SeanB]

As you are going to be hitting the battery hard with regular discharge cycles to a deep level I would recommend getting a Deltec lead crystal battery. I am using a 60Ah one with my UPS, and it will run the load for the regular Eishkom dropout fine for 2 hours, and I have had some times when I came home after 4 hours and the power was still off ( yay cable blew on turn on) so turned on the generator for around 5 hours more till the power came back on. UPS was still running the PC with monitor off, and I know it will do around 2.5 hours with it on.

With your PC you might want a larger ( or 2 60Ah units in parallel) battery to keep you running. Bit expensive though, the price is around R1600 direct from Deltec for the 60Ah one. More expensive than the standard deep cycle RV version, but a lot longer lasting. Do not use a car battery, it will die in around 15 cycles, designed for short high current discharge, and will shed oxide badly when deep cycled.