[RE-wrenches] Battery bank during peak times

[SunHarvest]

Hi Jesse,

If I read this correctly your customer is wanting to go off-grid during peak hours. I recently went through this same thing with a customer. I received feedback from an old SMA tech and some Wrenchers (thread titled "Battery Back-up and Grid Tie"). Here's what I learned:

1. Inverters are more efficient at inverting than charging, anywhere from 5-10%. Add to that about 10% transmission loss...you'll always have significant energy losses associated with trying to put back that power that you're drawing out during those peak hours. Energy loss = $$ loss.

2. Compare the utility peak kWh cost against the cost of cycling the battery bank. Just for an example, let's say for a 48V system you're using 150Ah 12V Trojan AGMs $2000 for 8 batteries. If the specs say those batteries, at 50% discharge, get 1000 cycles in their lifetime, that's $2.00 per cycle. 300Ah X 48V X 50% = 7.2kWh. $2/7.2kWh = $0.28/kWh. Add to that 20% round-trip efficiency losses, you're up to $0.33/kWh. What else does daily, unnecessary cycling add: Extra maintenance? Shortened life so probably should factor in some replacement cost?...

3. Murphy's law says that the power will go out at the end of one of these non-essential discharge cycles. Then what?

Probably not worth it.

Eric Stikes
SunHarvest
530-798-3738
  ----- Original Message ----- 
  From: Jesse Dahl 
  To: RE-wrenches 
  Sent: Monday, June 24, 2013 8:50 PM
  Subject: [RE-wrenches] Battery bank during peak times


  Hello,


  I was contacted by a local co-op about installing a PV system at their office.  At first they wanted a straight grid-tied system, and after they received the bid they changed their minds and now want a battery based system price. What they want now is a system that will allow them to draw the battery bank down starting at 5pm during their peak demand time. Has anyone worked on or installed a system of this type?




  Thanks as always!




  Jesse


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