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8293Re: [hreg] Grid-Tie with Battery Back-Up

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  • Stephanie Edwards-Musa
    Nov 1, 2008
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      Your email reminded me of an article I read yesterday.  'Solar credit changes the math for homeowners'

      I don't agree with everything in the article, but it does have some very interesting information.


      On Sat, Nov 1, 2008 at 10:17 AM, Chris Boyer <boyer.chris@...> wrote:

      Grid Tie PV systems with battery back up are a solution worth looking into for homes.
      A grid-tie PV system most economically generates solar electricity, running about $8,000 per rated kW size (and each rated kW makes about 1250 kWh per year in Houston).  However, when the grid goes down, the system goes down with it.
      To keep the lights on during a power outage, your options are a back-up generator, or a battery back-up solar system. 
      A portable gasoline generator works well and conomically (~$800 for 5 kW) to keep the fridge and some lights on for the occational long term outage - if you can find 10 gallons of gasoline every day and if you don't poison yourself with carbon monoxide. 
      A permanant natural gas generator with an automatic transfer switch is an elegant solution to keep the whole house running (including AC) .  Reliable ones run from $10,000 (15 kW) to $30,000 (45 kW) installed (requires plumbing & electrical permits).  There is about a 30 second delay between the time the grid goes down and the time the generator starts powering the house.
      PV grid-tie systems will not work with most back-up generators - I tried it.  It doesn't work because the sine wave and voltage limits of generators are not "grid" quality.  There are high-end, electronic governered generators that may sync with a PV system, but there still needs to be a dump load for excess PV power. 
      Grid-Tie solar with battery back-up will run about $12,000 per rated kW of solar power.  The power output of the inverter in not the same as the rated solar power, usually it is sized to be almost double.  Loads powered by a battery back-up system will not see any outage when the grid goes down - the switch is instantaneous.  SMA, Outback and Xantrex have good equipment for this purpose.
      The recent tax credit makes back-up systems economically attractive.  Look at the following example:
      CASE A: 
      PV Grid-Tie Cost:  $28,000  for a 3.5kW (no batteries)
      Generator Cost:   $12,000  for a 18 kW nat gas generator
      PV  Tax Credit:  - ($8,400)
      NET Cost: $31,600
      CASE B:
      PV Grid-Tie/Back-up:  $42,000  (including batteries)
      PV Tax Credit: -($12,600)
      NET Cost: $29,400
      So, as you can see, if you are thinking about solar and you want back-up power, then a battery back-up system is the way to go.
      Sizing your system is a simple excercise, but does take some work and expertise.  Designing and installing a system to integrate into the home takes real expertise - ask for experience and references when choosing a contractor.
      Chris Boyer

      Stephanie Edwards-Musa
      Realtor, Certified EcoBroker
      Mobile:  281-635-9444
      Prudential Gary Greene, Realtors
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