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8294RE: [hreg] Grid-Tie with Battery Back-Up

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  • Russell Warren
    Nov 1, 2008
      I think this example is missing some key elements.
      #1 very few solar systems are sized for a households daily needs (even without including AC)
      Therefore a battery backup system is also unlikely to provide the needs of the household.
      Furthermore there is a time factor...yes for a blip or even an hour, the system will be fine, but then again my food won't go bad in just an hour either.
      Lets say you have the worst case scenario like the hurricane.  Even if you are without power for (only) 3 days, a solar power backup system is unlikely to sustain that kind of downtime.  You have no charging at night, and during the day, your collected energy is re-charging the battery system...and you won't have spare power for actually running your house.
      On the flip side, your natural gas generator is going to last a lot longer for those folks who have a natural gas line running to their house.  It will run day and night in this scenario, and provide a ton of power.
      I just think for the money, you are better off getting the gas generator, if you need that, or buying a bigger solar system for the 360+ days you are going to be grid tied.
      If my power goes down, I will just rough it.  I can replace the contents of my fridge about 50 times before it would equal the investment of a generator or backup.
      -----Original Message-----
      From: hreg@yahoogroups.com [mailto:hreg@yahoogroups.com]On Behalf Of Chris Boyer
      Sent: Saturday, November 01, 2008 10:17 AM
      To: HREG
      Subject: [hreg] Grid-Tie with Battery Back-Up

      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

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