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8295Re: Grid-Tie with Battery Back-Up

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  • Jay Ring
    Nov 1 11:43 AM
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      You make a good point.

      If you size a solar install for, say, 75% of your normal use (as a
      grid tie usually does), when the battery doesn't help much because you
      depend on external power. Batteries never charge since you are still
      a net-consumer. If you are going to depend on external power, you
      might as well just depend on a generator.

      I think I might have to reluctantly agree that if you have an
      undersized system, money might be better spent buying extra panels
      before adding batteries and counting on a generator in an emergency,
      but I am not 100% sure yet.

      One thing to consider is buying just one day worth of batteries, so
      that the solar can operate about 1/2 of your house, or maybe just the
      refrigerator. That way you don't lose the entire benefit during that
      duration. Then instead of sizing your battery bank for the worst
      expected case of cloudy days, you are just sizing it for one
      refrigerator and maybe a TV after a storm. Might be cheaper than a
      generator after all.

      One thing that analysis leaves out is that power outage is not the
      only risk solar can protect you from. Solar also reduces the risk
      from rising energy prices which is a big part of my reason for buying!
      That would tend to push you even further towards "extra panels before

      Anyone else have any thoughts on this? The more I think about it the
      more complex it seems :)

      --- In hreg@yahoogroups.com, "Russell Warren" <rrwarren@...> wrote:
      > 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
      > 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
      > 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
      > 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
      > 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
      > poison yourself with carbon monoxide.
      > A permanant natural gas generator with an automatic transfer
      > 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
      > 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
      > 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
      > and expertise. Designing and installing a system to integrate into
      the home
      > takes real expertise - ask for experience and references when choosing a
      > contractor.
      > Sincerely,
      > Chris Boyer
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