Can I add a battery backup to my grid-tie?

System
System Posts: 2,511 admin
in Grid Tie and Grid Interactive Systems #1
The PV system isn't installed yet. It's going up next month. It's a 6KW DC array (2 strings) with a SunnyBoy.

As far as I know, if the power goes out, the inverter will turn off and even though it might be the middle of the day and might be generating 5KW AC, we'll be SOL while the power is out.

Is there a small battery backup that is high current and only needs to last 10 minutes or so at full draw? We get a lot of short power outages where we are.

Alternatively, depending on cost, a system that would last a good hour or two at full draw and charges only during E6 off peak would be really cool.

The idea here is to take advantage of E6 and only charge the batteries during the cheapest time of the day. Even better yet, is if the system could draw on the battery during peak should our usage temporarily, like for only a few seconds or minutes when the tankless electric water heater is running, exceed the output of the inverter and then recharge only at night when we're off peak, allowing us to sell as much peak power back onto the grid. It would also be really smart if the power went out during the middle of the day and the inverter is outputing more than we are currently using to charge the batteries on the spot if they need to be charged. Since we wouldn't be able to put the power back on the grid, we might as well use it to charge the batteries during peak time in this one instance. Even better would be a system where a threshold could be set so that if there was a power outage that drained the battery past a certain point that it would be recharged even during peak time. The idea here is that if there are multiple short power outages during the day, which happens here from time to time, that at some point the system says screw maximizing the selling back of peak power in favor of keeping the power up during the outages.

This may be asking a lot and may require software that hasn't yet been written or integration that hasn't been considered yet. But I throw these scenarios out there so that those that are informed might educate me on what is out there and what is currently possible.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,637 admin
    #2
    Re: Can I add a battery backup to my grid-tie?

    Change from the Grid Tie system to a hybrid off-grid / grid tie system.

    Xantrex has a new one that just came out that would do probably everything you would want. Outback and others probably do to...

    http://www.xantrex.com/xw/

    I assume that the XW system will not be cheap (actually, looking on the Wind-Sun store--not too bad)--you are making an entire off-grid capable system... Batteries, charge controllers, and a 4-6kW or so inverter (on-gird / off-grid capable with transfer switch).

    Really a cool way to go...

    Solar Guppy could probably give you some good suggestions on how to configure (min/max batteries, programming, etc.).

    Beware that cycling your batteries costs and extra 20% or so in power, plus cycling from night to day will wear out your batteries faster than just leaving them on standby/float charging.

    Lastly, E6 (if PG&E) is a tiered pricing plan--you start drawing a bunch of power at night (~300-900kW+ per month), and your rates start to rise too if the solar panels do not run the meter backwards enough during the day (I am not quite sure how the whole use at night, charge during the day works on TOU--my load even without solar are under the 300 kW per month baseline tier)...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • System2
    System2 Posts: 6,290 admin
    #3
    Re: Can I add a battery backup to my grid-tie?

    You mean charge at night and use during the day, right :)
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  • System2
    System2 Posts: 6,290 admin
    #4
    Re: Can I add a battery backup to my grid-tie?

    So it looks like the Xantrex xw6048 streets for about the same as the SB6000US. Even if I don't add the batteries immediately, is there a reason why I wouldn't want to go with the XW over the SMA SB given the price and that the XW seems to be able to do so much more?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,637 admin
    #5
    Re: Can I add a battery backup to my grid-tie?

    Yes, talking about load shifting (charge at night, inverter supplies AC power during the day)....

    Regarding batteries--As far as I know, you must have batteries installed for the system to operate (400 AH at 48 VDC battery bank per inverter)...

    Solar Guppy talked about the XW system here.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • System2
    System2 Posts: 6,290 admin
    #6
    Re: Can I add a battery backup to my grid-tie?

    If batteries were required from the very start, then ti puts the cost out of my reach currently. I'm confused as to why batteries are required as the charge controllers are optional components and it looks like, according to the literature, you could have an XW on a grid-tie with just a generator backup and that the optional generator auto switch will turn on the generator when the grid goes down.

    But what about simply having power if the grid goes down but your array is still producing more than you're consuming? Why would we even need batteries? Why can't any regular old modern inverter just disconnect from the grid but still provide the system power as long as you don't exceed the current array output?

    Failing all that, I can't believe there isn't a much smaller, simpler equivalent of a whole house UPS that doesn't require such a large battery capacity.
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  • n3qik
    n3qik Solar Expert Posts: 741 ✭✭
    #7
    Re: Can I add a battery backup to my grid-tie?

    From the install manual:

    Battery Bank Requirements

    Important: The minimum recommended battery bank is 100Ah. The inverter is designed to operate with batteries and should not be operated without them.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,637 admin
    #8
    Re: Can I add a battery backup to my grid-tie?

    A Grid Tied system takes the energy from the solar panels, inverts it, and sends it out to the utility line.

    An off grid system, and the Xantrex XW, takes the solar energy, sends it to the battery bank, and then from the battery bank to the inverters. With the XW system, it takes what it not needed by the batteries to maintain/charge, and the inverter sends that to the grid.

    There is another way of getting Grid Tied with backup power--but it will require similar components and complexity and offer no advantage for you installing a new system.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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