Battery backup, or not...

Hi all. I've been lurking around this forum for a while but this is my first post.

I had my mind all made up about a solar installation - I was going to do grid-tie with battery backup. However reading posts in this forum have changed my mind - it's a lot of extra expense and loss of efficiency to add battery backup when I really only need it for 0.5% of the time. So I'll go with a standard grid-tie inverter.

However, I do live in earthquake country where we're overdue for a big one, and an extended loss of grid power is a very real possibility someday. I have a generator, but the gasoline will only last so long. So while my primary goal for solar would be offsetting my utility bill, I would like to design something that can allow me to convert over to off-grid on a temporary basis when needed. If this involves doing some manual re-wiring (within reason), that's OK with me.

I can get as far as a bank of batteries that can be maintained by a plug-in charger during normal operation. What I don't know yet is the most effective way to be able to charge those batteries from the solar panels if the grid power is down.

Given that, any advice on possible designs?


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery backup, or not...

    Welcome to the forum.

    For what you're looking at - grid-tie with battery back-up - really the simplest thing to do is go with a "hybrid" inverter set-up like the Xantrex XW series or perhaps one of the new Outback Radian inverters.

    If you go with a "standard" grid-tie inverter re-arranging the panel configuration to supply the much lower array Voltage used by battery charge controllers is a pain. The easiest solution there is to spend the large money on the Xantrex 600 Volt MPPT controller; that's what it was made for. Otherwise you'll be disconnecting a lot of panels at one end and changing the output at the other. Most GT systems run in the 480 Volt range whereas battery charge controllers normally operate below 150 Volts. It's a pretty big jump.

    You might want to consider having a completely separate off-grid system if you feel the need for emergency power is going to be that great and long-lasting. Regrettably there's no inexpensive answer to having both GT solar and battery-based emergency power when needed.
  • BB.BB. Super Moderators, Administrators Posts: 32,015 admin
    Re: Battery backup, or not...

    It all really begins with you defining your loads.

    For me, I have defined my emergency loads to fit a Honda eu2000i genset (use fuel stabilizer and recycle into my vehicles once a year). That is around 400 watts average load and 1,600 watts peak.

    If I was going to do a battery system of similar performance, I would be looking at a ~1,600 to ~2,000 watt inverter system.

    If the inverter was to support a 4kW surge (starting load from a well pump, for example), and I assume that the battery bank should be ~2.5x larger than the starting surge (and for >1,200 watts, I would be looking at a 24 volt battery bank):
    • 4,000 watts * 1/24 volt battery * 2.5 surge rating for battery = 417 AH @ 24 volt battery bank
    That would be ~8 golf cart batteries in series/parallel connection (2x strings of 4x6volt 225 AH golf cart batteries) for a 450 AH @ 24 volt battery bank.

    Solar panels would be ~5 to 13% of rated charging current. Pick 10% as a good number (fairly large array to battery ratio):
    • 225 AH * 2strings * 29 volts charging * 1/0.77 solar panel derating * 0.10 rate of charge = 1,695 Watt solar array
    Or, if you went with the minimum 5% array, that would be ~850 watts of solar to support the battery bank off grid.

    Another option would be to design for a 300 watt TSW 12 volt inverter. That would give you enough power for a few lights and TV/Radios, plus recharge your cell phones and laptop computer. You would get away with ~2x golf cart batteries (12 volt @ 225 AH) and a much smaller solar array (much less costly system--quiet power at night, run the genset during the day to cool the fridge/freezer) and add power back to the battery bank.

    Either of the above battery banks could be recharged with the Honda eu2000i genset and an AC battery charger (the 450 AH @ 24 volt is about the maximum size bank I would suggest for a Honda eu2000i).

    One way to approach the system... Start with the Battery bank + Inverter (or inverter/charger) plus an AC battery charger plugged into 120 VAC household power to keep it recharged the 99.5% of the time you don't need emergency power.

    If you run your battery system at night and recharge during the day (or every second day), you will have a nice backup emergency power system. Keeping your loads small and using an small/efficient genset can keep fuel use down to a couple gallons a day (20 gallons could last you a 10 day outage). The larger 3.5 to 5kW noise maker (aka cheap) gensets will drink a 1/2 gallon of fuel per hour--almost impossible to store enough fuel in a suburban home for any multi-day outages.

    And you can add solar panels later as the funds/projects permit.

    If you go through "2manytoyz" website (and he is a poster here), you can see how he made his emergency power system by adding a component at a time.

    You can also get simple automatic transfer switches that fail over from Utility Power to backup power for not too much money.

    Unfortunately, if you want to run your refrigerator on solar power, that is already a fairly substantial off grid system system (1,500 watt minimum inverter, typically).

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