solar bank back to grid - is there a switch available?

Hi folks. Usually we think of an automatic transfer switch to go from grid to emergency battery bank when the power fails. I have a system that is mostly self sufficient during the summer but not so much during the winter. Does anyone make a switch that would transfer back to the grid when the battery bank goes too low? I have a manual 240v switch now to support my system but when the batteries dip to a certain level the inverters kick out. I would appreciate any suggestions.

Comments

  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: solar bank back to grid - is there a switch available?
    digger59 wrote: »
    Hi folks. Usually we think of an automatic transfer switch to go from grid to emergency battery bank when the power fails. I have a system that is mostly self sufficient during the summer but not so much during the winter. Does anyone make a switch that would transfer back to the grid when the battery bank goes too low? I have a manual 240v switch now to support my system but when the batteries dip to a certain level the inverters kick out. I would appreciate any suggestions.
    There are plenty of automatic transfer switches that will switch over to generator when AC is not present. Just reverse the GEN and LINE connections.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: solar bank back to grid - is there a switch available?

    What inverter have you got?
    A lot of the systems done for places where utility power is poor utilize the auto gen start circuit to control a relay to switch utility power to AC IN. In effect the grid takes the place of the generator. It requires an AGS system of course, and often an inverter-charger will have this.

    One question (rhetorical): if you've got grid, why have you got an off-grid system?
  • ldiorioldiorio Solar Expert Posts: 47 ✭✭
    hi all,
    reading these posts -have the same questions -have a Magnum 48V 240V inverter charger with panels and 415AH batteries
    using panels to feed sub panel thru inverter-this is the first week of operation for me --very sunny everyday so batteries get to float even with some ckts working off the Panels-today i got up and very cloudy so i had to switch over to the grid tocharge the batteries--
    can i use the inverter with a AGS unit to switch from solar to grid when batteries get low
    i didnt go net metering bec local electric co not really pro solar and paper work a mess--just wanted to try some solar thats why i went with this grid interactive plan

    on some good days the panels have produced 6-7kwh per day

    lou

  • BB.BB. Super Moderators, Administrators Posts: 31,159 admin
    You have a couple of "simple" options that I can think of... One is to use a voltage controlled switch controlling an AC transfer switch (aka relay).

    When the battery voltage is >~52 volts, the transfer switch is set to AC inverter for your load. When the battery voltage is below ~48-50 volts, the transfer switch is set to AC mains.

    A second more accurate method would be to use a Battery Monitor that has programmable alarm contacts (like the Victron units). Set to turn "on" when battery is > 90% state of charge, and turn off the transfer switch when battery bank is less than 75% SOC (use your own numbers--like 80% on, and 50% off if you want to do deeper cycles on the battery bank).

    You still need to monitor the battery bank state of charge with a hydrometer (if flooded cell batteries)--Either method is an approximation and battery monitors can easily drift if not programmed correctly, or even just over time.

    Note that batteries are not "cheap" and off grid power systems are not that efficient--So if you put pencil to paper for initial system costs and long term maintenance (new batteries every 3-8 years, new electronics every 10+ years, etc.)--Your system may not really be saving you any money.

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