Combining grid tie and battery system

Hello fellow Greenies!

I am currently installing a 1.8 kw grid tie system (eight 255 watt Sunpower panels configured as 2 strings of 4 panels each) with a 7 kw grid tie inverter (oversized for future expansion of fourteen 255 watt Sunpower panels).

I also have 4 Sharp 175 watt panels and two 255 ah batteries I would like to combine with the grid tie system. This system is set up as a 24 volt system with an Exeltech XP-1100 24 volt 1100 watt inverter.

A couple of questions –

Is it possible to add a transfer switch so that the 4 Sharp panels will power the grid tie inverter (making 2.5 kw) most of the time while just switching it over to the battery system during power outages and to keep batteries fully charged?

If that is possible, would it also be possible to add a transfer switch for all 2.5 kw to switch over to the battery system or would I need to increase the size of the battery bank and inverter? I'm assuming that as long as your load is below 1100 watts the inverter should handle.

Lastly, is there any way to “fool” the grid tie inverter to make the system produce power during a utility power outage?

Thank you in advance!


  • a0128958
    a0128958 Solar Expert Posts: 316 ✭✭✭
    Re: Combining grid tie and battery system

    If you're going to use a 7 KW (DC) inverter with the 8 Sunpower panels, I think you're going to have to do a couple of things to make it work:
    1) put all of the panels in the same orientation / plane;
    2) wire the panels as 1 string - i.e., all panels in series.

    For example, using Sunpower 225 W panels (I couldn't find 255 W panels so I thought maybe 255 was a typo) and an SMA Sunnyboy SB7000US inverter, operating at 240 VAC:

    The inverter's PV Start Voltage is 300 VDC. You'll need 8 panels in your string to stay above this voltage. And at higher temps, i.e., upwards of 120°F, you'll be marginal without at least 9 panels in your string.

    If you really need to have 2 strings of 4 panels, and you want to stay with a 7KW (DC) inverter, you'll need to find the inverter with half of the PV Start Voltage, i.e., around 150 V. I don't know if such a (grid-tie only) inverter exists.

    Hope this helps.

    Best regards,

  • BB.
    BB. Super Moderators, Administrators Posts: 33,404 admin
    Re: Combining grid tie and battery system

    The quick answers:
    1. Take a look at the Xantrex XW Hybrid Inverter system. It does both Grid Tied and Off-Grid. Basically, a 4-6 kW (depending on model) whole house UPS with alternative power sources (solar, generator, A/C Mains).
    2. Yes, it is possible to "trick" a Grid Tied Inverter to run Off-Grid. Basically, set up an isolated circuit (behind a transfer switch). Install a GT system and a Off Grid System (the OG system can have solar panels or not--your choice). Pick the Off-Grid true sine wave inverter such that it is at least as large as the GT inverter. Normal operation, the GT inverter feeds back on the AC Mains. Mains, fail, transfer switch opens and connects Off-Grid Inverter to GT inverter. Now solar panels power GT inverter which back-drives TSW Off-Grid Inverter which charges batteries with excess power (OG inverter acts like AC Mains power). Note, only a few TSW inverters have been tried in this mode. Probably none are tested/approved as back-driven battery chargers. And there is no charge controller--batteries can over charge if nothing else is done.
    Some notes... Is the GT inverter matched to the Sun Power panels? Depending on which "Sun" Brand panels you are using, they may need Positive Ground Referenced GT Inverter.

    Also, again depending on which GT Inverter and Sun panels, there are not many pure GT inverters out there that will run on 4 panels in series... Most require a much higher voltage to operate (Vmp minimum ranges from around 200 to 300 VDC on the more popular inverters in the US).

    Regarding switching of panels from GT to OG and combinations thereof... Yes, it is possible to setup a panel with jumper cables or even switches to do this... However, it usually becomes a wiring nightmare. Most GT inverters are configured to operate around 300 VDC +/-. Most MPPT Type solar charge controllers are configured to run around Vmp=24-100VDC range (140-150VDC max). So, you cannot just swap over the +/- leads from GT inverter to DC charger. You have to play with the series/parallel configuration too (long a complant of us people with pure GT systems).

    And, you have a similar problem going back to the GT inverter with your other brand/model of solar panels. To get the optimum series/parallel connections--you have to match Vmp and Imp (appropriately) between the strings so that you get the most power from the arrays (parallel need to match Vmp's, series connections need to match Imp to within ~10% or so. More mismatch and you will loose power/may cause damage to your arrays).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • dlenox
    dlenox Solar Expert Posts: 42
    Re: Combining grid tie and battery system


    The purpose of the battery bank and grid tie inverter is so that when the grid drops off that you can still keep generating power, and/or drawing off batteries.

    BB referred you to the Xantrex XW inverter, I have a 6048 and it is one reliable unit and highly recommended by me.

    Dan Lenox