Grid Tie

Hello All,

I'm rather new to PV and have built/am building a small system to gain insight as to how PV works and it's cost/benefits. Thinking of augmenting with wind since I have a fair amount living near Edwards AFB in the Mojave desert

I have spoken with CA authorities and they have indicated I can apply for multiple rebates as I build a larger system, but, I must have an initial system approved and Grid-Tied.

I currently have 4 KC 130 panels with an MX60 controller feeding 800 AH of Walmart batteries and a Harbor Freight 2000 W inverter. This is working fairly well powering fans, a swamp cooler, a TV and a few applicance chargers such as cell phones, game batteries, etc. and I am getting a good feel for how much panel power I will need and the batteries as well.

My main electric panel appears to have been designed for a total of 300 amps but is using only 2 of the main breakers leaving the third one empty.

If I want to divert some of my small system to grid tied, can I run the grid tied inverter off the existing batery bank and can I attach the output of the Grid inverter to one side of the open main breaker? Do I need to connect to both sides?

I do have a few 220v appliances, kiln and a well pump

Thanks in advance for opinions



  • BB.BB. Super Moderators, Administrators Posts: 32,599 admin
    Re: Grid Tie

    I don't quite understand your main breaker setup... Generally, there is one (dual circuit) or two (ganged) breakers for single phase 240 VAC (two hot and a neutral). Three breakers (ganged?) should be for three phase power--and in California, that is generally 208 VAC rather than 240 VAC--not a huge deal, but something that you need to be aware of when connecting 2xx equipment (with 220 VAC it does not sound like you have three phase power).

    Anyway, you should not need to add a third breaker at your main disconnect--instead you would add your breaker(s) to the distribution box where the rest of your breakers are all installed (one for a 120 VAC inverter, or two ganged for a 240 VAC inverter circuit). You can add solar/grid tied inverter up to 20% of the rated current of the panel(s) (including upstream panels) where you are adding the extra breaker(s)--for residential systems.

    And yes, you can add a grid tie inverter to your battery backed system--but I am not quite sure that it would make economic sense for such a small system. I am pretty sure that Xantrex makes such an inverter/grid tie module for their battery backed system, other manufacturers probably do too. You can program them to supply excess charging power to the grid and they will automatically transfer to battery power in a power failure.

    Overall, if your power is reliable, a plain grid-tied system is going to be the least expensive and most efficient at power conversion. You will find that adding panels and inverters (or oversizing the inverter for the first install to allow additional panels later) is going to be kind of expensive and as you add panels--and it will be a pain (you must add panels according to the connection rules of the inverter manufacturer and, for most inverters, the panels ideally should be almost the exact same model number--or at least ratings when adding to an existing grid-tied string--something that may be difficult with the current solar panel market).

    If your power is unreliable, a battery based grid-tied system is very nice to have. You will have similar issues with adding panels as with grid-tied inverters--plus you will have the issues of upgrading your battery bank size as you add panels/inverters. Adding new batteries to existing banks is generally not recommended as the new batteries will (usually) last no longer than the old existing batteries in the bank. And significantly adding to a battery bank usually would need new wiring with configuration issues too.

    That is about all of the general information that I can think off at the moment. You should probably start talking about specific inverters/systems as each manufacturer/model system has its own issues/requirements.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: Grid Tie

    Let me chime in here real quick about your choice of inverters as I have had the same inverter from Harbor freight working for me in the past. While at first it seems like an incredible value, it might prove to fail you in the long run. These inverters just weren't designed for solar applications. My 2000w inverter worked flawlessly for months, which kept me happy. Then the inverter's alarm kept sounding and shutting down. I thought my voltage in my bank was too low causing the problem. It wasn't. Removing the inverter and running it off a separate 12v source proved the inverter wasn't faulty, just not suited for my application. I have replaced the "Chicago Electric" inverter with one from xantrex and all is good so far.
    The Chicago electric inverter was also pulling way too much power in idle and was sending my meter into a frenzy. I'm keeping the inverter and will use it sparingly, just not with my small solar bank. My experiment into solar energy has proven useful and fun. I just wish I spent the $$ first and got the right gear, not just jumping into this as cheap as I did. :-D
  • System2System2 Posts: 6,290 admin
    Re: Grid Tie

    Thanks shawndecker1 and BB for your responses. They are appreciated and useful. I'm going to go back and look at that panel closer. I just glanced at it and haven't actually looked under the breaker cover to see how it is wired.

    I know that the HF inverter is not the long term solution. I just wanted to get started into solar and see what it had to offer. It was on sale and I had a 20% off coupon for any single item. One thing I have found out, Alternative Energy certainly isn't cheap! Conservation is sure a better buy dollar for dollar.

    I am getting a feel though for what it will cost. I'm getting the KC130's for $579 and they seem to be readily available so I can expand with the same panel, at least for the present. I've seen a number of formulas for calculating how many panels, wire size, inverter size, etc. and the best way for me is to jump in and measure actual's on a small scale.

    I'm also getting a feel for how much I can expect from batteries albeit, Walmart for the moment. A 125AH marine/deep cycle is only $64 plus $7 case. Even with what I would have called a lot at 800 AH's, it don't last long with a TV, an amplifier, and a fan. I started with 2 130's and they kept up with and slightly ahead of the fan for most of the day.

    The MX 60 is nice and I can expand a bunch of ways with it and more MX60's if needed. Outback seems to have several solutions as well as xantrex.

    What I'm wondering is about feeding only one side of the power panel to start with. Do the 220v items care about in-balances or are they OK as long as they have the minimum they need? If I feed only one side, I then need to make sure the things I want to power are on that side. That's why I'm feeding those items with a large extension cord for the moment.

    I want to apply for both State and power company rebates but I need to grid tie first. So, once again I'm trying to get in small if I can.

    I'll likely oversize the grid tie inverter since I believe I will ultimately go full bore with solar/wind. Living in the Mojave desert gives me a bunch of sun most of the time and a considerable amount of wind most of the year, at least according to Edwards AFB wind statistics for the last several years.

    Again, thanks for you replies.

  • BB.BB. Super Moderators, Administrators Posts: 32,599 admin
    Re: Grid Tie

    Several points...

    First, are you going with battery (back up or primary power?) or Grid-Tie inverter? Almost nothing (other than the solar panels themselves) is the same between the two systems. Up north here in California--there has been quite a change in Time Of Use metering rates--the E-7 PG&E rate is no longer open to new customers and the E-6 TOU rate does not appear to be near as solar friendly (to me, E-6 seems to reflect the utility's true costs of operation better than the E-7). The E-1 residential rate appears to be unchanged in how it works with solar.

    Second, 240 VAC / out of balance load question, etc. kind of indicates that being an electrician is not your primary job (ain't mine either). Connecting an inverter to a home's load center is a job for an licensed electrician--both for safety and liability reasons.

    To answer your question--the standard US grid tied inverter is usually a 240 VAC device and does not care about 120 VAC connections at all--and you will not care either (assuming that you have 240 VAC single phase power to your home and the circuits/box(es) are large enough to "legally and safely" manage the loads). The utility will take care of the 120/240 VAC issues for you.

    If, you are attempting to design a battery backed system (with or with-out grid tie inverters), then you need to understand your loads and the options out there... The "typical" inverter setup from Outback and Xantrex (that I have read about) is done with 120 VAC inverters. You can parallel two (or more) 120 VAC (say 3kW) inverters into one 120 VAC 6kW circuit--or you can wire two 120 VAC inverters between separate hot legs (Hot-Common Neutral-Hot), and interconnect the inverters so that they are exactly 180 degrees out of phase and generate a true mix of 120 and 240 VAC circuits that total to 6kW (however it would be a 3kW maximum on each hot leg). And these would be separate circuits with respect to the incoming AC Mains.

    Third--If you want wind (and have a large enough windy property to support a good size tower), I believe you will need to find either an exact solution for grid-tied wind (wind generator-and possibly mating inverter--that are designed for grid tie), or you can use wind as an add-on to a battery based system (either with or without grid tie).

    My suggestion is to read up on the Outback and Xantrex systems to get a better understanding on how all of this works together... It is not to bad to understand once you see how all of the various components fit and play together.

    The Battery systems that use grid-tie as a backup (and can even dump excess power back to the grid) look really neat. However, they appear to be expensive to install and there is the ongoing expenses of owning batteries).

    If you have reliable power and just want a little battery and/or generator backup--then I would suggest Grid-Tied inverters plus solar and just keep the small battery/generator system for emergencies.

    If you have unreliable power and/or other issues with the utility company--then a battery based primary power solar/wind with gen-backup system is pretty hard to beat. However, it will probably never be as cheap and easy as existing utility power (at least for the near future).

    Good Luck!
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • FrankFrank Solar Expert Posts: 54 ✭✭✭
    Re: Grid Tie

    These guys have all made good points. I'd just like to add that any grid-tied inverter must be UL approved for such. There may be other requirements required by your local utility.
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