ASK THE EXPERTS: Grid-Tied Inverter Approval

Trace SW Inverter
A Trace SW inverter.
Trace SW Inverter

I’m setting up a 2,600-watt grid-tied, battery-based solar-electric system (using SunPower 320-watt modules). The guy I talked to at the electric company said he was not familiar with the inverter that I’m using—an old Trace SW4024. Is there a way of getting a list or some kind of information on electric companies that have approved Trace inverters?

Martin Mladenka • via email

Electric utilities take the safety of grid-tied power systems very seriously. Any such inverter must meet UL 1741 and IEEE 1547 standards, which are intended to protect utility workers from electric shock as they repair power lines during a blackout, and also protect a homeowner’s equipment and appliances from damage due to power quality fluctuations. To meet the standard, inverters must:

  • Immediately disconnect from the grid if power quality falls out of specifications.
  • Detect and prevent “islanding”—feeding electricity to the grid when the grid source is no longer present.
  • Wait for five minutes of clean power from the grid before trying to reconnect.

In your case, it’s likely that your utility simply isn’t familiar with your inverter because the Trace SW series was discontinued a few years ago. The SW4024 inverter was first introduced in 1994, and thousands of them are still in use for both off-grid and grid-tied applications. The SW series has gone through many different hardware and firmware versions since then, and most of them meet UL 1741 standards.

Some utilities think UL 1741 isn’t stringent enough. They may require that all grid-interactive equipment be chosen only from their list of approved models, which may also specify only certain model years or firmware updates. In other cases, it is the state utility commissions that determine allowable equipment. Unfortunately, I don’t know of any master database of inverters approved by different utilities, though many provide that information on their websites or will mail it to you upon request.

Utilities and electrical inspectors like to see new installations that are tidy; contain modern, new equipment; have ample documentation and schematic drawings; and were designed and installed in a professional manner. That puts the do-it-yourself homeowner with old equipment at a disadvantage. Your vintage Trace SW inverter is an old workhorse that will likely continue to perform for years to come—but is best placed into service in an off-grid situation.

Dan Fink • Buckville Energy Consulting

Comments (12)

SolarServiceGuy's picture

I'm have my whole system set up, did some great research and followed ALL the coding etc. and I get my interconnection application and I need UL 1741 GTI, I bought a decent one but isnt, where can I find a decent 500wat to just get my inspection approved and my application done?

Michael Welch's picture
I doubt if you can buy a "decent" GT inverter that is not approved to UL standards. My guess is you have an inexpensive inverter, one that may not stand up to the test of time. I don't know of any useful 500 W inverters, the smallest string inverter is 1.5 kw. Another option is microinverters, which can potentially handle about 240 W each.
littleharbor2's picture

I have a Trace/Xantrex Grid tie interface for this specific inverter if you still are looking to integrate your inverter with the grid.

Adam Lockert's picture

Hey y'all. I've been performing a great deal of service work on batt based grid tied systems, almost all of which are SW-4048's. Mostly battery replacements and an occasional charge controller upgrade. Based on the night time current draw potential, and the desire to be sure the system is selling as much as possible throughout the day I have been setting the sell volts around half to one volt below the float voltage setting on the SW. Am I thinking about this right? Are there any articles in the archives you could point me to that covers this in detail? Thanks.

mike werth's picture

"The Trace 4024 is gaining acceptance by utilities. It has been approved by Pacific Gas & Electric for utility intertie in Northern California. The Trace SW4024 is ETL certified to UL spec. 1741 (residential service), so there will be no problems with installations on grid."

Michael Welch's picture

Hi Mike. Thanks for sending that page, the company [ahem] "borrowed" that entire page from a 1995 article published in our magazine, without giving us or the authors any credit.

But the operative word here is "1995," and these inverters are excellent workhorses for off grid homes. But they are the oldest of grid-tied battery backup technologies, and do not perform up to snuff compared to newer grid-tied inverters.

Michael, Home Power

mike werth's picture

hi michael,
so even though it was accepted at one time (1995?), pg&e would no longer accept it today? would they disconnect someone who was previously approved? how does ul1741 differ from ieee1547 or whatever the current acceptable standard is?
since i have a relatively small system (1kw array, trace 4024, mx60 and 880ah battery bank @ 24v) i've never bothered to go through the hoops to qualify for net metering here in the vi. does this mean that the powers that be will not look kindly on my request if i should try?

Michael Welch's picture

The old Trace inverters are still on the CPUC's approved inverter list, so they would be accepted by a CA utility. While many states accept the CA list of approved inverters, there are no guarantees. It would be completely up to your local jurisdiction. (Though it is another story entirely whether you *should* do it, considering that those old Trace inverters did not do well at sellback. If I had one that I was considering tying into the grid, I would sell it and buy more modern equipment.)

I think both of those standards apply to all the inverters.

mike werth's picture

thanks for the reply.
when you say, "those old Trace inverters did not do well at sellback", do you mean from an efficiency or safety (or both) perspective and could you elaborate?

Justine Sanchez's picture

Hi Mike,

I just wanted to add to Michael's comments that in addition to pulling power from the grid every night to keep the batteries topped off (making those systems pretty inefficient), as the text states..."the SW series has gone through many different hardware and firmware versions since then, and most of them meet UL 1741 standards"... by "most" we are referring to the issue that came up a few years back some of them did "island" (i.e. pushed power out on the grid side, when the grid went down) and did not comply with UL1741 and lost their listing, however apparently units built before November 2000 retained their listing, for the other units I believe you must have the Grid Tie Interface (GTI) installed to comply.
Justine Sanchez
Home Power Magazine

mike werth's picture

thanks justine and michael. mine was installed in mid 1999 so i guess i'm ok. the system has required so little attention i really haven't kept up. i was under the impression that islanding was "not possible" with this inverter and have argued the point over the years with a few utiity folks/electricians. as the man said: "'s what we know that ain't so" that gets us in trouble. : >)
will be building a new house soon, so am starting to slog through what's currently available. any favorites for a grid tied, battery backup, net meetering system?

Michael Welch's picture

They are perfectly safe, that is what the standards are for. Now you are testing my memory.

As I recall, you have to manually take the inverter out of sell mode after the sun goes down, or it will try to keep the batteries at sell level, instead of float level, which means the charger in them would be on all night, running off the grid.

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