Grid-Tied with Backup - question

[Bruce Bon]

In Home Power magazine (Oct-Nov 2010 issue, p. 61), Flint Richter says "...backed-up loads must be separated from non-backed-up loads and placed in a dedicated service panel". I don't understand the rationale for this, and would appreciate further explanation if anyone has it.

I know that at least some load analysis must be done to size the system; i.e. what is the maximum power loading expected when the grid is down and you're in backup mode. But I presume that the hybrid inverter (e.g. Xantrex XW6048) will shut down if you overload it. Why can't I connect such a system into my main service panel and, in the infrequent case where the grid goes down, simply flip non-essential breakers off or, even simpler, not turn non-essential loads on? If I overload it, wouldn't the hybrid system shut down without damage and then allow me to reset it after I removed excess load? I can't believe such a system would not have the equivalent of a master circuit breaker to handle overload situations.

[Steven Fahey]

Maybe you could do that, but
a) a licensed electrician couldn't sign it off, and
b) you would be missing the advantage of the system

The XW can make a seamless switch (they say) from grid AC to inverter AC, so your essential loads don't even blink. If they are in a separate panel, with the inverter able to feed them independent of the grid. The grid can go down and you don't have to jump and frantically flip breakers!

The grid can go down at 2AM while you're asleep, and you can wake up on time for work tomorrow, with a heated house and toast for breakfast. Okay it might not be so utopian, but properly designed, the back-up system will keep you going for a while without interruption. The 6048 offers this. It has to be part of an appropriate system to work out that way, of course.

So if you can have that, why would you deliberately cause it to shut down, leaving you with NO BACKUP power?

[Rob Beckers]

Hi Bruce,

You can, legally, do what propose. The inverter will indeed shut down in case of overload. What will not work, as Stephen pointed out, is using the build-in transfer switch of the inverter. It's not rated at the currents needed for a whole house (ie. 200A service usually). So, you would have to either use a manual or automatic transfer switch, much like those used for generators, to switch from grid to inverter power during a power outage. Those are far from seamless, as the one inside the inverter is. That's why a sub-panel is used with just critical loads, to power in case the grid goes down.

-RoB-

[Bruce Bon]

I think I get the picture, that the internal transfer switch of the inverter has pretty smart electronics, to keep everything running like nothing had happened. And I guess that the external transfer switch, whether manual or automatic, would have to shut off the grid connection before turning on the backup connection, so there would necessarily be a momentary blackout, perhaps enough to mess up clocks and computers unless they were equipped with their own UPS.