Grid-Tied with Backup - question

[WantsItAll]

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.

[greenHouse]

Because the pass-thru relays have limited capacity. I have a twin stack of GVFX 3648's feeding a 60A sub panel. This is acceptable since the pass-thru rating of the GVFX 3648's is 60A. But if you have a 200A panel, you can't pass that potential current through a device with a 60A limit, you have to create a subpanel that's limited to 60A and only feed that panel with the inverter(s).

[WantsItAll]

Hybrid inverter pass-thru capacity

So, if I understand you correctly, in non-backup mode (i.e. grid is up), all of the current needed for the backed up loads (your 60A sub panel) passes through the hybrid inverter stack (GVFX 3648's, in your case). This would explain the sub panel requirement, but I didn't know that was how it worked.

This would imply that, if I wanted to backup a 200A main panel, i.e. all loads, I would have to have hybrid inverter(s) with a total pass-thru capacity of 200A. This would not necessarily mean that the inverter(s) would have to be able to furnish 200A in backup mode. To fully answer my question, and to figure out what is financially feasible, I need to know the capabilities and costs of available hybrid inverters. I will ask that in a separate thread.

Thanks for your helpful response.

Because the pass-thru relays have limited capacity. I have a twin stack of GVFX 3648's feeding a 60A sub panel. This is acceptable since the pass-thru rating of the GVFX 3648's is 60A. But if you have a 200A panel, you can't pass that potential current through a device with a 60A limit, you have to create a subpanel that's limited to 60A and only feed that panel with the inverter(s).

[russ]

Hi WantsItAll - Welcome to Solar Panel Talk!

Thanks to Julie for the reply for your first question!

Looking forward to comments on your second post!

Welcome again,
Russ

[Sunking]

I see you made it over here now. Same answer as at the other site. Batteries cannot supply the large wattage loads. You have to separate the the loads to bare minimum when on battery. No such headaches with a generator.

[greenHouse]

I see you made it over here now. Same answer as at the other site. Batteries cannot supply the large wattage loads. You have to separate the the loads to bare minimum when on battery. No such headaches with a generator.

That's not at all the case. I can supply all the "reasonable" loads on a 60A subpanel with a twin stack of GVFX 3648's. I've done it for days and weeks on end because the system was properly designed in the first place (I designed it ...)

And it's not at all the case that "no such headaches with a generator" since the system still has to be designed so that the currents don't exceed the ampacities of the components.

More people in the solar biz need to get out and study residential wiring methods and system designs. This isn't hard stuff, but the answers are more involved than "you need a generator" and "it can't be done."

[Sunking]

And it's not at all the case that "no such headaches with a generator" since the system still has to be designed so that the currents don't exceed the ampacities of the components.

I never said anything about currents. My reference is the limited finite energy a given battery can store. If you have a reasonable inexpensive battery bank of say $5000 gives you around 5 to 6 Kwh per day. You have to count and conserve every watt hour you can, otherwise you will deplete and do permanent damage to the battery. With a 5 to 8 Kva Generator of the same cost, no worries as it can produce up to 192 Kwh each day.

[greenHouse]

I never said anything about currents. My reference is the limited finite energy a given battery can store. If you have a reasonable inexpensive battery bank of say $5000 gives you around 5 to 6 Kwh per day. You have to count and conserve every watt hour you can, otherwise you will deplete and do permanent damage to the battery. With a 5 to 8 Kva Generator of the same cost, no worries as it can produce up to 192 Kwh each day.

First off, 8KVA isn't enough to power a 200A panel, which is what this thread is about. Backing up a 200A panel (24KVA), at 100% of load, is a pretty phat prime mover, and that's a lot more than $5,000. If you want to put in a cheap-o generator that isn't rated as a prime mover, that's fine.

Secondly, you're comparing apples to oranges since a generator has fuel and maintenance costs that well exceed what solar costs over the lifetime of the system. For example, backing up a 400 watt load (that other thread you responded to) requires about 8 =gallons= of gasoline per day. At $2.50 / gallon for "regular" here in the States, that's $20 per day in fuel costs, or $600 / month.

[Sunking]

First off, 8KVA isn't enough to power a 200A panel, which is what this thread is about. Backing up a 200A panel (24KVA), at 100% of load, is a pretty phat prime mover, and that's a lot more than $5,000. If you want to put in a cheap-o generator that isn't rated as a prime mover, that's fine.

Well for 1 thing a 200 amp service panel for a residence can supply 48 Kva, not 24 Kva. Second if you have ever worked load calculations for distribution and home with a 200 amp service will never demand 48 Kva or even 24 Kva for that matter . If the a utility sets a transformer to serve 1 home with a 200 amp service, they install a 12 to 18 Kva transformer which is a bit of overkill in most circumstances unless the customer has resistive electric heating. The purpose of having a 200 amp service panel is so the user can have up to 42 branch circuits of up to 30 amps each.

Secondly, you're comparing apples to oranges since a generator has fuel and maintenance costs that well exceed what solar costs over the lifetime of the system.

No the initial cost and maintenance cost of a generator are far less than a battery back up system. The generator only uses fuel when it is running. With utility availability of greater than 98% of the time, at moist you might run the generator 1 week out of the year, and that is only after a significant event like a hurricane or tornado. Otherwise outages are measured in a few short hours. After 5 or so years with battery back up you are replace a very expensive set of batteries. A generator would never use that much fuel in 5 years operating in emergency stand by service.

If solar battery back up was less expensive and more reliable than a generator, commercial industrial would use it rather than generators.

[greenHouse]

The purpose of having a 200 amp service panel is so the user can have up to 42 branch circuits of up to 30 amps each.

The purpose of having a 200A service is telling your neighbors you have a 200A service. That and satisfying the load calculations. The number of slots is incidental -- a 42 slot panel protected at 100A can have 42 30A breakers and be very code compliant. A waste of a 42 slot panel, yes, but that's irrelevant to the OPs question.

No the initial cost and maintenance cost of a generator are far less than a battery back up system.

On a 5KW system, the incremental cost of adding batteries is about $5K. Whether or not that $5K is worthwhile is more a function of frequency of outages. It really is only about $1 / watt more, done right.

Nor are batteries on a 5 year replacement schedule. Even in off-grid environments (worst case), well-designed battery systems can (and do) last 10 or more years.