Why "critical load panel only" when grid is down?

[ButchDeal]

ButchDeal can you expand on that...I think you may be onto where my thinking is wrong... thanks

Sure I will try to plane this out.
Firstly though others comments are correct. your system is not large enough to run your whole house.
So the radians are similar to my Flexpower except they have 3 AC connections. So lets look at it like one inverter system for simplicity.
There is this box with 3 AC connections:
AC1 goes to the GRID
AC2 goes to a generator if available
AC3 goes to critical load
DC1 (only one) goes to the battery
PV is connected to Charge Controller on one side and to the battery on the other.

Your system is using net metering and from your descriptions you have:

GRID <--------> Meter <-------> MSP <-------> Radian <-------> Critical load panel

Now when everything is going along good, grid up and all. then AC1 and AC3 are tied together inside the radian and the radian is feeding power into them both.
The inverter has a computer (MATE3) which looks at the CC (Charge Controllers) and how much power they are generating, as well as the battery monitor to determine if charge is needed. It enables the inverter to invert above what is possibly needed to charge. (this is a simplification). the power flows out AC1 and AC3. In most respects the inverter works like any non-battery grid tied inverter as long as the grid is up. Like my system most grid tied net metered bimodal systems like this will have the batteries fully charged almost all the time. Since the batteries are charged nearly all the power goes to the inverter.

Now say the grid goes down: The radian disconnects from AC1 so that it doesn't fry anyone working on the lines. It however keeps AC3 up and running, using the DC bus. The CC is used to feed the DC bus as much as possible but if not enough power comes from the batteries (no logic here there is just one DC bus).
If there is a generator, the MATE3 can start it when batteries get low and it will feed in through AC2 to charge the batteries and run load, till the batteries are charged.

[kingofbanff]

You seem to think that you can feed power out of the inverter from solar only on one lead and out the other lead from battery. It doesn't work that way the solar is tied to the battery.

ButchDeal can you expand on that...I think you may be onto where my thinking is wrong... thanks

[gmanInPA]

... To duplicate that with batteries you would need another addition to your house to hold 48 volt 4000 AH, 12,000 pound $42,000 battery for you to replace every 5 or 6 years.

What's wrong Sunking - aren't you in the market for a $42k, 6-ton battery? I keep a few dozen laying around just in case A generator is heavy and expensive, but FAR less than batteries!

kingofbanff - Running more than critical loads is just not feasible for an average home with batteries and PV. Also, keep in mind the load that would be immediately placed in your inverter is say... you have a roast in the oven, a load in the dryer, the microwave running, and the iron heating up. That is a tremendous amount of wattage not only for batteries, but for an inverter. You would need quite a few inverters to handle such which would add to the $42k/6-ton batteries you'd need.

To try to get the most out of my own system, I installed a generator selector switch in my power panel. Under normal conditions, it provides backup generator power to my critical loads (and inverter, charger, and non-6-ton-battery). If for some reason I want to run something non-critical, I can toggle a switch in my power panel, flip a main breaker interlock, and run the whole house on the generator. Because my inverter is on a branch circuit, this then ultimately delivers power back to the inverter to charge batteries, support loads, etc - but prevents any possibility of back feeding, etc.

[Sunking]

If you would have used a generator you would have avoided that, and not have to replace those batteries every 5 years. Would have been a lot less expensive and more reliable.

The main issue is your system is a SOFT SOURCE with very limited power. It cannot run everything in your home like the grid can do. The grid is a STIFF SOURCE and for all practical purposes unlimited power. The limitation is you ATS and batteries. The ATS is most likely 60 amp and that limits what you can power during an outage. Mostly restricted to a few light, refrigerator, and a few outlets. It comes down to this, your batteries are not capable of supplying much power for any length of time. To power a large luxury home like you have, would likely take a 20 to 25 Kw generator. To duplicate that with batteries you would need another addition to your house to hold 48 volt 4000 AH, 12,000 pound $42,000 battery for you to replace every 5 or 6 years.

[solarix]

Your house's main panel is probably 200A right? Even a Radian 8048 and 4048 (12kW total) can output a maximum continuous of 50A or one quarter of what the load could be. Now, granted - your house will rarely go over 50A, but your inverters would shut down at those times. The purpose of the dedicated subpanel is to limit the load to what the inverters can handle.

[ButchDeal]

Thanks for the quick reply. If my inverters (Radian 8048 and 4048 probably) are large enough why can't I power both panels with my PV when the grid is down?

If you would like to you could by doing a lineside tap and making your existing service panel be the critical load panel.
I do not think that your two inverters are large enough though. You do have a pretty large system so you will have a pretty large critical load panel.

You seem to think that you can feed power out of the inverter from solar only on one lead and out the other lead from battery. It doesn't work that way the solar is tied to the battery.

[kingofbanff]

If you make more the excess is fed back to the grid. If you make less then the grid makes up the difference.

When the grid is down, your main panel and critical panels are isolated, and the inverter runs only critical loads.

Thanks for the quick reply. If my inverters (Radian 8048 and 4048 probably) are large enough why can't I power both panels with my PV when the grid is down?

[jflorey2]

Most designs I see isolate certain loads into the critical sub panel and only have that fed when the grid is down. Is there any reason I can't have both my main and critical panels fed from my modules and have my batteries back up only the critical loads?

You do; that's how the system works. When the grid is up, everything (main panel, critical panel, inverter output) are connected together. If you make as much as you use, your inverter is supplying your entire load. If you make more the excess is fed back to the grid. If you make less then the grid makes up the difference.

When the grid is down, your main panel and critical panels are isolated, and the inverter runs only critical loads.

[kingofbanff]

Forgive my newbieness...

I'm installing 13kW grid tie with battery back up in Denver. The set up ensures that if the grid goes down I can still use any power from my modules.

Most designs I see isolate certain loads into the critical sub panel and only have that fed when the grid is down. Is there any reason I can't have both my main and critical panels fed from my modules and have my batteries back up only the critical loads?
I will have more than enough power for everything on a sunny day so it would be a shame not to run my blast furnace (attempt at levity).
I'm not getting a huge battery set up so I don't want that attached to the whole house.
Thanks in advance.