# Switching between grid and battery backup



## greg273 (Aug 5, 2003)

Hi all...

I'm basically wondering what sort of switch I would need to manually switch from a 1.1KW solar array with 770ah battery bank at 24volts, to grid power and back if necessary.
Basically I can get an underground grid connect for $1700, labor included. We have an electric co-op, and apparently there rates to connect are reasonable. They are willing to run 800 ft of underground cable for that price. Thats tough to pass up, considering the convenience factor of having a grid tie. 
Now, the current inverter is a DR2424... and oldie. I havent checked into any newer grid-tie inverters, but I read somewhere I would need a higher voltage system on my end to make true grid-tie work...I am not envisioning selling power back to the utility. With such a small system, it would be tough to justifiy the investment in a new, very expensive grid tie inverter.

I am envisioning a transfer switch... that way, I can have the convienience of grid power, while still maintaining the old system for battery backup and occasional use on long stretches of bright sunny days.


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## TnAndy (Sep 15, 2005)

You envision correctly.....you will need a transfer switch. 

You'll want a manual version, like I have, since you want to control the "switch over", whereas an automatic version is designed to only transfer when the grid goes down. Also ( and the reason I chose this version ), they are a LOT cheaper.

You'll need one like this:










I take it you don't have grid power now, so I don't know you distribute/use AC power now. I already was grid power based, then added the solar. My 200amp main panel is the white one on the left. I added the 100amp subpanel in the center, and moved the circuits I wanted to run off solar into them ( lights/fridge/gate opener/etc ). With the transfer switch arm in the upper position, as shown, those circuits run off the grid....when thrown to the down position, they run off solar/battery.

That's a Cutler Hammer transfer switch, it ran about 600 bucks.


Also, when you said: "but I read somewhere I would need a higher voltage system on my end to make true grid-tie work."

I had to put in a pair of Outback inverters, 24v, 2500watt, (GTFX2524) to put out true 240v AC ( master/slave setup, controlled by an Outback Hub/Mate you'd also have to buy ) because my utility would not accept 120v AC input....they would have had to buy a special meter to handle it.....they did that on the first grid tie system in our utility, then said "no more.....all future connects must be 2 wire, 240v"


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## greg273 (Aug 5, 2003)

Thanks Andy. That looks like the device I'm looking for. Harbor freight of all places had the cheapest deal on a Cutler-Hammer 200 amp switch. I was getting worried...the 'Square D' ones were almost $900!


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## adamtheha (Mar 14, 2007)

Outback inverters will accept grid power, pass it along to the house panel, and will automatically switch to battery when the grid fails.


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## TnAndy (Sep 15, 2005)

greg273 said:


> Thanks Andy. That looks like the device I'm looking for. Harbor freight of all places had the cheapest deal on a Cutler-Hammer 200 amp switch. I was getting worried...the 'Square D' ones were almost $900!


Actually, the 100amp one is the one I have, and will probably do you unless you plan to set up a REALLY big solar power system. Mine is 5,000watts on the twin inverters, and when you divide by 240v, you see that's not even 25 amps.

The way you'll want to set it up is pretty much what I modified mine to now:

Set a main AC panel for "normal" grid use....size based on your expected use and number of circuits needed. Then set a small subpanel ( like a 60 to 100amp, depending on the number of circuits and any growth you might anticipate in your solar down the road ), with the transfer switch able to toggle between using those circuits in the subpanel off the grid, or off solar/battery as you decide, and flip the switch.


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## greg273 (Aug 5, 2003)

Andy, just curious as to what prevents you from running your entire main panel from solar? 

I've got a transfer switch, found one locally at an electrical supply store. I am planning to run the output from that into the main panel, with input from either solar and grid. Dont anticipate too many problems, except where to bond the neutral to the ground?? The main panel already has a ground/neutral bond... but it looks like the meter base has a lug for a ground wire also... Would this not violate the 'neutral/ground bond in ONE place only' rule?


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## nadja (May 22, 2011)

Andy can run his whole house on solar, but his wife will most likely beat him if he does Then she couldn't use the hair dryers and gadget like that. Ask him to show you his whole system. It is quite impressive to say the least


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## TnAndy (Sep 15, 2005)

greg273 said:


> Andy, just curious as to what prevents you from running your entire main panel from solar?


Assuming you mean while NOT connected to grid power, it's not possible. 

My main panel is 200amp. Multiply that times 240v, and you get 48,000 watts one COULD draw, in theory.

I have 5,000 watts of max inverter power.

See the problem ? 

Somebody flip on the electric dryer while the fridge was running and it would be all over.

Now, while CONNECTED to grid power, the solar DOES flow to the main panel IF my use requires it....otherwise, my regular "buy" meter turns backwards as power flows out to the grid.


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