I am building am off-grid solar power system for my home. In a previous home I did the same, using a transfer switch from inverter to service panel. I want to avoid the complexity of a transfer switch this time, connecting the pure sine wave output from a large capacity inverter directly to my service panel, taking advantage of the branch circuit breakers and in-home wiring. I'll use an interlock switch to isolate the 200a main breaker.
1) Am I correct that I can simply add the proper sized branch circuit breaker and connect the inverter output directly to that? Of course, the interlock switch will prevent the 200a breaker from being on at the same time. 2) Must I use a split phase inverter? If the full power from the inverter (8,000w to 10,000w) is input with the one breaker I install, will 240vac items draw from that as well - or do I have to run a separate breaker for 240vac? I would truly appreciate anyone's assistance with this. John

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    Hello, and welcome to Stack Exchange. A diagram or picture of what you're planning would really help us understand your question. Feb 25, 2016 at 1:43
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    You probably need a transfer switch to meet code. Remember, the purpose of such switches is to ensure you don't electrocute a power company worker who has every reason to believe the line has been disconnected. This is NOT something to cheat on unless you will ne hoing so completely off grid that there is no longer a connection to the grid at all.
    – keshlam
    Feb 25, 2016 at 2:52
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    Ditto. Do it right. Do it safe. DO NOT cheap out on this. The nature of the questions being asked here also seems to indicate that there are reasons to bring in a professional to do the proper installation.
    – Michael Karas
    Feb 25, 2016 at 10:15
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    If the safety is listed it could be used, the way you are asking about the feed bothers me. With a large inverter set for 240v you would feed both L1 & L2 and your 240 devices would use that power. If you only have 120 to feed both sides you would have to transform the power up to 240 with a 1:2 transformer quite expensive at 8-10kw, a 240v inverter would make more sense and have less system loss than transformers.
    – Ed Beal
    Feb 25, 2016 at 14:43
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    I concur that listed safety interlocks should suffice since that is their purpose. A transfer switch is just a gold plated, automatable and remotable way to do that same thing. Feb 25, 2016 at 17:24

2 Answers 2


Note that the simplest solution may be net metering.Where permitted, this uses inverters that monitor and synch with the power lines, and therefore can be connected in parallel with the grid. At the same time, your meter is replaced with one that can count both forward and backward; if you are producing more power than you need the excess gets sold to the grid, and when the balance goes the other way (eg at night), or when clouds cut your production) you buy power.

Major advantages include continuous aitomatic balancing of the two sources, no need to maintain a battery bank, and the joy of the midsummer bill when the power company may owe you money. (I haven't quite gotten a negative bill yet -- I've gotten negative grid usage, but not enough to overcome the $6/month account fee.)

Disadvantage is that, for safety reasons, these systems are designed to shut down when power is lost on the grid to avoid the risk of electrocuting someone who would have expected the downstream end of the break not to have voltage on it.

Theoretically it would be possible to run disconnected (in "island" mode"), but them you'd need to re-introduce transfer switches for safety, and you'd need a 60Hz 240-split reference for the inverters to sync to. I can imagine a setup where you'd disconnect from the grid and connect instead to a generator as "local grid reference", with the solar panels and the generator sharing the load... but that has complications in protecting the generator from reverse current, and it isn't a approved/supported configuration for any manufacturer I'm aware of. Maybe someday...

Anyway, I just wanted to toss the out as an alternative to explicitly switching between solar and grid. Depending on your goals and needs, it may or may not be the best answer.


1) Yes. you may simply add a properly sized breaker for your inverter.

2) Yes, You must use a split-phase inverter. NorthAmerican homes are generally split-phase.

The breaker you use for your inverter will also have to be a double-pole breaker.

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