Grid Tie system with enphase M250's

essenz

Hi all, I am new to the forum and had a question that I have been pondering.

I am looking to build a 5.5KW Grid-Tie system using the enphase M250 micro-inverters and 20x SolarWorld 275W panels.

The issue I keep coming back to is this topic of being able to "use" the PV power when your utility is down. Last winter, we had a few multi day outages following snow storms, and those days were 100% clear and sunny.

I know the enphase M250's, like most grid-tie inverters, are designed to not operate when they cant sense grid power, this is a safety issue so power doesn't leak back into the street where there might be a lineman worker on live wires.

There are grid-tie systems with battery backup that get around this, but they are very complicated on the wiring side and dont work with micro-inverter AC systems, they want a traditional DC bus PV system.

Now my utility requires an outside disconnect after their meter in order to be granted net-metering, so that means I can break my link with the utility during an outage - safety issue resolved. Now I just need to "trick" my enphase M250's to think there is grid power. I was thinking, why couldn't I create a mini-grid just to fool the M250. The mini-grid could simply be a 240V 2-ph inverter with a few batteries that feeds into by AC bus. Would this trick the M250 since it would now "see" power and allow it to put my PV power on my local AC bus, or is there something else going on that would prevent this? My concern is dumping 5KW of PV power onto my local AC bus without there being a place for it to go, i.e. my house load is say 1KW, and I have 5KW being dumped on the AC bus, that excess power cant go back to the utility grid, it sticks around, would that fry something?

Its just really annoying that I will have 5.5KW of power above my head, but during an outage I cant use it...

-John

solar_dave

Re: Grid Tie system with enphase M250's

We hear this quite often and usually the answer is always more trouble than its worth. Yes there are Hybrid systems that have battery and grid tie, however the cost to have one of these for an occasional extended outage is pretty prohibitive. Your generally better off getting a small generator to carry your really essential loads until the grid comes back.

inetdog

Re: Grid Tie system with enphase M250's

John,

The unfortunate fly in the ointment is that whenever your local loads are lower than the output of your panels, your GTI will do its best to feed power back into the small off-grid inverter, which will not survive very long.
The only practical ways to do what you want are:
1. Use a newer model SMA TL-x000-US with secure power supply feature.
2. Pair an off grid inverter and a GTI from the same manufacturer which are designed to work together. (SMA among others offers this option)
3. Get a hybrid inverter in the first place instead of your current GTI.
All of these are big buck scenarios that do not make use of what you already have.

jebatty

Re: Grid Tie system with enphase M250's

I had the same wish and the same question, and I got the same answers which make a lot of sense. I have a 2000 watt inverter generator when only minimal power is needed during an outage, and I use an extension cord. I also have a 5500 watt/240volt generator with a transfer switch to power the most important house circuits when they may be an extended outage.