Zero Export Grid Tie

They keep changing the rules on grid tie. Avoid dual meters, special fees and changing deals by mixing your power with that from the power company, without backfeeding to the grid.

Many years ago, a fellow came up with a gadget he called a synchronous inverter. Today we call it a grid tie inverter, and it changed everything. It allowed you to mix sun power DC current with the utility AC current. If you were using a heavy load, the solar power would replace some or all of it. If you were not using much of a load it would flow out through the meter, spinning it backwards to reduce the power bill. As long as you used a little more from the grid than you made with the solar, things were ok. This was called guerilla solar.

If the power company came by to read your meter and got a negative number because you made more power than you used, well that is when the trouble started. Some power companies forbid you to do grid-tie, some encourage it, some pay you to do it, some make you pay dearly and they’ll pretty much all require some special equipment. Your mileage may vary. Check with your local power company before you start.

My friend Courtney got lucky with his system. He lived in South Carolina where they actually paid him to add solar, plus he got Federal tax credits, so the whole thing came out pretty much free. There was a price to be paid in paperwork and jumping through hoops. He and I did the work, but he had to have a licensed electrical contractor and a NABCEP solar installer inspect the work before it could be commissioned. He also had to have a mechanical engineer sign off on his pergola being strong enough to hold the panels. It was strong enough for all the panels and the two old guys putting them up there, but it had to have the engineer’s stamp of approval. There was lots of paperwork.

On the other hand, there’s my power co-op that throws up every obstacle to grid-tie. I know of three people who have done it, but I can’t see where they are saving much. Others have had their power companies welcome grid-tie, only to change the rules later and jack up the fees.

Being the rebel I am, I have spent about a year studying how to use your solar power and not get the power company involved. One lovely way of doing this, used by Tom, is to use a grid interactive inverter. These can usually do grid-tie, but can also provide standalone power for use if the grid is down. During the day, Tom’s array charges his batteries and they feed the DC to the inverter that makes the AC. The inverter also has an input from the grid. As long as the batteries have plenty of power, the house runs on solar. Tom uses loads of power. Computers, TV, lights and coffee pot are always on. The inverter lets him select just how much he will draw from the batteries. He sets a fairly shallow draw to give the batteries longer life.

To accomplish this, Tom has a “generator plug” and a transfer switch. There is an actual diesel generating setup, but for everyday purposes, his solar system is the generator. So, a grid line goes to the inverter and the inverter runs the house. At night, the A/C will drag the batteries down pretty quickly and the unit automatically switches to grid power. If a hurricane takes out the grid and blows away the solar array, he can fire up the generator and charge the batteries with it or run directly into the house.

Tom’s stuff is not grid-tie, though. Courtney brought over a pallet of LG 300 panels and a case of Enphase microinverters and challenged me to see what I could come up with for operating an air conditioner from solar. No problem, except that I can’t do grid tie.

I played with the microinverters on my off grid system, which goes to my “generator plug” to see how far I could push it without a visit from the big white truck. I figured a plan and calculated how much my A/C compressor used and selected enough panels and micro inverters to match. The output of the microinverters went underground to the MOTOR SIDE of the big contactor under the hood of the A/C. When the A/C comes on, the microinverters are energized and they connect sunlight power to the cooler. Success!

Limited success, I should say. By Federal law, the microinverter has to wait 5 minutes before it does its thing. If you have a short cycling A/C, then that is a problem. Most HVAC contractors oversize their gear, but in my case I have a system for upstairs and a system for downstairs. The smaller upstairs unit is only 1.5 tons, but it keeps the house comfortable downstairs, if a bit chilly upstairs, while running nearly constantly on a really hot day. In this configuration it really saved a lot on my power bill and I had the house a lot more comfortable than if I were buying power. Best of all, I did not have to deal with the power company.

So here are the limitations. We are only saving power on the A/C, we are saving nothing at night and very little on cloudy days. Can we improve on that? Why, yes we can! It turns out these particular Enphase inverters have a form of limiting in them. Though they are being fed with 300 watt panels, they can only feed a bit over 200 watts into the power line. That isn’t as kookie as it sounds. It sort of levels things out when the clouds pass. It also means you can connect batteries to the microinverters and use the panels to charge the batteries through charge controllers. These micros are designed to operate from 60 cell panels, so a 24 volt battery bank works fine as a feed. Now the microinverters can microinvert at night or when cloudy, within limits of course. We saved even more on A/C.

Can we do better? Is there a way to feed the power to the whole house without spilling any out the meter? Yup!

So I ripped out all the microinverters and started over. First, the panels were arranged in series pairs and the DC was sent underground to the “dog house” next to the A/C units. In the dog house, two Grid Tie Interfaces, Limiter (GTIL), sometimes known as Zero Export Grid Tie, were installed, each plugged into a 110 plug from each leg of the 220 line. Some of you may be thinking, “Hey I’ve seen those blue boxes and they make one for 220. Why not use that one?” There’s 220, Euro style and there’s 220 American split phase. The bigger blue box does not understand the American Way.

Anyway, you plug these GTIL boxes into 110 outlets, add solar and, voila, grid tie. But not limited. You’ll get the visit from the big white truck and probably quickly if you have a smart meter. It’ll call home and you are busted! However, there are sensing coils that go in the big gray breaker box. They can sense if the power is coming or going. With the sensor coils in place, turned the right direction and plugged into the right box, the solar power will come rushing into the house wiring, filling every light bulb and small appliance, but when it tries to squeeze out toward the meter, the sensors stop it in its tracks, leaving maybe 4 watts coming from each leg of the utility.

In my case, the boxes are called “1000” units, but actually rated at 950 watts each. That may be a bit of an exaggeration. Still, under ideal conditions, they will handle most of the background load, but get overwhelmed a bit when the clothes dryer or oven are used. The dryer, for example uses 5000 watts, normally, but if the GTIL pair can knock 1500 watts off that, then that adds up to some savings.

Let me interrupt for a moment, in case you are thinking of getting some of these wondrous boxes. If you already have a grid-tie system, it is quite possible that adding a simple gizmo will allow you to do the same tricks with your existing gear.

Back to the GTILs I use, am I perfectly happy? No. For a couple of reasons, or more. First of all, the two inverters are connected to the same solar array. Solar panels are a somewhat high impedance source of power, which is to say that when you put a load on them, the voltage goes down. If one of the boxes is providing 800 watts to the coffee maker and someone starts up something substantial on the other leg, the solar source voltage drags down a bit further causing the first unit to trip out while the second tries to take charge. Before you know it a full-on fight ensues and nobody is contributing power. Sometimes both inverters will settle down in harmony and then a puffy little cloud goes sailing by and everything trips out again. Can this be fixed? Yes, to some degree, by simply dividing the array into two separate branches. The thing is, the solar panels are 200 feet from the point of use and digging trenches and pulling expensive cable is a bother. No, it isn’t quite as dire as that, in truth. I do have several cables in parallel that could be separated, but I have bigger plans, so no use getting involved in wrestling with heavy gage wires.

The next phase of the plan is to add batteries. Again, we will have power available around the clock, as we did with the microinverters. Batteries are a low impedance power source, so the two GTIL units should not fight.

The next step took some time to complete because I needed batteries. I looked far and wide for just the right batteries for my Solar Yacht project with the idea of running the house on them as a test and putting the old batteries, Sun 230s from Sun Electronics, on the GTIL project. Batteries, cash and opportunity converged and I had batteries.

The lines from the solar panels, LG 300s, operated in parallel pairs, for a total of 2400 watts, were disconnected from the GTIL units and reconnected to a 60 amp MPPT charge controller I found in a drawer. The output of that was connected to terminal blocks with connections to the battery and the GTILS. Oh, yeah, there were some fuses, circuit breakers and a surge protector thrown inline for safety. It worked.

Then Courtney wanted his panels back, so I put up a kilowatt of panels from a boat project. Less power, but since they were charging the batteries there is usually enough. Now the only limitation is high usage on a cloudy day, but isn’t that always the case?

Now the batteries can run loads any hour of the day or night. Cooking on the stove top? Covered. Electric clothes dryer? Knocks off nearly a third of that. Washing machine? Covered. Baking blackberry cobblers? Uses only about 1 kwh from the grid for a 5000 watt oven. There were two mystery loads that showed up on the monitors. They turned out to be a fridge and a freezer that had been left on the grid. They have been moved over to the all solar system, but they were being covered by the battery GTIL.

There was 72 kwh use from the grid this month, about half of last month. I had projected that it would only be 50 kwh, but in the corona lockdown, my wife deals with the situation by doing laundry. Many more hours of dryer time are being consumed than are actually needed, by my calculation, but my calculations don’t seem to count. I keep quiet about her laundry obsession and she makes me cobbler. I don’t mind losing a kwh to get a bowl of cobbler! The limited grid tie took care of an additional 67 kwh. The grid tie solar would have provided more power, but about the 6^th day I discovered I had an unexpected load of a fridge and a chest freezer that had been overlooked. They were moved to the main solar circuit so they would be left running if the grid went down. They would have used another 72kwh from the grid tie.

OK, so if I have explained this properly, you may be wondering how you can get some of this limiting grid tie stuff, am I right? If you already have a grid-tie power system, you may be able to add the limiting system.

Most GT systems have an optional power monitoring module available that will allow you to keep up with your power use and grid interactions. Some of these have sensing coils available and they have the software to convert to limiting operation. Check your manual. It’ll probably be a very small mention. I know that SMA/Sunny Boy has a power meter accessory that can do this. Elgrispower.com has one they claim works with about a dozen different grid tie inverters.

No, if you are running a 600v string inverter system, you probably don’t even want to think about running batteries. If the power company decided to charge you a lot of money to connect grid tie, you can be rid of that nonsense because the power is being mixed in your house and never going out to join forces with their kilowatts.

If you have net metering or some arrangement that is satisfactory, then never mind. If your power company is trying to discourage solar and jacking up your bill with fees, then this could be for you. It is the rebirth of guerilla solar and may be just the thing to save you some money on your power bill.–Neal