I was looking at Chris Olson's videos on youtube on the use of a moderately sized inverter, with a generator backup to handle the peak loads. Was thinking in my case, of use of a moderately sized system that handles nominal loads entirely on solar but calls from the grid for peak load help. Is this possible and practical? Would this be considered grid-tie?
We are off-grid, but over time have gone to one of the biggest inverters you can buy, combined with generator support to eliminate the need for dual inverters and more batteries.
The same inverter we have can do grid support as well as it does generator support, and it is not considered grid-tie because you are not selling back to the utility or net-metering power. Basically what the inverter does is sync the sine waves on both legs (L1 and L2) to the utility, then uses your solar power to offset how much you use from the grid. If you have enough solar (and battery capacity for overnight) it will use a minimal amount from the grid (just enough so it can sample the stability of your grid power and stay sync'd with it) and most of your loads will be powered by solar. If the solar isn't sufficient it will continue supplementing the grid from battery power until the batteries are discharged to the point you set, then stop using renewable and start using all grid power until the solar comes back. If your grid power goes out, the inverter takes over automatically and all your loads are on battery/solar until the grid comes back.
The inverter has dual AC inputs - one for grid, one for generator. So you can also incorporate either a manually started genset, or an auto-start genset controlled by the system, for extended power outages. If your power is out then you are off-grid just like we are and the inverter will use generator support automatically (with an auto-start genset) if the inverter overloads. These inverters (Schneider Conext XW/XW+ series are capable of handling extreme overloads for quite awhile before they shut down due to thermal issues. We use 20-30 kWh/day in our off-grid home, all totally electric including water heating, range, electric clothes dryer and my workshop with two welders, air compressor, lathe, vertical mill, etc.. and it powers the whole works with no problem.
This is a current photo of our power room for the house and shop. All the system inputs are on the left, including generators, wind turbines and solar. AC power out is on the right with the inverter, of course. Load centers are in the middle. The load centers are dual 100A SquareD QO, one for the house, the bottom one for the shop. The battery bank is down below in an insulated and sealed case with a duct fan blowing warm air into the case in the winter, and cool air in the summer from the basement in the house.
So the DC power flow is from the left to the battery bank and DC bus panel (with ammeter on it) to the inverter. The AC power flow can be switched with the 200A Ronk transfer switch on the left, and with the SquareD QO Load Center panel immediately to the left of the inverter. The QO panel has two 100A breakers interconnected with a SquareD QO2DTI interlock, and a third two-pole breaker for the water heaters. The combination of the two transfer panels allows me to connect any one of three generators either direct to loads, or to the inverter's AC2 input.
This is the system controls on our kitchen wall - the system is totally automated, including water heating controls, and basically all we have to do is monitor it. We interact with, or mess with settings, on our power system very little anymore, as it provides us with pretty much the equivalent of having grid power @ 20-30 kWh/day energy consumption.
I recently put together an estimated total cost figure for our system and came up with around $76,000. Off-grid power is not cheap. Even if your utility tripled their prices, it is still cheaper than off-grid power. Our system is considerably, by several times, more reliable than utility power. But the batteries alone, on a 10 year replacement interval, cost 18 cents for every kWh they store and put back into the system. It should be noted that not all the DC power goes thru the battery, as a good portion of it goes directly from DC sources to the bus to the inverter. But the batteries are the Achilles Heel of an off-grid system.
