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Posted by Steven in Colorado Springs, CO on 1/2/2011

I'd mentioned a couple of days ago that Solar LeRoy had put on an adapter such that I could plug a generator into the system if necessary. Today I decided to test this connection while up working on the Punch List.

Note that this is not the "ideal" way one's supposed to tie a generator into one's solar system. The Outback hardware (and I presume the other various brands as well) have a mechanism for hooking up a nice big house backup generator (like these) to serve as backup power in the event that you demand more than your batteries can provide, or if you are using lead acid batteries to occasionally make equalization runs on them to extend their operational life. For an off-grid house like Tanglewood, these are almost universally propane or diesel devices, though for an in-town house I've seen natural-gas varieties as well. These are always larger, beefier generators a step up in quality (and price!) from your standard "construction generator" that provide cleaner and more stable power, but they're built to handle an entire house rather than just a few tools. They will have a remote start capability (sometimes called an Automatic Generator Start or AGS) designed for them to be switched on by some remote device (either a sensor noting that you've lost grid power or, if you're off-grid, the system inverters deciding that the batteries can't handle the load by themselves). Price-wise they're not unreasonable (in my opinion anyway) and I plan to add one to Tanglewood this coming summer. 

However backup generators are not, except in rare circumstances, gasoline powered. That's because they don't typically have the lifespan and you won't always get a reliable start on a gasoline generator. An automated backup generator needs to start on a moment's notice and come online with a few hundred milliseconds of receiving a signal from the system or you'll lose power at the far end. Most gasoline generators just aren't that reliable at starting; I've got two of them (a 4000W Generac and an 8000W Rigid) and while they're both fine generators and have proven to be quite reliable, they certainly couldn't be called "instant start". Even if they did have an AGS connection, I frankly wouldn't trust it, and as it happens most gasoline-powered generators don't have AGS for exactly those reasons.

However all of that doesn't stop me from hooking up one of my generators to the system manually to see how well it all works, and so that's what I did today. The process was simple enough in theory--the system is pulling power from the PV panels, and if a new supply comes in on the generator line (it's a separate input), it'll just start taking that power too. Normally if you're in full upcharging mode, it would take as much power from both sources simultaneously as it could (within the limits of system amperage), but if your batteries are already nearly full, there's logic in the charge controllers and inverters to ramp down the supply from either one or both. This  helps to protect the system from overcharging, and keeps the power loads inside that big gray box on the wall where they need to be.

So I walked through a series of snapshots this afternoon, starting with the system as it was towards the end of its charging cycle (just after 2 in the afternoon) through the evening when I headed out. I used my smaller Generac 4000W generator mostly, because I'd positioned it up near the solar shed for emergency use should it ever be necessary (and besides it's probably a third the weight as my Rigid generator is), so I knew that at best the system would pull 4,000 watts on the generator line. The results are kinda interesting (at least to a geek like myself).

First off (and I really should have remembered this) the Generac is a construction generator and so puts out what is called a "modified sine wave" on the power. That means that it doesn't provide the nice "pure sine wave" electrical signal that a house backup generator will give you, so my inverters would whine that they were receiving a "low input voltage" from time to time (and you can see that in one of the snapshots). This "modified sine wave" is common with most construction generators because it's frankly a lot cheaper hardware-wise,  and when you're dealing with a big circular saw or something you never even notice it. It'll play merry hell with your more more-sensitive stuff though like electronics, clocks, computers, that kind of thing. Some off-grid folks have tried to go the cheaper route with generators (and I guess there are a few inverters out there that do this too) that provide modified-sine-wave power since that costs less, but unless they have very simple requirements and no electronics to speak of, it's not a satisfactory arrangement.

Secondly, I may or may not have something I need to have Solar LeRoy investigate--when I turned on the generator and began pulling power from it the power coming in from the PV array went down. Now, this could be because the batteries were already full enough that the system didn't need the extra power, so I'm really not sure at this point, but I'll pass it on and let him double-check everything. This is something that could easily be a setting in the bowels of the inverter or charge-controller hardware (did I mention that I'm underwhelmed by the control interface on these things?) too, and simply something we've not looked at before. 

Third, I found it was a piece of cake to fire this up and that was a good thing, too. If necessary (say, several days of cloudy weather or an unusual load in the house) I can fire up one of my generators to charge up the system even after the sunlight window has passed and things will work great. The system will whine about the incoming power's sine wave, but that won't hurt it for these limited uses, and it's good to know I've got the option either way.

Very neat. I look forward to installing my more-proper system down the road, of course, but for now this will serve well for emergency situations over the winter.

Steven in Colorado Springs


Snapshot before I brought the generator online. You can see the panels are producing about 2,500 watts.
The generator's online now; you can see it putting about 1,300 watts into the system on the left of the upper window. Note how the PV input, however, has dropped by just about exactly that amount.
By about 4:00, the system had to be just about full as it's only pulling 1,456 watts from the generator. The PV array is barely supplying anything at all this late in the afternoon.
Last snapshot just before I shut things down. You can see the yellow "CHARGE" indicator on the upper right; that's showing that the system is unhappy with the low voltage the generator is providing.

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