For some years now, my grid tied, battery based 3500 watt Outback inverter (24 VDC) has sent power into the grid, the main soure of this power is my micro hydro setup, which puts out a high of 34 amps, to mid 20's later in the season as the flow drops. The way it works is once the two 12 volt batteries reach 26 volts, the inverter acts as, in effect, a charge controller and syphons off any "extra" power, after transforming it to good old 120VAC, into the grid. When I set this up, the 3500 watt inverter was about the only battery based grid tied one around, and I was familiar with Outback gear so I didn't look any further. Even though I really only needed about 1 KW of inverter power to handle my hydro's output. A few years go by, and I accumulate a few odds and ends of solar panels, and it hits me: I can wire them up in a 24 VDC configeration, and get a MPPT charge controller and add more power, as I have a surplus inverter capacity.
So, I do so, simple enough, I eventually end up with an additional 1000 watts approximately or so of three different modules, all putting out 24 volts (these are all older 12 volt panels, used in gangs of two, one group being wired as a 48 VDC panel and ran through a MPPT controller to get it down to 24, one group of two is simply hooked directly to the battery).
A few more years go by, and last week it hits me, I STILL have excess inverter capacity, and even though I currently have a 9,000 KWH credit with my utility, and on an annual basis never actually use more then I produce, so I don't actually NEED more power, I happened to have a few 290 watt newer panels laying around, 72 cell types, the kind that put out a higher voltage and are meant to be operated through a MPPT controller to step it down to the system's battery voltage (if off grid), or if used grid tied, daisy chained to get several hundred volts. I did not want to buy yet another MPPT charge controller, as mentioned I didn't really need the power, but then it hit me: I didn't have to buy another charge controller, I already had one, in the way the inverter rides herd on the battery voltage, preventing an overcharge! Sure, I won't get the advantage of the MPPT function, producing the most efficient combination of amps and volts, in effect I'll lose 5 to 10 % (the MPPT function is most effective, up to 20+% added power) only with low rate of charge on the battery, less of a help on a fully charged one which mine always are, 24/7) of the total power of the 290 watt panel. So, I did a quick and dirty install using scrap metal I had laying around the shop, to the 4" steel tube that I use as a power pole to elevate my line that goes to my meter base a hundred feet away. Not ideal as it will get shaded a tad by the main over head cable, but quick and easy and real solid. So today, I hooked it up and even in the overcast conditions immediately starting putting out extra power into the grid, the battery will clamp down the 30+ volts the panels "like" (is most effective) to operate at, all without adding the expense of another charge controller. I will check it tonight, after the sun sets, and I kill the hydro, to see if I get any backflow, but I believe these modern panels have diodes to prevent that? Making the only purpose of a charge controller, being to regulate the voltage, and I'm letting the inverter do that. Critiques welcome, seems to work fine!