I have had a go with this kinda thing for awhile now and have a few questions.
Have you tried actually varying the load with the PWM controller, while load was connected to the UPS?
Yes, many many times. I can up the output when it's sunny and the other day I had it running at just 125 watts on the cloudy day which didn't boil the kettle all day to my surprise. I match the PWM output to the power the panels are producing and go out and adjust it many times a day. I try to set it so as to maintain a constant value on the batteries but sometimes I turn it up so the batteries drain to their very low level drain before the controller kicks out and recharges the battery's so the process cycles. I have put in an amp meter and can gauge by the load gauge on the UPS how much I am putting out. The other day I had it running at .5A and I have had it up to 5A which is the UPS limit.
I have a hard to believing that the UPS is handling the load being pulsed on and off without out tripping out.
Mate, you believe what you want. I have been doubted loads of times on things I have done over the years usually by people that think unless something is expensive, complicated and with a shipload of failure points built in, it can't work. get that every single day on what I have on my YT channel even though 50 other people have built my designs and posted them working and more still have told me about what they have built and done with them.
I'm not trying to sell you anything or claim I'm smarter than anyone else, just telling people of whom I know there are some out there like me that don't have the knowledge to build one from an arduino, the cheap and low tech way I found of making it work using off the shelf bits cobbled together.
If you have, what is the PWM base frequency and which SSR are you using?
No idea on the frequency. Didn't even give it any thought. I bought the thing as a prebuilt unit I just had to wire up off ebay.
Initially I wanted it to control the blowers on my oil burners but I didn't like the way it made them buzz and I wasn't sure if some of them were universal or Capacitor start and didn't want to fry the windings.
What I needed for the heater was a way firstly to ramp up the 24V DC to 240V Ac. Been doing that for years with inverters for my work on location and had some UPS units I was given when the batterys were stuffed which were better because they were pure sine wave where all my cheapie inverters were square.
The next thing was how do I limit that current? Water heater is 3-6Kw here. Didn't matter if I didn't give it full tilt, just have to give it what I can which will be the inverter rating and not overloading it. Logical way, chop the power. What do I have or can get to do that? Motor speed controller.
Now all I need to do is switch the thing off when the sun goes down or a cloud comes over so I don't drain the batteries and the UPS shuts off which won't restart automatically.
The cheap arse PWM solar controller has that function built in like many of them do so all I need to do is get a relay that has a Low DC side and an AC high side. Initially bought some of the little blue mechanical relays but then saw the SSR's so bought them which arrived first.
Cobbled it all together and I have now made a board up with it all on so it doesn't look a complete dogs breakfast and that was it. Put in a breaker for the panel feed and another one on the battery charge and that was it. UPS feeds direct through some joined car battery cable. As it's going to pull around 100A at full load, it's unfused and I couldn't find a breaker nor want to muck around wiring up multiple on a bus bar.
The SSR is one of those white square ones on fleabay. I think I have a 25A unit connected but it could be a 10. I bought a few different capacity's of the same output and wired it up and put it in a junction box so I didn't accidentally fry myself. I have seen it mentioned they should be heat sunk but this one never gets much more than being detectably warm. Maybe because it's being so under driven even if it is a 10A unit.