I have been playing with the panels some more and learning a lot.
I found my old urn I have been testing with is 1800W which is half the element in the HWS and a size I can get for it.
In bright but slightly hazy sunlight I got up to 212V with the urn connected. This was near as dammit full output and had the urn boiling as it would on mains power. While observing this, some cloud came over and the voltage dropped to 30. The boiling stopped being a roll and the element was probably only replacing heat loss.
Clearly the direct coupling would be fine with the 8 panels in good sunlight but the ramp up in the morning and fade off in the afternoon would be pretty poor.
I started wondering how much power I needed for my water heater. If I needed to heat the 250L of water my tank holds from 50 to 70 as I guesstimated on average, I'm going to need 6KW. If I need to heat from 40, it's going to be 9Kw. As I have around 1500W in the array, I clearly wasn't going to get 6 kilo out of it directly coupled but I could get that and maybe a fraction more when I had the panels on an inverter.
As I am on grid and my main array is to wind the meter back I thought the cheapest, easiest and simplest thing is to hook the array to an inverter and run the heater off the normal mains which I am backfeeding into rather than the offpeak I can't rewind on the digital meter.
Then another thought occurred..... the normal power costs me .30C Kwh. Off peak is 11c. As I am probably going to be using a bit more than I am generating once the warm weather comes and the AC kicks in, I'm financially better off, nearly 3 times better off, to put all my generated energy into the normal circuits and just pay for the offpeak to heat the water.
That still left me with 5Kw of 250W panels and 1.5 Kw of 190W panels. Very different open and loaded voltages, amps etc.
I looked up mismatched arrays but could only find info on mis matched panels within and array, not different arrays or strings. I worked out that the loaded power point for the 190 array of 8 panels was 293V and the 10X 250w panels was 306V. Close enough I thought.
Figured I had nothing to loose trying to parallel them up so I grabbed some cable and hooked them together.
The 250W panel array was doing about 1500W output. The 190 panel array was doing about 880. I hooked them together in parallel and maxed out my 2kw inverter which was really pleasing to see. I tried swapping the 2Kw inverter with teh 3kilo one I have and it showed about 1880W output.
Lot of possible reasons for this comes to mind but I'll wait and see what I get tomorrow in hopefully better light. Even a 500W gain is probably going to average out better than directly hooking the panels to the HWS.
It will be interesting to see tomorrow what the 3Kw unit maxes out at but I think there is a gotcha in the plan. The 3Kw inverter has dual inputs. I am pretty sure that neither input will take the max load. I seems to remember they overlap, as in you can put 2Kw on each side but not 3 Kw.
I'll see what sort of output I get tomorrow.
I'm also thinking I can probably offload these 190w panels for the same price I can wheel and deal 250's so I'm probably better off getting rid of them and getting better matched panels and being done with it. That way I can get max output from them in any config and it will be much easier to incorporate them in with the other panels.
As I'm on grid, the cheapest way for me to do anything with solar is to just backfeed my meters with what I generate and run whatever I want from there. I have inverters and panels are always going to be cheaper and easier than more controllers and setups. Not as much fun or as interesting but efficient from both a power and cost POV.
For the moment I'll backfeed all I can and just pay for the offpeak power to heat the water. If I start getting into credit on the meter's, I'll put the hot water onto the normal power and use that. I can see where possibly spring and autum when I don't use the AC much I might be able to run the water heater off the mains and then in winter and summer when I am using more amps I can run it off the offpeak. Or.......
I can put another few KW of panels up and loose the off peak all together.
See how I go when it's properly set up and running but I am having fun and learning a lot with this along the way.