Hi all,
I hope this isn't too basic for such a well educated community, so my apologies if it's off topic.
I'm planning to relocate to a rural area in the next year or two and the idea of generating my own power really appeals to me. I don't necessarily want to go off-grid, but I would like to have power available during grid blackouts.
I recently picked up a second-hand 5kVA UPS for $50, and I was thinking I could repurpose it as a small residential inverter (single circuit probably.) It's a "dual conversion" unit, meaning in normal operation the inverter runs all the time even if mains power is present, so having it run 24/7 on DC only should be no problem. It also runs off 192 VDC (nominal) which is supplied by 16x 12V 5.5 Ah batteries in series, which I like as there should be less loss than a similarly sized system running on 24 or 48 VDC.
Firstly, is going down the track of a ~200 VDC solar system more trouble than it's worth? I imagine there aren't too many issues doing DIY work with a 24 V system, but are there any hurdles around a 200 V system? I don't mind getting a sparky out to handle any grid-tied parts of the system and to check over the thing initially, but I'd like to be able to do things like replace batteries and non-grid-tied inverters on my own.
I am also wondering how I would set up solar cells to get power into it. I thought maybe trying to run at 192 VDC would be difficult if most equipment is aimed at 12/24 V users, but I have seen a few realistically priced MPPT solar charge controllers that will support a 192 V battery bank and 400+ V worth of panels, so I guess this is what I'm after. When connecting solar panels in series, is there any maximum rated voltage you have to be aware of? I am not terribly familiar with how the solar charge controllers work. It looks like they adjust their voltage to get optimal current into the battery, which makes me wonder how high of a voltage will they put in, and could this damage the inverter? I am guessing that since it's a UPS designed to charge batteries while the inverter is running then it should be able to handle some higher voltage than the nominal 192 V, but I'm not sure what the upper limit is.
I presume these MPPT controllers are designed to have inverters running off their outputs as well as batteries, so having one or more inverters "stealing" current before it gets to the batteries should be no problem? What happens when the inverters pull more current than can be delivered (e.g. cloudy day or dawn/dusk) - do the charge controllers just deliver whatever they can and then switch off once they can no longer supply any meaningful current? What happens if you have a 5 kW load and a 1 kW charge controller? Will it overload, shut off completely, or will it just deliver 1 kW and the rest of the current comes from the batteries? Can you connect multiple solar charge controllers to the same DC bus (e.g. from panels on two different buildings) or will they compete?
Lastly, is it possible to have both an 'island' system (that stays powered when the grid is off) as well as a grid tie-in system? I'm thinking along the lines of multiple inverters running off the same DC system, with one (or more) powering residential circuits and one feeding any excess back into the grid. If so, how exactly does this work? Logically to me, the solar controller would raise the voltage as more power becomes available, and the inverters would drag it down again as they use current. An inverter feeding the grid from the same system would switch on once the DC voltage went above a certain point, and it would essentially cap it there, preventing it from going any higher by feeding more and more power back to the grid (until it reached maximum load of course.) Is this simplistic understanding roughly accurate?
Sorry for the long post and all the questions but any info would be much appreciated!