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4 Solar panels and salespeople that want sales
July 3, 2014 8:14 AM   Subscribe

So I find myself with 4 185 W Solar panels AND 4 enphase M190 Micro inverters (that onboard connect to each panel). The system from what I can tell is "made" for a grid connected solar system. Which is great but NOT what I want/need (Also I am too far from a station to qualify for microfit in Ontario).

SO, I have been trying to make a solar island/charge station to at least provide a partial off-grid solution.

Problem: Solar energy businesses seem to be the "new" tin-men and I am sick of "we will come give you a quote" which always ends up being $10K plus and is really just their own system installed with my four panels added on as an after thought. THEN doing research really just leads to either lists of equipment or really high level stuff like Solar->Charge Control->battery->inverter->save so not really helpful!

I am handy enough and know enough to do it myself. Except some very simple "WHAT do I need equipment wise to make this come together.

As stated I want to make a sort of island station in our house to run whatever we can from the station so it is not a "help me replace hydro" question.

So Questions
1) In the non-grid tied world, CAN I use these micro inverters? if not so be it I can always e-bay them. If I can how would it look? I can see doing Panel->Micro Inverters->Battery charger->battery->INVERTER!->Power... but um that seems stupid(or tell me different?)
2) Regardless; How many batteries should I get?
3) Inverter and charger recommendations?

Other info that might help in my quest:
I would like to add panels in time at least up to 8.
I can literally disconnect my house from the grid and plug in using a 240 North American plug to power my whole house and have had to with our back up genny this intrigues me.
I have LOTS of places to put the panels and would even one day like to add a tracker
posted by mrgroweler to Technology (5 answers total)
 
Your instincts are generally correct that more links in the chain would be less efficient, but there's some magic that overrules it.

Basically your setup in #1 is it -- Panel -> Micro Interter -> Charger -> Battery -> inverter for household AC supply. I'm used to working with this stuff at a pretty technical level but I'll try to shove the mumbo-jumbo into footnotes...

Those micro inverters have MPP (Max Power Point) trackers built in that make sure the solar cells are always operating at peak power1. They also seem to deal with partially shaded cells2, though I can't see from the data sheet exactly what they do. You can connect all the micro-inverters end to end to create a single AC circuit; they will evidently sync up with each other to give 208 or 240VAC.

At that point you can tie in any charger and battery that meet the power specs. To make the calculation easy, call it 200W/panel (a vast overestimate given you're in Canada), so that's either 208V/1A or 240V/0.8A depending on which M190 inverters they are. That's amps-per-panel -- for 4 panels, you'd get 208V/4A or 240V/3.2A output peak.

I don't know how people size real-world batteries and output inverters but as a first draft I would say: 10 hours of sunlight * 800W = 8 kWh of energy to potentially be stored in a single day. Then I might double or triple it (or more, who knows) so I had the ability to store up several sunny days. I'd size an output inverter that could fully supply whatever circuit breaker I was using -- 240V/15A would be a ~3600W inverter. (Important note: you definitely need a circuit breaker on the output side!) Without a doubt there are tables out there that lay all this out with "appropriate" margins.

There is a decent amount of info here, in case you haven't found it yet: Enphase M190

1 For a given amount of incoming light, there's one specific voltage/current combination that will extract the maximum power from the panel. That's basically impossible to hit without active electronics, and that's what these things do. Finding the MPP far, far outweighs any efficiency hit you take by adding another link to the chain.

2 If a panel is partially shaded, the shaded part acts a a "clamp" that constrains the output of the rest of the panel. The data sheet says these inverters help deal with that, which I don't quite understand -- if true, very helpful. That sort of compensation is usually built into the panel itself. It may be that what they're saying is that they can deal when one of several connected panels is shaded, which would make more sense to me.
posted by range at 9:05 AM on July 3


1. As a general rule, you can't use the micro-inverters if the grid is down (or you're disconnected from it). Be aware that I'm not familiar with the M190s and this isn't a 100% rule. Generally component sizing and anti-islanding behavior prevents using them without a grid: imagine the powerline in the neighborhood gets cut and your inverters keep feeding power back into the grid, keeping it hot -- the serviceperson is going to get a shock from a line they thought was dead, etc. Grid-tie microinverters like you have are supposed to detect grid disconnection and take themselves offline if there's no grid power. Off-grid (and some stand-alone grid-tie) inverters do not do this, and that's what you want.

2. Battery sizing is absolutely impossible to do without knowing what you need to run and how long you need to run them for. Running a TV overnight is entirely different from running a refrigerator + lights + computer + fans + well pump for a week because it's really rainy out. There are a lot of online calculators that will walk you through "loads I want to run + for how long worst-case" sorts of steps. The number of watts of panel you have will limit your total loading (you can't use more power in a year than you produce), but the number of batteries will determine how long you can ride out dark times.

Let's say you have 300 W of load on all the time and need to be able to survive a week of no charging: 300 W * 24 h == 7200 Wh. Let's say you use some 12 V batteries: 7200 Wh / 12 V == 600 Ah. For reference, a typical car battery (not what I'd use) is about 40 Ah.

3. I work for a company that makes some of the involved hardware, so I can't ethically make recommendations. Remember that, with power electronics, you generally get what you pay for up to some "knee" in the cost curve. (Think: Lada vs. Honda vs. Bentley.) Avoid going cheap cheap if you can't stand the system breaking when you need it.

You're also going to need to buy some "BOS" (balance of sale) stuff: circuit breakers, switches, conduit and boxes, etc. Wiring up a system safely and to code takes a good bit of research, but lots of people have done it before you.

If this sounds like there's a lot of research and experience and some math involved, it's because there is. I'm very sorry that you've only experienced the charlatans so far. If you don't want to / can't do all this figuring, you need a professional and I highly recommend talking to people on some off-grid solar power fora. Off-grid solar people are generally about 95% less profit-motivated than the hot-new-industry grid-tie solar people. They're often green energy evangelists who were doing this before it was popular. You may be able to get free help on fora if you're willing to wade through the wastrels; you may also find the names of some companies that people trust to do good, honest work.

Optional #4: adding panels later to increase your power input isn't hard. You can run (almost all) charge controllers in parallel. (That is, 6 panels on one charge controller wired to the battery bank, 6 different panels on another charge controller wired to the same bank, etc.) The battery input voltage is so resistant to change moment-to-moment that each charger has no idea the others are out there also charging the battery. It may be cheaper for you to buy one bigger charge controller capable of doing 8 panels now, or it may be cheaper for you to buy a small one now and a second small one later. Similarly, adding batteries isn't hard. Getting more inverter capacity, however, is. Size your loads up-front!

Optional #5: Trackers are generally a mug's game. Unless you're space-limited or playing games with your electric rate plan, just putting more panels down is more efficient than a tracker system. (When panels were very expensive per watt, this was less true.)
posted by introp at 12:18 PM on July 3 [1 favorite]


To be clearer on #1: if you want both grid-tie (sell excess power back) and off-grid (run the house when the grid is down), you'll need an inverter + BOS setup that allows that. A transfer switch to take you off the grid, etc.
posted by introp at 12:19 PM on July 3


Thanks Range and Introp for the advice. Introp: for our Load this is specifically for off grid "island" so there is no actual demand from essential appliances like fridge or furnace. so I am not as worried about sizing I would like to charge all "chargeables" and run my main computer on it as well as some entertainment things like rokio/appletv and a tv etc but this would be when NOT charging etc so not parallel.

I have NO ABSOLUTELY NO desire to feed back into the grid even if I could offset the cost to me screw Hydro one (ontario)

Thanks for the advice on the M190s as well I read through and it "seems" to be as you state which sucks but truly no harm.

I already assumed I would be doing some electrical(breaker etc) but again I am not worried about that (Brother-in-law is certified)

The other thing is I don't actually want to "tie" into the house electronics that is why I said an Island where we use it for whatever it can sustain and the rest will be billable... So if I cant use the micro inverters I suppose at this point I need just batteries charger and inverter? and some math to figure out what I need... is there a worksheet with Panel watts = x batteries + x size inverter?
posted by mrgroweler at 1:15 PM on July 3


I've built a couple of larger systems using Enphase micro-inverters. They only work in grid connected systems - basically they won't start up unless they see the grid connection. So the short answer is no you can't use them off-grid.
posted by Long Way To Go at 9:27 PM on July 3


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