Fun and educational off grid project in my office

[funeralsinger]

Hello solar gurus

[SunEagle]

Hello funeralsinger and welcome to Solar Panel Talk

[funeralsinger]

Hmmm

[funeralsinger]

Oh, for heaven's sake. Why aren't my posts making it up to the board? I composed it in MS Word and copied and pasted into the body of the post. Can I not do that? Are there characters that I can't use?

[funeralsinger]

Okay. Let's try it again.

First, some background.

A couple of years ago I purchased a 10kW grid-tie system for my house roof, installed by a pro. It's working great so far! We've generated 23MWh in just under 24 months. It's a Solaredge system with Jinko panels. Cost was around $28k CAD, and so far we've made back about $9k selling power back to the grid. We're located just west of Toronto, Ontario, Canada. Now, to the point...

My 12 year-old son has become interested in renewable energy through school programs (his school has 120k array on the roof) and what I'd like to do is start a little off-grid project at home for fun and education. I thought it would be nifty to power my home office by 100% solar energy. I've done some research, and I know it's going to cost me a few bucks, but I'm okay with that.

Using a killawatt meter, I determined that my office uses an average of about 1.3kWh per day between the internet modem (20W...WOW!), desktop computer, printer, phone scanner, and LED lights. Some of these are small, continuous loads that can't be shut off, and the larger loads are only on when in use. Max continuous load never exceeds 200W.

I will not be totally dependent on the system for power, so I don't need to allow too much battery capacity for emergencies or cloudy days.

I have an empty expanse of roof above my office that faces south, so the install shouldn't be too bad. I'll get help with this.

I'm hoping you'll all kindly indulge ignorance. Here are my questions:

How much solar, inverter and battery do I need to make this happen?

Does it make sense to have the inverter/battery in my office closet and run an extension cord to my loads?

Can I use grid-tie panels instead of 12v panels?

PWM or MPPT charge controller?

12v or 24v system?

How much maintenance should I expect to perform on the system?

Am I crazy for doing this?

Just to reiterate, I don't mind spending on quality components that will cause me the fewest headaches in the long run, and that might allow for some growth in the future.

Thanks in advance for your patience. My son and I look forward to your replies.

[AzRoute66]

First, well done on so clearly defining your loads. I'll try to address things in the order you brought them up.

The type of panel - 12V, grid tie (20V), or 24V are mostly a question of sizes. For instance most (but not all) grid tie panels are 1m x 1.6m. Have your son make a drawing showing how the roof area will be utilized to support X watts (perhaps 1000W to start) of panels with the different 'standard' sizes. We can refine that number later.

The question of PWM controllers has become moot. That was the first type commonly sold, but it has gone the way of the buggy whip. MPPT is virtually always the clear choice.

The number of panels and the size of the battery will be largely determined by your solar 'insolation' (I didn't misspell that). These numbers, and some other nice design numbers can be obtained with a program called PVWatts. It is simple enough that your son can get quite involved with this. At this stage you are mostly interested in the insolation for the worst case, which is winter/December. This is found at pvwatts.nrel.gov/

If you want to keep the battery in your office, you will want to go with AGM batteries instead of FLA. You can certainly run your equipment via extension cord as long as it can be done without being in front of a doorway or other obvious safety hazards. Have a power strip that can be plugged into your extension cord, or into a wall socket when needed. Note that this will not be seamless, you will have to re-power/boot up whenever you change over.

The maintenance will be minimal with AGM batteries. (So long as you don't get sucked into our currently quite interesting review of solar panel cleaning considerations...).

Your son will get a lot out of this project, and you will learn some things as well. I commend you on your choice of 'hobby' for you both

If I mistook the intent, and you really wanted us to just calculate the end numbers for you, just say so. I assumed the process was more important.

Edit: If the roof area is not free of shade between 0900 and 1500, this is also a great time to consider shading.

[funeralsinger]

Thanks for your reply. After going in circles for a few days online, some guidance would be greatly appreciated. The dollars are large, and I don't want to make any costly mistakes.

According to PVWatts, 650w (two 325w panels) will get me around 30kWh per month in production, in November and December, which I think I can live with. I'll have to supplement a little in those months, but I should be fine for the rest of the year. It seems there is quite a dance to do to size a charge controller to an inverter and battery properly to the pv capacity.

As I said, I think my max AC load will be around 200W, so I'm thinking maybe a 400-600W inverter should give me plenty of headroom. Do I need pure sine or mod sine for these types of loads? The prices vary considerably. Any recommendations?

Is a 30amp MPPT charge controller overkill? Again, there are so many out there, it would be nice to get a recommendation for something someone has used with success.

Storage. Yikes. Not sure where to go here. I think I need around 120Ah to get the kWh I need for my loads (around 1.3kWh per day), but does that mean I need a 240Ah battery so that I'm only discharging to 50%? Also, do I have enough PV to charge a battery of that size? And do I have enough battery to get peaks of 200-300W out of it without damage?

Round and round...

[Mike90250]

What AZRoute66 left out, was that 12V panels are made in small quantities and are expensive. higher voltage grid tie panels are made by the bazillions and are much less expensive.
The trade off is at about 400w of panel, below that, 12V & PWM controller is fine. Above 400w, and the cheaper grid tie (GT) panels are cheaper, even factoring in the more expensive MPPT controller.

Battery systems have the disadvantage of several layers of inefficiencies stacking up, which make the system more expensive.
Panels produce 80% of nameplate. Batteries are 80% efficient when recharging, inverters have standby losses and perhaps 85% efficient. Now, you find you need about 2x the panels you originally thought,

Shaded panels - even just the shadow of a twig or overhead wire, will have greatly reduced output.

[AzRoute66]

Very well then. You started with your panels, which at 650W would supply 30 kWh /month according to PVWatts, which implies 1000 Wh per day. Yet later you suggest the load is more like 1300 Wh per day. That is a significant difference. I'm pretty sure that you don't want to go to all of this expense for a designed 70%-ish solution. I will go with the 1300 Wh per day. However, it doesn't really matter, what is important is the template and the logic of the solution which I will attempt to provide.

Rather than go to PVWatts myself I'll just bump up your load to 1300 Wh by multiplying your 650W by 1300/1000 to get 845W panels. A sanity check would be to divide your load by the sun-hours/day (December = 2.5, I looked that up), then multiply by 1.5, so 1300/2.5*1.5=780 W panels.

Close enough but let's go with the PVWatts number of 845W for 1300 Wh, I've have yet to fully buy into the 1.5 fudge factor, and have no idea of the accuracy of the 2.5 insolation number I found for your approximate location. I do like PVWatts a lot.

Next, let's calculate the max current off of that array.

845W / 24V * 80% = 28.2 A (24V system)
845W / 12V * 80% = 56.3 A (12V system)

I have recently been shown that 70% is probably a better number than 80%, and that is for the American Southwest. It probably is for you as well, but I am very comfortable using the 80%. (I am still new to this, just slightly less new than yourself...) Note, if we get better production than the 80% at any given time it will not harm the downstream components, you are just briefly bouncing photons off of the panels.

Since we have already decided that the batteries, due to being inside the home, will be AGM we will want to bulk charge them at a C/5 max rate. This number is very manufacturer dependent, and should be revisited during battery selection. I think C/5 will be workable for most all of them.

28.2 A x 5h = 141 Ah (24V)
56.3 A x 5h = 242 Ah (12V)

Note that both options are 2904 Wh. (1300 Wh / 2904 Wh = 45% Depth of Discharge, which is within our 50% rule of thumb.

Here you see the problem (and what is probably making you go round and round although that might not be evident at first) This solution does not allow for a cloudy day. Cloudy days are a problem for off grid systems, which is what you are building. In order to build in some autonomy we must increase our storage, in order to support that we must increase our charging amps (as we only have so many hours of sun), in order to support that we need more panels. The question is - how much. Some would advocate 2.5 days to achieve some odd sense of 'system availability', others have suggested 1.5 days to have some, but not that much. I had intended to take some shade photos in my backyard on Dec. 21 of 2016 to use as a reference for sanity checks on some self shading calculations and counted 21 days IN A ROW of mostly cloudy days starting on Dec. 21 - so take that you 2.5 day people!!!

Let me hear your thoughts on that, and the rest of what I've done so far, and we'll proceed (in fact, we'll pretty much start over but it won't hurt much). It essentially means adding to your battery, then adding to the panels in order to get enough amps to charge them, then getting a charge controller for that increased amperage.

The calculations won't hurt much, but the result will. I am sorry that you live so far north. I live 13 miles from the Mexican border, and I can assure you that winter numbers for places like Seattle and Minnesota and and Michigan just confound me.

Just to put a bow on this while you think about it, a 30A or 60A MPPT charge controller will work nicely for our 24V and 12V systems respectively.

Your 400W-600W inverter will do fine. As Mike suggested, the inverter has efficiency losses (not much) and standby losses (which can build up significantly if left on all day) and should probably be added to your 1300W load figure right from the start. The inverter is just another load component as far as the batteries are concerned.

You asked about modified square wave (MSW) vs pure sine wave inverters. About the only time it makes sense to answer that with MSW is if it is just powering one or two known tolerant loads. You have quite a variety of things plugged in, and damage to any one of them is not really acceptable, and it can take a while for that type of damage to manifest itself. If you go MSW, and you blow out your printer in four months you will always ask yourself if the power was the problem. With pure sine wave you don't usually have that question. But, you can save that money by researching the MSW tolerance of all of your loads to your satisfaction.

Your last question related to how quickly you can take power out of your batteries. That has two components. Taking power out of a deep cycle battery is very mysterious. Even the best manufacturers of deep cycle batteries do not usually publish I-V curves as are frequently done for panels and other battery types like Lithium, NiMH, etc. As a result, it seems that the community ignores it if you are not drawing it out any faster than the max rate you are pushing it in. For us, that was 28.2A x 24V (or 56 x 12) = 677 W so our 300 W, or 400 W is well below that. They do fuss considerably about voltage drop over the wiring, but the upshot on that is if we keep the wires from the controller to the battery, and from the battery to the inverter from being ridiculously long, we can find a practical wire size to mitigate the loss.

Did I 'not answer' your main question to your satisfaction? Some more input from you on the autonomy issue, and we can hammer this down. Then again, I'm pretty sure you will already be doing just that as you ponder the nature of autonomy. As I said, I am new to this. I like to take a stab at the easier questions knowing full well that the experienced folk will check it out and chastise me without reservation.