Questions on new offgrid implementation

DaveLB

I am new to the forum, but long time lurker and have some questions. I have searched the forum for answers to my questions, but found many. I apologize in advance for the length of this post, and if there is a thread I should read first.

We have built a cabin in Northern MN (okay, just the shell right now) and will be putting in a small solar system. Here are some facts:

• Cabin is water access only and built on Canadian Shield and off-grid.
• Plan to wire to NEC 2008 (2011 not adopted yet) as utility may be in next 10 years and don't want to uncover wiring later on.
• Fridge & Oven are propane and all spark igniter lighting.
• Plan to use only CFL or LED lighting.
• Expect 3 ceiling fans to be largest draw, but bought new fans rated at 65 watts on high (not counting lights) to help mitigate.
• No vacuum will be run off of the solar, also no tv or PC as no internet (or good cell coverage).
• Have a 2500 watt gas generator to run power tools, vacuum, charge batteries, etc.
• Have a small Honda gas pump for water
• Solar panels and batteries will eventually be 30 feet from cabin

I have estimated usage of 575 watts (everything on at same time) and 2400 watt hours per day.

Here is what I have so far that I got when I purchased a highway construction solar trailer:

• 2 - Kyocera 123 W panel (may be able to get 1 more of same panel)
• 12 - 6V Trojan T105 batteries (configured as 12v currently)
• 1 - Steca Solarix charge controller (can be either 12v or 24v)
• 1 - Iota Power Converter/Charger

Questions, but am sure there will be more:

• Thinking of getting the Exeltech XP600 as inverter, is this cutting it too close or should I go with XP1000?
• Want to make the DC side be turnkey for my wife, so would something like the Midnight Solar mini DC disconnect be a good choice or build my own disconnect box?
• Would it be better to go with 24v system and reduce wire size and breakers or stay with 12v and get the 3rd panel?
• May be difficult to put in a grounding rod with the bedrock only being 3-4 feet deep (in some places). Any ideas?
• Any need for PV combiner box with only 2 or 3 panels?
• Any issues with the inverter generating 5-10 amps, but AC Service panel has 100 amp main with 15-20 amp AFCI breakers? Other options I should consider?

I have learned lots so far from this forum, and looking forward to your help.

mike95490

Re: Questions on new offgrid implementation

> Have a small Honda gas pump for water
What's your backup plan ? How deep is water, how high is lift? Purification plant ?

Too many batteries in parallel ! ! I'd suggest a 24V system, and have room for growth.

If you stay with 12V, consider the Morningstar SureSine 300-600W inverter.

Are ceiling fans AC or DC ? (AC will need pure sine wave)

DaveLB

Re: Questions on new offgrid implementation

Mike,
Lake water is being pumped up to a small tower (12 ft) for a minimal gravity feed. Won't have any septic system as we have a composting toilet and will build an outdoor shower or sauna. Drinking water will be brought in via 5 gallon jugs. We have played around with various filters that are gravity style, so not planning on any power use for drinking water.

I am concerned about number of batteries, but that came with the trailer (along with a 25 foot telescoping mast). I may either go 24v or reduce number of batteries.

Ceiling fans are AC and I want a pure-sine wave inverter. Would like the Morningstar based on what I've read here, but was concerned about possibly being above 300w. Thing that is nice about the Exeltech XP is that it will start clipping the sine wave when the load gets above its rating and will stay in that mode until the load drops back down. Seems easier than having the power go out.

Cariboocoot

Re: Questions on new offgrid implementation

Having just calculated the 12 to 24 Volts switchover threshold for someone else ...
2400 Watt hours per day is just under the wire for going to 24 Volt. But it sure is close.

Mike is absolutely right about the too many batteries - and not enough panel. But the Morningstar 300 won't work for a 575 Watt potential peak load. I think I'd pick the Exeltech 1100 over the 600 - not much price difference.

So how much battery do you need for 2400 Watt hours on a 12 Volt system? At least 400 Amp hours. That would be two parallel strings of two of the T105's giving 450 Amp hours. You can easily see why that is the threshold level; if you want 25% DOD instead of 50% DOD you double that and end up with four parallel strings. The 24 Volt version of the inverter mentioned above is about $50 more. Worth it for the simplification.

Panels.
If you go with 450 Amp hours at 12 Volts you're looking at roughly:
14.8 Volts charging * 45 Amps (Trojan specs) = 666 Watts, less derating = 865 Watt array at least.
24 Volt would be the same @ 225 Amp hours:
29.6 Volts charging * 22.5 Amps = 666 Watts.
Or you could double it and have 25% DOD or an extra day's worth of power in case the sun doesn't shine.

And at that point you are definitely in MPPT charge controller territory. Morningstar 45 MPPT at least.

DaveLB

Re: Questions on new offgrid implementation

Thank you for the replies.

If I'm understanding Cariboocoot correctly, the magic target to charge the batteries is 865 watts. Is this how many watts the panels need to generate in one day? If I have an average of 4 hours of sunlight then the panels need to generate 216 watts per hour (865/4)? Or do I need to have 865 watts worth of panels?

For batteries, it sounds like 4 would cover my useage for 1 day and 8 for 2 days regardless if it is 12v at 450amp or 24v at 225amp, correct? It seems like long term 24v would be the way to go, even if I cut down my usage as it will reduce my wiring and circuit breaker costs and allow for future growth.

What about using the mini disconnect box vs. building my own disconnects?

Thanks,
Dave

Cariboocoot

Re: Questions on new offgrid implementation

Okay, Watts vs. Watt hours.

865 Watts would be the "nameplate" rating total for the whole array, if such a configuration were possible. In practical terms you have to pick from the available panels and for preference size a bit larger than that. For instance you might get four Kyocera 215 Watt panels and have 860 Watts which might do but you'd be better off with four 235 Watts = 940 Watt array or some other combination that would end up being above that target Wattage.

That calculation is based on meeting the potential peak charge current. The other consideration, which usually but does not always follow, is the Watt hours harvest per day. You have to get enough to replace what was used. For 865 Watts you could get (based on 4 hours of equivalent good sun) 3.4 kW hours. That would cover the expected 2400 Watt hours use.

Or you can use the "Icarus Formula" short-cut: nameplate rating * hours of sun / 2 = AC Watt hours out.
As in 865 Watts * 4 hours = 3460 / 2 = 1730 Watt hours AC. That is a "worst case estimate". You can see why you always want to round your battery capacity up to the nearest available amount, then calculate your panel size, then round that up to the nearest available amount. Loads are always bigger than you expect and, regrettably, PV production is always lower.

You have to start with basic calculations, then refine for the particular situation. Take into account the available equipment, likely insolation at the site (PV Watts is good for this http://www.nrel.gov/rredc/pvwatts/), losses specific to the install such as long wire runs or high operating temps, et cetera.

I disparage over the whole "several days capacity" battery bank idea. Batteries don't like to sit discharged for even a short period of time. My preferred method is to shoot for 25% DOD and daily recharging, knowing that you've got another 25% in reserve and that panels will produce something even on cloudy days. Then the generator gets run if Ol' Sol still refuses to co-operate on day 3.

As for disconnects ... you need to figure out what is necessary for your particular system first. It makes a big difference whether you're trying to switch off 12 Volts or 48, and also how much current. Before you get those details worked out you can't determine what you need in switches, combiners, wire, and of course circuit protection.

Break down into stages. Write it down if it helps. Move carefully through each stage. That's the way to avoid unpleasant surprises. Like a system that doesn't work.

BB.

Re: Questions on new offgrid implementation

DaveLB wrote: »

If I'm understanding Cariboocoot correctly, the magic target to charge the batteries is 865 watts. Is this how many watts the panels need to generate in one day? If I have an average of 4 hours of sunlight then the panels need to generate 216 watts per hour (865/4)? Or do I need to have 865 watts worth of panels?

WE are looking at "Rated Watts" here... So 865 watts (STC) of solar panels.

How much useful energy per day will 865 watts of solar panels generate?

Assuming 3-5.5 hours of sun per day (winter/summer for Minneapolis Min), 0.52 derating (solar panels to AC power available for use), fixed array (not tracking):

• 3 hours of sun * 865 Watts * 0.52 derating = 1,349 Watt*Hours per day winter
• 5.5 hours of sun * 865 Watts * 0.52 derating =2,474 Watt*Hours per day summer

So, if your usage is ~2,400 watts--Basically, only during summer will you come close to breaking even:

Month/Average Hours of Sun per Day
1 3.85
2 4.72
3 4.97
4 4.91
5 5.74
6 5.91
7 5.89
8 5.64
9 5.21
10 4.28
11 2.95
12 2.85

For minimum array sizing--I start by assuming you will run 9 month of the year from solar only... So that would make the "target" hours of sun be February at 4.72 hours of sun per day:

• 2,400 Watts * 1/0.52 system derating * 1/4.72 hours of sun = 978 Watt Array minimum based on 2.4 kWH per day loads

For batteries, it sounds like 4 would cover my useage for 1 day and 8 for 2 days regardless if it is 12v at 450amp or 24v at 225amp, correct? It seems like long term 24v would be the way to go, even if I cut down my usage as it will reduce my wiring and circuit breaker costs and allow for future growth.

Batteries, say you aim for 2 days of "no sun" and 50% maximum discharge and 24 volt battery bank (although, 12 volts may be OK too--as you say, expansion and more efficient use of Charge Controllers at 24 volts vs 12 volts):

• 2,400 watts * 1/0.85 efficient inverter * 2 days no sun * 1/0.50 max discharge * 1/24 volt battery bank = 471 AH @ 24 volt battery bank
• [add missing 1/ to above equation. -Bill]

Then we go back and double check the sizing of the solar array vs battery bank size. 5-13% rate of charge rule of thumb. And 10% being Trojan's recommendation (assuming 0.77 solar panel + charge controller deratings):

• 471 AH * 29 volts charging * 1/0.77 derating * 0.05 rate of charge = 887 watts minimum of solar panels
• 471 AH * 29 volts charging * 1/0.77 derating * 0.10 rate of charge =1,774 watt array for Trojan recommendation
• 471 AH * 29 volts charging * 1/0.77 derating * 0.13 rate of charge =2,306 ~cost effective maximum solar array

So, those are the above rules of thumb--Plug in your "real numbers" (use a spread sheet if you wish) and you will get your sizing.

What about using the mini disconnect box vs. building my own disconnects?

What are you looking at? Fuses, Breakers, Rotary Boat disconnects, etc...

High amperage fuses, breakers, and switches are not cheap... So, going with a higher bank voltage (i.e., 24 volts) helps keep the copper/switches/over current protection a bit smaller.

Depending on how complex your system becomes--the prewired boxes can be a real time saver (hunting down all the little parts and getting time mounted).

Midnite solar is known for their "cost effective" solutions:

Breaker Boxes
Midnite Solar E-Panels

-Bill

DaveLB

Re: Questions on new offgrid implementation

Thanks for the clarification and breaking it down for me. Things are getting clearer in my head. I will get it all down on paper and work it out piece by piece. I did find your thread on the conversion of 12 to 24 volt so looking that over also.

This site has been a wealth of information so far and it gets a little overwhelming going through it all.

Thanks.

DaveLB

Re: Questions on new offgrid implementation

BB,

Thanks for the calculations. This will cabin will not be used in winter (maybe 2-3 weekends at most) and will be used May-Oct. Looks like I will need more panels or be more strict on usage. I did exaggerate on my usage as I had figured closer to the 1200 watt-hour.

I do have a question on your battery amp hour equation as I only get 118 AH if I use these same numbers:

2,400 watts * 1/0.85 efficient inverter * 2 days no sun * 0.50 max discharge * 1/24 volt battery bank = 471 AH @ 24 volt battery bank

What am I missing?

BB.

Re: Questions on new offgrid implementation

DaveLB wrote: »

2,400 watts * 1/0.85 efficient inverter * 2 days no sun * 1/0.50 max discharge * 1/24 volt battery bank = 471 AH @ 24 volt battery bank

What am I missing?

I am missing my brain. :roll:;)

Should have been 1/0.50 (i.e., 2x capacity to use only 1/2 of the battery storage capacity).

I will fix the original equation.

-Bill

DaveLB

Re: Questions on new offgrid implementation

Not a problem, I'm often surprised when I find out I have a brain.

BB.

Re: Questions on new offgrid implementation

For occasional heavy power usage (i.e., big family party weekend)--A Honda eu1000i (or the larger eu2000i) can be a great for the extra power boost. A couple extra gallons of fuel a couple weekends a year vs up-sizing the system can be a huge savings in costs and maintenance.

Plus, you probably can use a smaller backup generator at the cabin and home too (for home backup, an eu2000i 1,600 watt inverter generator will keep you with enough power to get through a blackout or poor winter sun at the cabin).

-Bill

DaveLB

Re: Questions on new offgrid implementation

Bill,

I already have a Yamaha 2500 watt generator that we use to run the power tools, so we're covered from that end. Looks like I may want to get (at least to start) that 3rd Kyocera panel that matches my other two and keep the system at 12 volts. As we grow in our electrical needs I can increase the voltage and use the 12v pieces to run separate power to a small bunkhouse and biffy.

Dave

BB.

Re: Questions on new offgrid implementation

Is that a Yamaha 2,500 watt inverter/generator (i.e., quiet and fuel efficient)?

Those Honda and Yamaha quiet generators are worth their weight in gold vs the standard 3-5kW noise makers (unfortunately ).

-Bill

DaveLB

Re: Questions on new offgrid implementation

I don't remember the model, but it is 10-15 years old. It is quiet, easy to start, runs forever (it seems), I can still lift it and has been bullet proof so far. It will produce both 12v and 110v. It has run our metal 3 cu ft cement mixer (although you have to start with 1 bag get it rolling then add the second bag) when we were pouring our piers.