Help me finalize my off grid system please

[sensij]

My array is pretty optimally placed for my Lat and on a 35 degree cloudless day my panels exceed their rated capacity by as much as 8 percent. I've never has an off-grid system where it gets as cold as where the OP is located nor tilted at 18 degrees but my feeling is that he could certainly approach his rated output.

What you observe for your system is totally consistent with PVWatts, using the same parameters I described earlier... it shows a south facing 45 deg tilted array in VT (edit: or Portsmouth, VA, as it turns out) as reaching STC power in winter. I don't know the specific orientation of your system, but as tilt increases, extra power beyond what the base model shows is entirely believable when there is snow cover and the albedo increases. NREL's SAM gives the ability to tweak albedo to improve the model accuracy in those situations. An 18 deg tilt isn't going to see much of that, but since it sounds like the OP is designing for mostly summer / fall use, the flatter tilt is not necessarily a bad design. His system will not produce more than 30 A, and almost always, much less.

Discussions in the grid-tie section of the forum frequently move beyond simple rules of thumb and dig deeper into the consequences of specific array locations and orientations. I think the off-grid discussions could sometimes benefit from that... rules of thumb get you close, but when it comes time to spend thousands or tens of thousands on equipment that will fail prematurely if not used correctly, a little extra work is worth the effort. That is not meant as an attack on you... everyone who contributes to helping others is doing some good, although sometimes more well-intentioned good than actual good. It is mostly a warning to those who come seeking advice... rules of thumb are frequently not good enough to design an optimal system, and if the advice you get is based solely on those rules and is not specific to the circumstances, it may not be right. If the only advice being received is based on those rules, it may be because not enough information is being provided.

[J.P.M.]

What you observe for your system is totally consistent with PVWatts, using the same parameters I described earlier... it shows a south facing 45 deg tilted array in VT (edit: or Portsmouth, VA, as it turns out) as reaching STC power in winter. I don't know the specific orientation of your system, but as tilt increases, extra power beyond what the base model shows is entirely believable when there is snow cover and the albedo increases. NREL's SAM gives the ability to tweak albedo to improve the model accuracy in those situations. An 18 deg tilt isn't going to see much of that, but since it sounds like the OP is designing for mostly summer / fall use, the flatter tilt is not necessarily a bad design. His system will not produce more than 30 A, and almost always, much less.

Discussions in the grid-tie section of the forum frequently move beyond simple rules of thumb and dig deeper into the consequences of specific array locations and orientations. I think the off-grid discussions could sometimes benefit from that... rules of thumb get you close, but when it comes time to spend thousands or tens of thousands on equipment that will fail prematurely if not used correctly, a little extra work is worth the effort. That is not meant as an attack on you... everyone who contributes to helping others is doing some good, although sometimes more well-intentioned good than actual good. It is mostly a warning to those who come seeking advice... rules of thumb are frequently not good enough to design an optimal system, and if the advice you get is based solely on those rules and is not specific to the circumstances, it may not be right. If the only advice being received is based on those rules, it may be because not enough information is being provided.

A very wise engineer once told me that rules of thumb are good for designing thumbs, but since thumbs have already been designed and seem fit for purpose, such rules are less than optimum for professional engineers, or most anyone else, as a serious choice in design. Maybe as a 1st cut elimination aid, but not much else.

[lkruper]

A very wise engineer once told me that rules of thumb are good for designing thumbs, but since thumbs have already been designed and seem fit for purpose, such rules are less than optimum for professional engineers, or most anyone else, as a serious choice in design. Maybe as a 1st cut elimination aid, but not much else.

I have been told that engineers carefully calculate everything to a few extra decimal places and then when they are finished, double it for good measure

[Mike90250]

.... his batteries are going to see in excess of 35A at times, which I figured would boil off his water levels too quickly between visits.......

Not going to happen. The batteries are going to bed daily, full. When the sun rises, there is at least a 2 hour ramp-up time as the sun angle improves. By the time the sun is optimum, those full batteries will, again, be full, and not accept the potential 35A charge. Maybe if there are a couple cloudy days, and the sky clears exactly at noon, then you might get 35A , for 10 minutes, as the batteries fill, the amps will fall off rapidly.

When the cabin is occupied, and batteries low at night, then the morning-noon charge may approach the limit, but that's a different situation.

For the idle months, absorb times can be set way back, since the batteries are not in "cycle mode", but more like Float duty.

[sensij]

OP here. I will be using the Trojan L16RE-A batteries. 325ah. People have reported the RE batteries use less water for whatever reason. I will also install water miser caps and set the absorption back when I leave. Hopefully the water use will be minimal.

If you are primarily using the 1710 W array as described earlier to do the charging, I think water loss will be less of a risk than stratification due to low charge current. Again, I'm calculating C/13 at most for all but a few hours of the year, but clearly modeling it differently than some might.

[hammick]

If you are primarily using the 1710 W array as described earlier to do the charging, I think water loss will be less of a risk than stratification due to low charge current. Again, I'm calculating C/13 at most for all but a few hours of the year, but clearly modeling it differently than some might.

Thanks. If I am having problems getting the batteries charged when we are staying up there I will have three more panels installed.

[Mike90250]

stratification due to low charge current.

With L16's in their tall cases, the electrolyte will tend to stratify out to different layers. When there is enough AMPS in the PV array, to charge the batteries hard enough to really gas the batteries, the bubbles mix the layers up, and there is no problem. But with a over size battery (under paneled) you don't get enough charge current to stir the layers up, then battery life suffers.

[hammick]

OP here with an update. My system is fully installed and so far I am very happy. For the seven days (1-26 through 2-3) experience I have using my system everything seems to work great. My living quarters isn't complete yet but I was running my travel trailer off the system (led lights, water pump, refrigerator, heater fan, etc) and I was down to about 80% SOC each morning. It was pretty cold and the heater was cycling a lot. I'm guessing when the living quarters is complete I will see about 85 - 90% SOC each morning (wood burning stove only for heat).

One of the fully sunny days I actually saw the CC putting out more watts than the panel wattage. I'm assuming this is because it was so cold. The day I left I was using my 230v well pump a lot to winterize the trailer. In the early afternoon full sun the CC was putting out 30 more watts than the well pump was drawing. Batteries were staying in float. That's good news for the potential of adding a washing machine for use during sunny days.

The batteries were in float when I left and I set the bulk, absorb and float voltages to 52.8v which is the low end of the float spec for the Trojan L16-REA batteries. I shut down the inverter. I still need to fuse the positive and negative cables at the battery terminals.

Had a couple cloudy days and my 2400w inverter charged the batteries just fine. It's only 120v so the charging is 50% vs. using a 240v genny but it worked well enough I sold my monster 7000w contractor genny to my builder.

Thanks for all the great advice.

[Logan005]

Fantastic, Looks great! Love the building. How far off grid is this place? Looks like a nice location.

[solar pete]

Hi hammick, thanks for the news on your system. Big WOW on the beautiful location, reminds me of when I was driving around rocky mountain national park last year, awesome country, cheers