Panel sizing questions? Winter vs. summer?

[PNW_Steve]

Hello Everyone,

I am going to install solar on my RV and find myself chasing my tail (circular logic?) regarding array sizing.

If I size my array to support my needs in November in Seattle, and then travel to Phoenix in August, I will be significantly "over-paneled" and risk charging my battery bank at a much higher than ideal rate.

If I size for Phoenix in August then I will be short of capacity when I go back to Seattle in the Winter.

In a perfect world I would Summer in Seattle and Winter in Phoenix. If anyone finds the way to that perfect world.... Please send me a map.....

What can you all suggest as far as dealing with the variables?

Is designing for Seattle Winter and having a "disconnect" on a portion of the panels to be used in sunnier weather a good option?

What do you all think?

[morgandc]

In Seattle in the winter you will need a lot of angle on your panels, not flat on the roof. Are you planning on adjustable panels on the roof? If not, you can do a ground set that you tie in during the winter at the proper angle. My 450watts of solar flat on the roof is worthless late fall and winter in E Washington.

[LETitROLL]

What your overall power needs are, and system size will make some difference in the best answer to this question, for a small simple system, panel oversizing is a good option, just make sure you use a quality controller with current limiting and have it properly adjusted in summer to protect your batteries. Undercharging is a much more common problem with solar, so i like more panels to minimize that, you waste production when sun hours are good, but panels are one of the cheapest parts of a system now and last very long, so while not perfect, it is the lesser of all evils. On larger more complex systems it will depend on the exact components used and overall needs, but there are a few ways to deal with the change. And as mentioned above, in winter in the N.W. keeping the panels at a good angle to the sun is critical (even if you have enough panels).

[Wy_White_Wolf]

As long as the amperage output of the controller is within the range of the batteries you will not overcharge it. Only reach full charge sooner in the day. So sizing the system for Seattle winter then moving to Phoenix will not charge it at a higher rate, But will reach fully charged at an earlier in the day.

No different than between summer and winter anywhere without moving. Any system that is designed for winter usage is over paneled for the summer. But that doesn't hurt anything.

WWW

[PNW_Steve]

Thank you all for the replies.

You are confirming what I "thought" I understood. Size for my lowest light days and leverage the current limiting capability of the charge controller to protect the batteries from excessive current.

I got yelled at on another board for suggesting such a thing and told that the World would certainly come to an end if I put too many panels on my roof..... I thought: "I must not understand this like I thought I did......"

I cannot, realistically, install enough solar & batteries to accommodate 100% of my electrical demands. I will need shore or generator power to some extent. I want to minimize my generator run as much as i can.

Tentatively, I am looking at 1200-1800 watts of panels, Midnight Classic 200 MPPT charge controller and 8x GC2 batteries in a 24v bank.

I can angle the panels but..... Where I am likely to park in the Winter I will be able to angle them to the East or West depending on which direction I park.....

[J.P.M.]

I can angle the panels but..... Where I am likely to park in the Winter I will be able to angle them to the East or West depending on which direction I park.....

Park E-W and face the panels as southerly as possible, north side (that is, one side of the RV) of panels higher. Avoid sawtooth pattern that will cause the more southerly panels to shade the more northerly panels unless a panel pitch that's ~ 2 to 3+ times greater than the highest panel point.

[zamboni]

I've spent one winter in the PNW (Seattle and surrounding coastal areas) with my RV and my experience aligned with the sterotypes: Rain and overcast, and once a month the whole city freaks out when the sun shines. Definitely not optimal for solar...at least it doesn't get too cold. I didn't see enough sunshine to think tilting would buy me anything - but maybe it was just a bad year or i was too close to the mountains. Rather than tilting, consider sticking with flat-mounting and use an MPPT charge controller with the array at 48V or 96V if possible. I was able to get 100-200W out of my 1200W array (@48V) on most overcast days, sometimes even while being rained on. It's very little...but it's not zero, and it lasts all day. Tilting would not have improved anything in those conditions.

This approach has worked well for me and i love not having to tilt the panels. I'm not saying it's the only way or even the "right" way. You'll have to consider your options and decide for yourself, but definitely give a lot of thought to shading before you commit to tilting -- roof space is almost always the limiting factor with RVs, and having a larger rig often doesn't help a whole lot because there's more "stuff" up there to shade the panels. Whether or not you go for tilting, put a lot of time and effort into locating the panels to avoid shading. Especially with low-angle sun in the PNW antennas and A/C shrouds can cast really long shadows that sweep a big arc. In case you're not already familiar, the "sawtooth" pattern J.P.M. mentioned is explained pretty well here - you will see that you run into problems on an RV roof quickly because at low sun angles the rows of panels have to be quite far apart from each other to prevent self-shading, and you've only got 8 feet to work with. I have also found this calculator to be quite comprehensive, although this one is a bit more intuitive.

While in Seattle pay attention to how long your batteries have gone without reaching "full". If you can't get them topped up for a few days go ahead and run the genny to get them back into float. Some battery monitors will track this for you and set an alarm after user-determined X days...i usually see 7 days as a max, but the lower you set that number the longer your batteries will live.




- Jerud
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1220W array (4x 305W Astronergy panels @ 48V)
1000Ah LFP house bank (5P4S HiPower 200Ah cells)
MPPT solar charge controller (MidniteSolar Classic 150)
2800W PSW Inverter (Magnum MS2812)
ME-RC, Trimetric, and JLD404
2001 Fleetwood Prowler 5th wheel 25 foot, self-rebuilt

https://www.livesmallridefree.com

[PNW_Steve]

Park E-W and face the panels as southerly as possible, north side (that is, one side of the RV) of panels higher. Avoid sawtooth pattern that will cause the more southerly panels to shade the more northerly panels unless a panel pitch that's ~ 2 to 3+ times greater than the highest panel point.

I currently have room to park N-S precluding me from South tilting the panels. If I want to park E-W I will have to tear down a building.... Not sure the property owner will be on board with this.......