Solar sizing rule of thumb

[technomadness]

I’ve been mulling over the question “how much solar do I need.” In the past, and I noticed everyone who talked about their solar array was happy with it when the array’s watts was equal to their amp hours.

Eg: if your usage works our to needing a 100AH battery, then a 100 watt panel should do it for you—- at least anecdotally.

I realized this became a rule of thumb in my head and wanted to post it Incase there was a real rule of thumb,

I stopped worrying about how much solar to get when I realized I could get residential panels for less than a dollar a watt (or close to it, prices are rising) and so the limit on how much solar I get will be purely on how much I can fit on the roof (which is going to be all solar.)


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[TedOnWalkabout]

Best rule of thumb? Shove as many solar panels and batteries on and in your rig as reasonable and affordable. With a good ratio of watts to battery capacity. I never heard anyone say that they wished that they had less. Besides, if you were drain your batteries a lower percentage because you have a greater capacity, you will substantially increase the number of cycles on that bank.
Ted

[Putts]

Someone better qualified may come along, but that's the number I've heard. Thing is, I think it works well when you're in full sunlight, but it's not really enough on marginal days.  I plan on 1000W solar and about 460AH batts. Same as you, just cover the roof and call it good. No tilting....'cuz it's just too far up for me.

[technomadness]

Someone better qualified may come along, but that's the number I've heard. Thing is, I think it works well when you're in full sunlight, but it's not really enough on marginal days.  I plan on 1000W solar and about 460AH batts. Same as you, just cover the roof and call it good. No tilting....'cuz it's just too far up for me.

Yeah, the fourth panel is my tilt. Adding an extra panel is effectively a %25 boost, with no effort required! Just money.


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[tx2sturgis]

I think 2:1 is a better number when the panels are flat-mounted.

Mine is 2:1 and it seems to work well so far.

Today is overcast here in Caprock Canyons, so, I really need 16:1!

[justjim]

How much solar do you need? Enough to cover your power usage on the bad days. Enough to cover your usage in the low sun and short days of winter which is twice as much if not more than the high sun of summer. Not only do you get the least amount of power for the least amount of time, you are in your rig later in the morning and earlier at night. While some sleep with the sun, most fill the extra time watching TV, Playing on the computer, turning on lights and in my case turning on a furnace. You get the least power and you use the most so it is at that time that you have to cover your butt.

A lot of people that were happy, happy, happy this last summer killed their batteries over the winter.

The 1 to 1 ratio is BS for a few reasons. First it has nothing to do with keeping your battery charged. That ratio is what it takes to equalize your bank at a certain temperature. You equalize your bank once it is fully charged and the amount of amps it accepts while equalizing is minimal. The second is a 100w panel does not produce 100w even on MPPT at peak. Flat mounted the panel may only get even close to peak mid summer at mid day, the rest of the time you are varying degrees behind the 8 ball.

So the amount of solar you need is enough to cover your needs in the worse conditions.

[sternwake]

The 1 to 1 rule of thumb is bullshit, and especially so in winter.

Other charging sources such as alternator or generator usage first thing in the morning, when batteries are lowest, have huge effects on how much solar one requires to get the battery to full before sundown.

Without regular empoyment of high amp charging sources early morning, I think one should have enough solar that the maximum charge rate of the solar at noon on a sunny june 21 is close to 15% of the battery capacity. 15 amps for a 100AH battery. that is ~2.5:1 ratio.

I know a lot of people get their warm and fuzzies from lots of battery capacity, but since I have an AH counter I am basically warm and fuzzying off my ability to feed a single group27 AGM high amperage from alternator or plug in charging source on grid, at the proper voltages tht causes high amperage to flow. I also have more than 2:1 solar and hopefully soon to be 3:1.

My AGM battery has well over 700 Deep cycles on it over 52 months of age, and it is my only battery, for house and engine starting. About 100 of those deep cycles are down to 30% state of charge. but pretty much after each one of those, the battery was immediately fed a diet of 40+ amps until absorption voltage was reached, and then held until amps tapered to 0.5% of battery capacity.

That would be Impossible on Solar alone

[frater secessus]

Rules of thumb are admittedly crude heuristics but they serve a purpose. IMO the 1:1 rule of thumb is there to keep newbies from grossly underpaneling their lead-acid bank system. By the time they understand how the heuristic is flawed they will not need it anymore. "Very good, Grasshoppah..."

I just calculated it and my system is a little over 2.5:1, flat-mounted. I've never been short of power and had enough to share at the RTR.

Full disclosure: I don't use much power at night; no TV, no inverter, no desktop, etc. I run a strip of LED lights sometimes although I use a luci when appropriate. I run a maxxfan on low any time the sliding door is closed. I bias my use to the daytime when power is abundant. Anything that needs charging gets it before the sun goes down.

The 2x duracell golf carts in here were bought [ab]used partially as a favor to a guy who was desperate for cash. They are definitely hurt but keep on chugging. When they do keel over I'm thinking 80A-100A of lithium will be the replacement. I've been snffing around prices of prismatic cells and the 80Ah seem to be the sweet spot for Ah/$. Maybe by the time I'm ready 100Ah will be the sweet spot.

[technomadness]

Exactly. That’s why it’s called a rule of thumb rather than a formula or a law. No rule of thumb fits every situation. It has to be applied intelligently.

What this rule of thumb needs is an average solar yield per watt of rated output, across the USA and across seasons.

I bet that stat has been calculated.

If we had that, though, one could say “if your usage is x watt hours a day, you need Y solar watts of panels”. And then if they are going to travel primarily in one region Y can be adjusted.

Both usage and solar output will vary from person to person and from day to day, season to season and region to region. This should be obvious.

As I said in my first message, my rule of thumb is that your entire roof surface should be dedicated to solar panels.

[frater secessus]

What this rule of thumb needs is an average solar yield per watt of rated output, across the USA and across seasons.

I bet that stat has been calculated.

I've collected some insolation maps that address some of that.  The reader would still have to derate the panels for temperature, and address controller and other losses.

I did such a calculation for projected solar yield at the winter 2018 RTR.