Article: Know how to determine your solar system

I started a comment to this article but quickly realized there was a lot more to say.

The article did not need to be longer; it just needed to be more accurate. I think some of the padding in the article is where it went off the rails.

Firstly, don’t think that “just a panel or two” will provide usable solar power. Any RV with “serious” solar capability has to have a _system_.

A panel or two (with even a $15 controller) can do a lot, particularly when augmenting alternator or generator charging.  See this list of actual solar installs to see what “just a panel or two” can do.

**Controller** – the electronic unit that converts solar panel power to power that can be absorbed by batteries

Using the word “converts” here is confusing and borderline inaccurate. The charge controller drops voltage to the appropriate level for battery charging. This nomenclature sets up a rather large unforced error later in the article.

Panels come in a dizzying variety of technologies, shapes and power outputs. They also are rated at either 12V or 24V output (nominal).

Panels used on RVs are generally 12v, 20v (60 cell), or 24v.

Divide the panel output power specification by 12 to calculate how many amps will be pushed to your batteries. This applies whether you are using 12V or 24V (nominal) panels, because the batteries are downstream from the conversion process. For example, 360W of panel power, regardless of type, will equate to 360 / 12 = 30A to the batteries.

13 is a more common constant than 12, and panel output needs to be derated from lab conditions.

360w * .75 derating = 270w 270w / 13 = 20.8A, a substantial difference from the 30A figure given above.

But recently, flexible panels have hit the market. Flexible panels have some unique advantages: They typically can be adhesive-mounted (no holes in the roof), and they are somewhat more shade- and shadow-tolerant (more power under adverse conditions)...  But they have a drawback, too – They don’t put out as many watts per square foot of panel, so you need more space for the same amount of power.

Amorphous/flexible panels are not new to the market.

Most of this paragraph is correct, but left out their high cost (about 2x that of framed panels) and relatively short life (1/2 to 1/3rd that of framed panels).  But if only flex panels fit on your RV then that’s what you need.

Some folks opt for “floating” panels, not attached to the RV. You can definitely place them for best exposure, tilted toward the sun. But they’re more fuss, they can blow over in the wind, and they can get stolen. To each his own.

They work great for folks with no space to mount panels.  They also allow the RV to be in the shade and the panels in the shade sun.

Choose 24V panels; these will work with smaller wire gauges, easing installation.
If you have space, flexible self-adhesive panels will be easy-install with no holes in the roof; otherwise, you’ll need framed glass units.

0/2 on this one.  Buy whatever is cheapest by the watt (which will NOT be flex panels).  Folks with PWM charge controllers (see below) will waste a ton of money buying 24v panels.

Controllers (sometimes called “converters”) come in a wide range of pricing

No, they are not sometimes called converters.  Only the author is doing that.

Cheap controllers do this by cutting off the extra power

This really should say “PWM controllers”;  not all cheap controllers are PWM (some are shunt) and vice versa.

The slightly more pricey controllers are the MPPT (Maximum Power Point Tracking) style

The average is 3x more;  that is not “slightly more pricey” in my book.

They also have input power specifications, and a 12V controller can’t be used with 24V panels and vice versa.

Most controllers are 12/24v and will do either one automatically, depending on the battery bank it’s connected to.

And you absolutely can use  24v panels with a 12v battery system;  it’s one of the best uses of MPPT controllers.  You can also do it with PWM controllers but it’s a waste of money as the controller won’t be able to use the higher voltages.

Choose an MPPT 24V controller, with an input-power rating _above_ your maximum panel output. If you might expand panels in the future, be sure to include those.

Again, 24v-only controllers are exceedingly rare;  most are 12/24v.

And while I personally use MPPT controllers it is a disservice to tell people to choose MPPT by default.  The price is too high for the returns on many systems, particularly in hot locations with nominal 12v panels.

IMPORTANT: Typical lead-acid batteries can only absorb about 10% of their capacity per hour. This means they can (theoretically) be fully charged from a 50% discharge state in about 5 hours. But that simple arithmetic doesn’t really work. These batteries develop a “float charge” and they strongly resist the last 10–20% of charge. It can easily take 10 hours for a full charge. Guess what, except for bright summer days, you don’t get 10 hours of full solar power in a day. This needs to be factored into your solar system strategy.

Much of the battery section is decent but this paragraph has problems.

  • The author is confusing the minimum C/10 charge rate with their maximum.  The max for  flooded LA during bulk charging is more like C/5 (20% of capacity).  AGM need C/5 and will tolerate C/3.

  • The word absorb here is needlessly confusing.  Absorption is the next charging stage.

  • No, the batteries do not ‘develop a “float charge”’.  Factually wrong, and more confusing word choice.  In the latter half of Absorption the batteries taper the amount of current they take while voltage remains high.

  • Float is the charging stage after Absorption.

  • if it takes “takes 10 hours for a full charge” that’s not the battery’s fault. It means bulk charging is happening at too low a rate.  Defer loads while in Bulk stage.

Buy as much battery capacity as you can afford and/or fit into your rig. Even the biggest battery banks will run low at one time or another, and large banks minimize those times.

No.  Buy as much battery capacity as you can charge fully each day.  Buying too much battery for your charging capacity will lead to battery murder.


This section was pretty good.  Credit where due.


Pretty good, but the fudge factor number was odd.

[edit:  author’s 360Ah is lithium, as stated in the comments.  This para is no longer needed.]

I will note that the author’s system seems to be underpaneled (or overbanked) unless he’s running lithium banks: RV has 406W of panels (about 25A output), and my batteries have 360AH of usable capacity. But with enough “bad luck” – overcast days, unusually high power usage – I’ve still had to use a generator every once in awhile.

Lead acid batteries with 360Ah usable would be 720Ah; a big heavy bank seriously undercharged by 400W of panels.  Maybe he means 360Ah rated, which would be be 3x 12v in parallel.  At least that would be a decent match for the panels.


This section and the graphic are well done, although I still object to the “divide by 12” method for Amps.