Solar Basics – Know how to determine your solar system

By Greg Illes
Books can be written (and have been) about all the nuances of solar power. For your RV, this article is meant to be a “thousand-foot view,” a way to see the entire landscape of solar power, without getting bogged down (yet) in too many details. I’ve tried to cover a lot of factors, and not too deep, but with enough substance to provide awareness. (There’s also nothing in this article about installation, and I’m not going to explore exotic, high-end capabilities – I’m trying to keep it under encyclopedia-size.)

[Before we get started, there’s a most-critical number that must be well understood – amp-hours (AH). It’s quite simple, being the number of amps consumed times the number of hours of consumption. Example: A light that uses 1A, burning for 5 hours, uses 5AH of power.

If you don’t yet understand the difference between volts and amps, please go study up a little on this topic before you read any further. It’s fundamental, it’s essential to understanding solar systems, and you need it to design your own system.]

So let’s get to it …

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. That system consists of three major elements:

• Panels – the photovoltaic unit(s) that convert sunlight to electricity
• Controller – the electronic unit that converts solar panel power to power that can be absorbed by batteries
• Batteries – the storage units (typically your existing coach batteries) that store the solar charge

These all have to work together, and be reasonably well-matched. For example, a huge array of solar panels would quickly overload a small-capacity charge controller, and neither could their full-day’s power be utilized by a small battery pack. Likewise, a huge battery pack could not be charged in any reasonable time by a wimpy set of panels.

PANELS

Panels come in a dizzying variety of technologies, shapes and power outputs. They also are rated at either 12V or 24V output (nominal). Unfortunately, panels are typically rated in watts output, and for several reasons this has to be converted to amps (approximate) for system design. So keep your calculator handy.

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.

RIGID or FLEXIBLE

Classic solar panels are almost literally sheets of glass in metal frames. 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). Also, debris and dirt won’t collect underneath. 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.

For my own system, I had lots of roof space, and I loved the peel-and-stick panels I bought. I can actually walk on them, and no debris or dirt collects underneath. But for a small trailer of a friend, there was no way to get enough power without using glass panels. It’s always a case-by-case decision.

TILTING or NOT

This is worth a whole article in itself. Suffice it to say that tilting is a lot of trouble, can lead to unintended wreckage if you drive off with the panels “up,” and it only provides about a 20-30% increase in power under the most ideal circumstances. I personally don’t recommend it for the typical user.

If you’re parked for long periods, and trying to get the ultimate power (like being a snowbird in the winter desert, with its low sun angle), the slanting might make sense for you. But if you’re moving every few days, and always parked in a different location and orientation, slanted panels might get you nothing except trouble and risk. And, of course, stick-on panels are fixed.

UNATTACHED

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.

BOTTOM LINE for PANELS:

• 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.

CONTROLLERS

Controllers (sometimes called “converters”) come in a wide range of pricing. These units have to use electronics to convert the panels’ output to battery-tolerant levels. Panels will output 15-20V (12V) or 30-40V (24V), and this can’t be used directly on the batteries. So the first thing the controller has to do is get the power down to 14.4V.

Cheap controllers do this by cutting off the extra power – and it simply goes to waste. It can also cause the controller to get very hot as it dissipates the extra power. The slightly more pricey controllers are the MPPT (Maximum Power Point Tracking) style. These electronically convert the wattage from the panels, and have 90%+ efficiency. It’s always nice to actually be able to use all that power you paid good bucks for.

The simpler controllers perform only the one function – “translating” power from the panels to the batteries. These controllers are priced in the few-hundred-dollar range. Far more sophisticated controllers are available, with the capability to merge power from panels, shore and generator sources. These are strictly for high-end systems, and we won’t explore them now.

Physical style of controller is quite variable. Some are solely intended for remote mounting, aka “ugly,” while others can be placed in the cabin within easy view, to check function and status. Many models have remote-control-panel options, which can be handy.

Some controllers allow dual-charging, of both coach and battery packs. But this can also be accomplished with a trickle-charger device, so it’s not critical.

IMPORTANT: Controllers are rated in output power, volts and amps. A typical unit might be 12V, 25A rated, which means it can deliver up to 25A at the 12V charging level (usually 14.4V). They also have input power specifications, and a 12V controller can’t be used with 24V panels and vice versa.

BOTTOM LINE for CONTROLLERS:

• 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.
• If you mount your controller remotely, be sure it has good airflow, and get a remote panel for it so you can monitor the system status.

BATTERIES

For most RVers, the ubiquitous lead-acid (either flooded or AGM) is the battery of choice. It’s familiar, cheap, available, and not fussy in most respects. In many cases of lower-cost systems, your existing batteries may be sufficient for your needs.

There is another choice, and it’s becoming more popular each year: Lithium Iron Phosphate (LFP) batteries. These have awesome capabilities, but they are finicky and not for everyone. They also require additional battery-management-system (BMS) circuitry and components, adding to system complexity and cost. The advantages can be well worth the trouble, but this discussion is for another article.

Batteries are rated in voltage and either amp-hours (AH) or cold cranking amps (CCA). If you find a battery that is only rated in CCA, it’s not the best for use as a coach battery because it’s not intended for deep/frequent discharge. However, you can approximate its AH capacity by dividing the CCA by 15. So a 900CCA battery would be about a 60AH battery – plus or minus.

That AH rating is critical for two reasons. One is that you can’t get any more power out of it than its capacity will tolerate. The other reason is that any lead battery must have its discharge limited to 50% of capacity, or its life span will be severely shortened (1/3).

The discharge curve of your batteries must be known and understood. It is the only tool (other than sophisticated electronics) for you to measure the level of charge/discharge. For lead-acid batteries, the most important number is the 50%-discharge level, and it is usually about 12.2V. Also important are full-charge (about 12.7V) and full-discharge (about 11.5V). Note that these voltages vary with manufacturer, battery models, temperature and measuring technique. These are just basic approximates.

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.

BOTTOM LINE for BATTERIES:

• 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.

WHAT DO YOU REALLY NEED?

This is by far the trickiest question to answer. But you have to be pragmatic, unless you have a small fortune to spend and an acre of roof space for panels.

• How long do you need/want to run without shore power or a generator?
• What is your daily power usage (in amp-hours)?

The answers determine how much usable battery amp-hour capacity you need, and how much solar power you will require to recharge those batteries in a reasonable period of time. Remember that your typical lead-acid batteries’ capacity ratings are perfect totals, and you can get only half of that without seriously reducing both usable capacity and life span. Here is a step-by-step example of one system calculation:

Let’s suppose you are using only your LED lights, water pump, fridge and water heater, and charging a couple of cell phones. You’ll probably go through 20–30 AH on a typical day. (There are ways to calculate this, or check it directly, but that’s for a future, more detailed article.)

You’d like to be able to boondock indefinitely without running the dang generator, so figure 30AH average is what you need from your solar system. A good rule of thumb is to assume 5 hours of good sunlight on a sunny day. So if you had a 6A solar system, you’d get that 30AH in your 5-hour day. Ah, but not all days are sunny. There’s shade and weather to deal with. That’s where your batteries come in.

Your batteries get calculated differently. They need to have capacity to last through your longest “dark” period – that is, when you’re camped in the shade or bad weather. Only you can make the decision on this factor, but 3–5 days is a reasonable starting point. So 30AH/day for 3 days is 90AH – and that’s 50% of your needed battery capacity. Now you need 180AH. This is why you see rigs with four golf-cart batteries (440AH rating, 220AH useful) in their power bays.

If you assume (for simplicity) that you’ll only get 50% sunny days, you’ll need to ramp up your solar output too. Charging your 90AH battery from the 50% point will need 45AH, which requires a 9A panel for those 5 hours of daylight.

IMPORTANT: INEFFICIENCIES

The devil is definitely in the details. There are so many ways that all these perfect numbers don’t exactly play out.

• Solar panels only produce their rated output under perfect conditions
• Solar controllers waste 5–10% of the power (even the good ones)
• Panels are often tilted away from the sun, or shaded, reducing power output
• Light overcast or cloud cover cuts back on usable power
• Batteries refuse to absorb all available panel output
• Panels have gotten dirty

For all of these reasons, and more, a “fudge factor” of 50–100% is often used to get a system that one can really live with and make reliable use of. For example, my 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.

BOTTOM LINE for the SYSTEM:

• A “light” RV power usage will be nicely augmented by 100-200W of solar capacity and 100AH of usable battery capacity.
• For heavier users (TV, coffee-maker, etc.), 400W and 200AH is more like it.
• Super-users (lots of appliances and extended use) have gone way up – 1200-2000W and 800-1600AH in some extreme cases. Obviously not for the faint-of-heart (or light-of-checkbook).
• See the chart for a typical range of selections for system capabilities

SOLAR TAKEAWAYS

• Except for simple, low-cost “newbie” setups, solar power is not just a panel or two, it’s an entire system.
• All the system components – panels, controller, batteries – have to be reasonably well matched to each other.
• Costs can run from the mid hundreds (100W/80AH) to several thousands (400W/200AH), depending on system capacity and who does the installation. Super-systems (2000W/1600AH) will be way more.
• With good installation and quality components, excellent reliability and long life can be expected.

FINAL CAVEATS and DISCLAIMER

I hope this hasn’t been too overwhelming (or too lacking in detail). Although I’ve covered a lot of topics, this is by no means an exhaustive treatise on the subject. What this article is intended to be is a “Solar 101” education. Before you invest in a system, please look into any and all details and be sure you are comfortable with your plan. Also, although I’m well-versed in the technologies, I don’t have a lot of brand-specific knowledge – PLEASE don’t ask me for reviews or model recommendations, I’m not qualified to answer.

Lastly – although I’ve made some strong suggestions throughout this article, these are simply my own opinions, and should not be regarded as instructions or mandates in any way. YMMV, as they say (Your Mileage May Vary). YOU are the only one to decide what you need, and how to achieve it.

Good luck, have fun, and travel safe.

Greg Illes is a retired systems engineer who loves thinking up RV upgrades and modifications. When he’s not working on his motorhome, he’s traveling in it. You can follow his excellent blog at www.divver-city.com/blog

##RVT815