You have too many solar panels! This should shed some light on our solar install. How did we design our RV solar and battery system? This was a comment in an email pertaining to our Zamp Obsidian Solar Install. The comment wasn’t as direct as my title statement and it was made with good intent. So that all my readers can benefit from the information, here are the answers to the comment.
System layout (so you don’t have to wonder)
We have installed eighteen 100 watt (1800 watts total) Zamp solar panels on our RV to feed our 900 amp hours of lithium batteries and our 600 amp hours of AGM batteries. The battery system is divided so that the 900 amp hours of lithium feed the inverter and take care of the alternating current needs of the RV. The AGM batteries take care of the direct current needs of the RV.
The panels are configured as the passenger side 9 panels and the driver’s side 9 panels. Each set of panels (driver’s or passenger) power a separate 50 amp-hour charge controller. In the winter, each array (driver’s or passenger’s side) can be tilted toward the center of the RV to expose the entire array without any energy-robbing shading. In the summer (to enable the winter tilt function) the entire array is in a lower travel position because it doesn’t need tilting to meet all our energy needs.
The design goal was to provide all our energy needs year-round with the exception of air conditioning in the summer, although running one air conditioner from the batteries is a proven feature. Because of the inverter circuit design, the water heater is not powered from the inverter, so we use our propane instead.
You have too many solar panels on your RV!
This one was easy, the answer is totally dependent on what you want to accomplish.
On a cloudy day, I probably don’t have enough solar panels. Yes, my system will produce a remarkable amount of energy on a cloudy day. Not nearly as much as it will on a sunny day but as long as it is somewhat bright, the system will still function well on a cloudy day.
Here is a link to our real test results operating our previous RV without lithium batteries on both good days and cloudy days. Testing the System In these adverse weather conditions, my 700-watt solar panels produced over 280 amps with clouds and rain over three days, and the only reason I didn’t get a full recharge on the battery was that I was using lead-acid batteries, not lithium batteries.
The Testing the System article also shows my energy production on cold winter days with bright sunshine. On winter days, with my new system, I hope I have enough energy to fully recover an overnight use by mid-day. (This will probably only be accomplished when tilting the panels.)
On a summer day, I have too many solar panels, unless I am trying to recover from multiple cloudy days — all in one day.
One of the cool things about my system design is that I can jam energy into the battery as fast as I can make it. Additionally, I don’t have to recharge the batteries to full every day. Here is an article about some test results with lithium but without solar last summer. Massive Electric Bucket
Remember we live in the RV all year, rain or shine, hot and cold. I can’t do much about cloudy days, but if I need more panels, I still have some unused roof space.
You have too many solar panels for your battery!
Again another easy answer. You don’t size solar panels to the battery.
The max charge rate that is acceptable for my lithium battery bank is more than 900 amps per hour. My solar panels on a great day are only expected to deliver a maximum of 100 amps per hour. I could even run the generator at the same time as the solar panels are maxed and deliver about 225 amps per hour. These charge rates are way less than my batteries would easily accept.
We measured our energy consumption carefully to determine our battery size. My battery is sized to sustain my electrical use for at least two days … less in the winter and more than two days in the summer. We tested it and in the summer we can go for about two and a half days. Here is another article outlining thirty days of off-grid camping (testing) without solar. Boondocking Without Solar
Hopefully, my solar panels will replace all the energy used from my two-day consumption all in one day. Perhaps by mid-day in the summer. Again easier done in the summer than the winter. Here is an article pertaining to measuring our energy needs. How to know what just-right means I’m pretty certain I know how much energy we use, the question remaining is how much energy can we make. Again, I still have some unused roof space
You have too many solar panels for your controller!
This comment was a little concerning. So I blasted off an email to the most knowledgeable RVer / solar guru I knew. This (with some editing) is Scott Helmann’s answer. I feel like tattle-tale… teacher-teacher, Johnny said… you get the idea. This is Scott’s answer turned into an article to share with you. I am publishing with his permission.
I think I have a real good handle on solar, perhaps I am in graduate school student on the subject. So, given my memory of the details, I went back to class and asked the professor. Here is the answer.
Scott’s answer –
Don’t worry — be happy
This is only a concern with cheap controllers with inadequate cooling.
Over-paneling is something that is routinely discussed and is standard practice in commercial remote applications. If you are running a high-quality controller (and you are) then over-paneling is an extremely good solution for RVers and off-grid applications.
Good engineering practice
This is the engineering aspect of designing an extremely well-functioning system and not about trying to sell you more equipment and make more money from the sale. I have sat through many online classes on this from both Morningstar and Enphase. (as I am an authorized installer of both).
You are building a system to use and not necessarily a system for bragging rights. Sure on some days, your controller will limit your max output when additional power could have been captured. (This wasted energy, while topping out, drives the armchair enthusiasts wild.)
As RVers, we don’t care about perfect efficiency. Our concern is maximum-continuous-sufficient-current and end-of-the-day totals. You have to charge on days with marginal solar on cloudy days and with imperfect sun angles. Maximum-continuous-sufficient-current is the way to charge batteries and get the job done. We don’t care about capturing a little extra — when energy is ample, but rather, we are concerned with getting enough to do the job on marginal days. As an RVer, you have to get – when getting is good.
High-quality controllers are designed to operate continuously at their maximum rated power output levels and will limit the output to that maximum design level unless things get too warm, at which times it will self-protect by downgrading the output. The best controller designs have big heatsinks and no fans (as fans will all eventually fail). I have tested and verified over paneling extensively and often find that when talking to “armchair experts” it is often easier to just smile and walk away. All they know is what is in the sales and marketing literature. I’m interested in the engineering design specs. Your Victron controllers will work just fine as they are high-quality MPPT controllers (make sure you have adequate airflow around them for cooling).
Panels in the laboratory
I don’t know if we ever discussed it but panels are rated at their flash output levels. The panels are at a standard temperature and are flashed with a bright standard light and their output is measured- that’s how they get their watt ratings. This light is at a perfect right angle to the panels.
Panels in the real world
Panels on an RV roof operated at all temperatures and almost always with an incorrect angle and azimuth. They almost never produce rated power. Typically they are also dirty. Panels that have reached full operating temperature under continuous light will not deliver their labeled output power. Often power output is 20% less than rated on the label.
You also will have wiring losses, connection losses, controller losses, wire length losses and after a few months of use slightly reduced power output from the panels and it is not 1000lm/m2. Add the degraded solar panel output in the real world, to the system losses and that’s how I come up with 70% of the rated panel power as power actually delivered to your batteries. On an average fun sun day with the panels facing the sun, you should expect to see an average of 70% of the rated power from the panels as continuous output.
Your system — really, you have two systems with a common battery.
You have designed 1800 watts of rated panels divided into a driver’s side array and a passenger-side array. Each array is 900 watts. Each controller has a maximum output of 50 amps. Maybe, some days you will see 50 amps, but it probably won’t happen very often.
The most important design aspect is shading
Your layout eliminates self-shading and shading kills output. Shade kills output for multiple panels in a series string even if only one panel of the string is shaded. There are some tradeoffs in terms of shade because you lose some efficiency based on the poor orientation of some of the panels at any one time. The only arrays that don’t have this problem, track the sun. Even tracking systems have problems because designers put the panels too close to each other, causing one array to shade the other array.
In the summer
Each 900-watt array should give you an average of about 45 amps of continuous power delivered to your batteries per controller when the sun is overhead. (Assuming zero shade and panels that are warm, but not too hot.) 45 amps would be a good expectation for the middle 3 hours of the day with less in the mornings and evenings. Thus two arrays should deliver about 90 amps per hour. Maybe a little more when the panels are brand new. You may see power output initially a little higher with your new system but in a few months of continuous use that will settle down a bit.
In the winter
You still have two arrays in the winter and need to consider them separately.
The array when tilted at a high angle toward the sun, you can expect to still get 45 amps of power delivered to your batteries by that bank during the midday. Production will be lower earlier and later in the day. Unlike the summer, the days are much shorter so the shoulder periods won’t produce nearly the power that they would have during the summer.
The panels that are not tilted should expect to see about 26 amps per hour in the middle 3 hours of the day (and less before and after).
It is unfortunate you can’t tilt the lower panels at the same angle as the higher angle panels. But of course, that means that the lower panels would shade the higher angle panels destroying their much better output. There is only so much room for tilting on an RV roof.
As far as energy spikes that occur as a panel exits shade from a cloud. Don’t worry about it, the duration is so short that it will have no effect on overall production. You will not even notice the spike and the controllers won’t have any difficulties.
Energy output will also spike when the panels are cold and hit with direct sunshine. Again don’t worry about it. You may not notice these increases and after they sit in the sun for only a short period, their temperature will rise. As the temperature goes up, energy capture will go down.
Wiring in series
Since you designed for three panels in series these rare situations will not cause voltage spikes more than the controllers can handle.
Not for bragging
Don’t forget that you designed your system for RV use and it is long duration high continuous power that gets the work done and your batteries charged. The peak stuff is only for bragging and when I meet those people I just tend to shake my head and walk away.
In my opinion, you have exactly the right amount of panels per controller so if someone gives you static just smile and say — well my system works extremely well every day and in every situation…
So the thing I take from this is that I don’t have too many solar panels or too many panels on a controller. Most of the time I won’t be at maximum on my controller. Given the multiple degradations to energy production, I will run out of roof space before I have too many solar panels. —- Scott Fox
Here is a link to the Zamp website explaining the Zamp Obsidian solar panel. Zamp