I actually read manuals and spec sheets for fun. And a big juicy technical white paper that dives into the engineering details can keep me up all night excitedly turning pages.
Understandably – not everyone enjoys curling up with a tech manual, spreadsheet, and 30+ web tabs open to cross-compare products.
And unfortunately, spec sheets are not written for average readers to easily comprehend. They turn into a sea of numbers that are at best confusing, and are often actually misleading once marketing departments gets involved trying to turn raw specifications into a reason to “buy now”.
Solar panel specifications can be especially hard to understand and compare. What numbers matter? Is a bigger number better? Smaller?
How can two different panels be compared?
Will you ever really get 100 watts out of a 100W panel?
What’s a Watt, Anyway?
Everyone has heard of a “watt”, but not many people have a solid grasp of what a watt actually is.
The “watt” is the standard unit used to measure power, named after Scottish inventor James Watt who developed the concept of “horsepower” for comparing the capability of steam engines back in the late 18th century.
To put a watt into context – 1 horsepower is defined as 746 watts, the average output of a typical draft horse. A puny human can sustain an output of around 75 watts (1/10 a horse!) over the course of a full workday.
The power required to run electrical devices is also measured in watts, as is the power produced by generators and solar panels. Thinking of things in watts is the best way to compare power production, and to balance it with power needs.
Solar panels come in a range of physical sizes and wattages – but the sizes most commonly used on RV’s range from 40W to 200W. Using 100 watt panels is particularly common, and by combining multiple panels on a roof larger systems can be built.
The largest RV solar installation we have seen managed to squeeze around twenty 100W panels on a roof for a 2000+ watt system. But most RV solar systems tend to be 500W or less. Often much much less.
A typical RV generator on the other hand might range from a 2000W portable (like the popular Honda EU2000) to a big 7500W built in unit, to even larger 15,000W generators in big Prevost-stlye buses.
On shore power – a 15-amp standard wall outlet can provide 1,800W to an RV, a 30-amp RV plug 3,600W, and a 50-amp RV shore power outlet can provide a whopping 12,000W via two independent 50A legs. This is what it takes to run multiple standard RV air conditioning units at once – each RV AC can easily draw over 1,500W while operating.
It takes an extreme solar system to even begin to hope to keep up with running an air conditioner, and it would take way more 100W solar panels than any RV roof could ever hold to equal the output of even a mid-range portable Honda generator.
When you start looking at the wattages, it hardly seems worth the effort to go solar – and for many usage scenarios it isn’t.
The advantage solar has is that it is clean, quiet, passive, and as long as the sun is shining there is no fuel tank to run dry.
Understanding Solar Spec Sheets
You’ll see a lot of numbers and details on solar panel specification sheets. Here’s what they mean, and how to calculate and compare the numbers that might not be so obvious.
- Power (Pmax) – This is the maximum wattage the panel is capable of producing, under absolutely ideal conditions. More wattage is always a good thing, all things being equal.
Cell Type & Efficiency – Solar panels are made up of solar cells wired together. The cells are where the magic happens – converting sunlight into electricity. A key point of comparison is just how efficient this conversion process is. The very best commercially available solar cells are still less than 25% efficient at this process, with 15% to 18% efficiency being much more common. In general monocrystalline cells are more efficient than polycrystalline cells, and they are both much more efficient than amorphous silicon thin film cells. (For more on efficiency, see the section below.)
- Dimensions: Length / Width / Depth / Weight / Flexibility – For installation onto an RV roof, the physical dimensions of a panel become critically important. Narrow panels in particular allow for more flexible roof layouts, working around roof vents and air conditioners. Thin and flexible panels are much lighter weight and have an advantage that they can be mounted directly to a roof, but the downside is that without an airspace underneath they can get a lot hotter. Hot panels are less efficient, and also heat up an RV interior more too.
- Watts / Square Meter – If you have limited surface area for panels, more efficient cells can cram a lot more wattage onto your roof. To compare panels in this way, multiply the length and width to get the area and divide the panel wattage by this value. The more watts per square meter, the better.
Voltage at Maximum Power Point (Vmpp) & Current at Maximum Power Point (Impp) – These are the key numbers for any solar panel, and describe the electrical point at which the panel is operating at peak efficiency – the maximum power point. Voltage times current gives you wattage, so (Vmpp x Impp) should always give you Pmax. When you wire different panels in parallel, you should try to match the Vmpp so that they work well together. And when you wire panels in series, try to match the Impp.
- Open Circuit Voltage (Voc) – If you test a solar panel that is not hooked up to anything other than the sun with a voltmeter, this is what you should read. It will always be higher than Vmpp – this is the maximum voltage the panel can ever produce. You need to make sure that the Voc of you solar panels is within the compatible input range of your solar charge controller – as long as it is, higher voltages can give more headroom above your battery voltage to help with charging.
- Short Circuit Current (Isc) – This how much current will flow if you touch the two wires coming out of the solar panel together, causing a short circuit. This is primarily important to ensure that you are using large enough wires and fuses for a safe installation.
- Warranty – A key number that should give you an idea how long the manufacturer expects the panels to perform up to specifications. Some rigid solar panels have warranties to produce 80% of rated power (under standard test conditions) for up to 30 years, but most flexible panels have much shorter warranties. Once a flexible panel has been glued to a roof, you will have a potentially hard time removing it intact for a warranty claim regardless. And of course a warranty is only useful if the company manages to stay in business long enough to honor it.
- Price Range & Price / Watt – All things being equal, cheaper is better. But there are always tradeoffs to be made. Solar panels currently range from slightly under $1/watt up to over $10/watt. Prices that seem too good to be true often are, so do look for reputable suppliers.
The key specifications to worry about for RV usage is the overall panel power, the price per watt, the watts per square meter, and the overall dimensions to make sure that the panels will fit where you have room for them.
“Standard Test Conditions” You Will Never See
Odds are you will never ever get 100W of power out of a 100W panel on your RV roof.
And in reality, seeing 75W or 80W is actually going to be a more common noon-time maximum.
Solar panels are rated to perform according to their official specifications under “standard test conditions” (aka “STC”) – a condition that can be created in a lab to allow for easy measurement and comparison. The defined STC for solar is a panel pointed directly at a bright sun with 1000W of solar energy landing per square meter, with the panel kept at 25 degrees celsius (77 degrees fahrenheit) with an atmospheric mass (a number that refers to the amount of atmosphere between the panel and the sun) of 1.5 overhead.
These are laboratory conditions – and the specs do not tell you how well a panel will work in actual real world situations.
A lot can impact the power a panel puts out – for example when the panel is not pointed directly at the sun, or the panel is hot, or scuffed, or shaded, or just plain dirty.
Heat in particular is a killer – the warmer a solar panel gets, the less power it puts out. Without any air space underneath to allow for cooling, heat becomes one of the prime concerns with using flexible panels mounted directly to an RV roof.
Angle is another big impact on power output – RV solar panels tend to be mounted flat, and even systems with rigid panels and panel tilting props tend to never end up with tilted panels without some fancy mechanisms to do it automatically. We had the capability to tilt the panels on both our prior solar setups on our trailers, but we rarely bothered because it was such a hassle.
Shade and dirt are always a concern – because even a tiny corner of a panel blocked from getting sunlight could end up diminishing the output of the entire panel.
And finally – time of day matters. There are only a few hours each day to either side of solar noon when a panel will produce the bulk of its daily output.
Understanding Solar Efficiency
If 1000W of solar energy is landing on a square meter surface, it is theoretically possible to convert 86% of that energy into electricity.
But so far – even the most efficient experimental solar cells developed using exotic materials have achieved at best less than 45% efficiency. Without using exotic materials, conventional silicon crystalline cells have a peak theoretical conversion efficiency is just 29.4%, and the best that has ever been demonstrated is 26%.
Commercially available panels are now available with greater than 20% efficiency – but there is not a ton of headroom for major new advances to dramatically improve solar efficiency all that much further.
Most of the focused solar innovation is now not on efficiency, but rather on driving costs down. A lot of the most promising technologies for cheaper solar panels are actually substantially less efficient – with amorphous silicon cells so far only reaching 13% efficiencies – producing half as much power per square meter than the best monocrystalline cells.
For many solar installations, this isn’t a problem – commercial and residential roofs and farms of panels on the ground generally have plenty of space to add more panels. The cost savings is worth it.
On an RV roof however – efficiency is usually more important than absolute lowest cost. There just isn’t space to add more panels to make up for extra energy needs.
One further note on efficiency… If only 20% of the solar energy is being converted to electricity, where is the rest of that power going?
A lot of it is turning into heat.
Solar cells tend to be black, and they absorb a lot of the excess energy as heat rather than reflecting it away like a white roof would do. This has the potential to make your RV interior warmer, and as that heat builds up the solar cells will become less efficient – producing even less electricity.
For more irony? That may actually necessitate the need for running the air conditioning to offset the interior heat caused by the panels – resulting in resorting to generators or electric hook-ups much sooner than otherwise.
Hard Solar Realities
A lot of people with solar on their RV have no idea just how little power they are actually generating, and they would be shocked if they knew that even on their best days they might be seeing just 80% of the rated power out of their panels – and that only for a few hours each day.
And on cloudy days, a 100W panel may only produce 10W or 20W – barely enough to keep a basic LED light fixture lit.
I sometimes suspect that this may be the reason that so few solar controllers provide a power readout in watts – instead showing just the amps going into the battery. If you have a solar controller that does not display wattage, you can calculate it by multiplying the charge controller output amps by your RV battery voltage. For example – if your charge controller is putting 3.7A into a 12.6V battery, you are generating just 47 watts of power.
On the other hand – nothing beats heading out on a sunny day running errands, and coming back to find your batteries fully charged without ever needing to even think about running a generator.
The goal is to design solar systems that make this possible.
And our goal in this testing project is to help demonstrate some real-world scenarios and results so that RVers are better able to set and manage their expectations.
Stay tuned for the next chapter…
Testing Update: We have added a 100W traditional rigid glass panel from AM Solar to our testing arsenal for comparison purposes.
We aren’t sharing any specific results or conclusions from our testing just yet though – one of the panel manufacturers suspects that we might have received a defective panel and they are rushing us a replacement. We want to give them the benefit of the doubt, and will need to re-run all of our testing once it arrives. And, now that we’re on the coast of Oregon, we have to also wait for a rare sunny day for that!
We hope you will follow along on our ongoing Solar Quest – you will be able to access all of our solar related posts from our new RV solar page.