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.
A flexible panel made up of 32 individual solar cells.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.
The “Maximum Power Point” is at the knee of this curve, Voc is at the bottom right, and Isc is at the top left.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.
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