Choosing Solar Panels: The Specs That Decide

Panels are the part of an off-grid system people compare on the fewest numbers. Wattage, price, and sometimes efficiency. Wattage tells you what the panel does at noon in a laboratory, and the two specifications that decide whether it suits your system are usually further down the page.

1. Open circuit voltage decides your charge controller

Voc is the voltage the panel produces with nothing connected to it. It is not the voltage it runs at, and it is the number that determines what you can wire the panel to.

The reason is the difference between the two controller types, covered in full on charge controllers:

• A PWM controller connects the panel almost directly to the battery, dragging the panel down to battery voltage. Any voltage above that is simply lost.
• An MPPT controller converts surplus voltage into extra current, so a high-voltage panel keeps its output.

That makes Voc a compatibility question rather than a preference. Two panels in the same manufacturer’s range, verified against their published figures:

The 200W is a 24V nominal panel whatever the listing says, and the same manufacturer’s European and Australian storefronts label that exact panel 24V. Put it on a 12V bank behind a PWM controller and you will pay for 200W and collect appreciably less. Behind MPPT it is fine.

This is the panel-side version of an argument this site makes repeatedly. A nominal label describes the market a product is sold into. The rating describes what it does. Where they disagree, the number wins. The same trap in switchgear is covered in why a 48V part is not for a 48V bank.

Buying the smaller panel does not by itself keep you on PWM

This is the part that catches people, and it falls straight out of the two tables above.

Series wiring adds voltages. Two 100W panels in series reach 45.58V, which is higher than the single 200W panel they were bought instead of. Someone who chose the smaller panel specifically to stay PWM-compatible, and then wired the pair in series because that is the usual off-grid advice, has gone further from PWM than if they had bought the 200W in the first place.

Parallel keeps the voltage at 22.79V and doubles the current instead, which is what preserves PWM compatibility, at the cost of heavier cable between array and controller. That trade is the subject of the series and parallel section on the panels page.

So the panel and the wiring decide this together. Neither one alone tells you whether your controller can use the array.

2. Voltage rises when it gets cold

Published Voc is measured at 25°C. Panels produce more voltage as they get colder, and a cold clear morning is exactly when an off-grid array is doing its best work.

So the figure your charge controller has to survive is not the Voc on the datasheet. It is the Voc at the coldest temperature the array will ever see, which is higher, and it is why controllers carry a maximum PV input voltage well above any panel’s nameplate.

Every manufacturer publishes a temperature coefficient for voltage to let you work this out. Use theirs rather than a rule of thumb, because the coefficient differs between cell technologies, and get the answer right before wiring panels in series. Series wiring adds the voltages together, so a string that is comfortable in July can exceed a controller’s input rating on the coldest morning of the year. That is a failure that destroys the controller rather than merely disappointing you.

3. Module efficiency, not cell efficiency

Panels are advertised on cell efficiency. The number that describes the panel is module efficiency, and it is always lower, because a module is not all cells: it includes the frame, the gaps, and the margins.

Both panels above are built from 25% cells. As modules they are 20.7% and 20.0%. All four numbers are true, and only two of them tell you how much power comes off a given area of roof.

You can check a module efficiency claim yourself, which is worth doing because it is the number most often quietly inflated. Multiply the panel’s length by its width to get its area in square meters, divide the rated wattage by that, then divide by 1000. The 100W panel is 865 by 578 mm, so 0.5 m² exactly, and 100W over half a square meter is 200 W/m², which is 20.0%. If that arithmetic does not land on the published figure, one of the numbers is wrong.

Efficiency is watts per square meter, not watts per dollar. That matters less than people expect:

• On open ground, where you can add another panel, the cheaper panel per watt usually wins. Efficiency is close to irrelevant.
• On a fixed roof, a van, a boat, a small cabin, the area is the constraint and efficiency decides whether the system fits at all. This is the only situation where paying for efficiency is straightforwardly correct.

Work out the array size you need from your loads with the system sizing calculator first. Then find out whether it fits. Doing it the other way around produces a roof full of panels that does not match the load.

4. Temperature coefficient

The other temperature number, and this one costs you output rather than threatening equipment.

Panels lose power as they heat up, and a panel in full sun runs far hotter than the air around it. A coefficient of -0.29%/°C means roughly a third of a percent lost for every degree above 25°C. On a still summer day a panel at 55°C is down about 9% before anything else in the system has done anything wrong.

Lower is better, and it is one of the genuine arguments for N-type cells over older p-PERC construction. It is also why a hot climate does not automatically mean a better solar site, which climate considerations covers alongside the opposite problem.

5. The things that decide the install rather than the output

Dimensions and weight, because the mount and the roof have opinions. The 200W above is 1262 by 764 mm and 10.6 kg.

Ingress protection. IP65 on the junction box and IP67 on the connectors is the normal standard for a panel expected to live outdoors permanently.

Mechanical load ratings. 2400Pa wind and 5400Pa snow on this one. If you are somewhere with real snow, that second figure is not decoration, and neither is the tilt angle that sheds it.

The warranty, which has two halves. A material and workmanship warranty covering the panel failing, commonly 10 years, and a separate output warranty covering gradual degradation, commonly 25 years at 80%. The long number in the advertising is the second one. It does not mean the panel is guaranteed to still be there in 25 years, only that if it is, it should still make 80%.

What to buy

Disclosure: This page contains affiliate links. If you buy through them I may earn a commission, at no extra cost to you. This never affects which products I recommend or what I say about them.

As with the batteries guide, this list is limited to products whose governing specifications have been read from the manufacturer instead of a retail listing. The scope note at the bottom says what that leaves out.

Between the two, the 200W is the better panel and the 100W is the more compatible one. If MPPT is already in the design, or is going to be, take the 200W: it is more efficient per square meter, lighter per watt, and one panel replaces two sets of brackets and connectors. The 100W earns its place when the controller is fixed, the roof is awkwardly shaped, or the array has to grow in smaller steps than 200W at a time.

Scope, and what this guide does not do

One manufacturer is not a market. Renogy is here because their published data is complete enough to check and because I run their panels. That is a starting point, not a survey, and a guide that pretended otherwise would be worth less than an honest short one. More will be added as their datasheets are actually read.

Panels are the most commoditized part of a solar system, which cuts both ways. It means brand matters less here than it does for a battery or an inverter, and a well specified panel from a manufacturer who publishes real numbers is usually a fine choice regardless of the badge. It also means the specification table is genuinely the whole story, which is why this page is mostly about how to read one.