How To: Choose A Solar Panel

Help! How do I choose a solar panel?  ... is one of the most popular questions we are asked.

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As with most things, there are different points to consider when choosing a panel:

What we'll do here is go through the different points in broad terms (and then cover the actual power calculation in Q6).

Q1. How much power (wattage) do you need?

The wattage of the solar panel you need is perhaps the most important thing to get right.

Why?  Well, if you underestimate the amount of power you need you could be very disappointed with the results.  (Like expecting a tiny car to pull a huge caravan).

Overestimate and you might end up spending more than you need to.  (It might be fun but most people don't buy an expensive Ferrari for a quick trip to the shops).

There are three main stages to a solar powered system.  

1. Power generation (the solar panel)
2. Power storage (the battery)
3. Power use (your items you want to run off solar power)

To work out the wattage correctly, the panel needs to be sized according to how much power you are going to use. (Later on you're going to need a bit of info on each electrical item you need to be solar powered).

The battery, no matter how large or small, is a storage container for solar power gathered in daylight for use immediately or for later on.  Highly important.  For example without a battery, a solar powered torch would be rather pointless in the dark!

Just remember that to maintain the power storage at a constant level the solar panel needs to put in to the battery the same amount of power as is going out of the battery now (or will go out of the battery later on).

However, at this point you don't yet need to consider the size of battery you are charging (unless you are sizing for an uninterruptible power supply (UPS) system).  You just need to know that you will need one.

Some quick ideas of power ratings needed:

As a general rule, if you have a caravan you'll need a panel between 20-60W, whereas most motorhomes are fitted with panels of 80W and above.  (There tend to more gadgets needing power in a motorhome than in a caravan).

For laptop charging you need at least 25W to provide a useful trickle charge.

See Q6 - How do you size a system for more details.

Q2. Does the panel need to fit a particular space?

One of the reasons we like to supply a range of different manufacturers panels is to provide lots of different sizes. (By that we mean width, length, thickness AND power rating).

You can also add panels together to get the overall size you need. So if a 120W panel is too large in area, but you have room for two 60W panels side by side, then this would work too.

Remember that for the equivalent wattage a crystalline panel will be smaller than an amorphous panel.  (If you haven't come across these terms before, you may want to scan through our Glossary).

For large orders we can have even have panels made to your specifications.

Q3. What is your budget?

Put simply, the more powerful the panel, the more it will cost.

Rigid panels cost less per watt than flexible and folding panels.

There are also differences between equivalent panels that may reflect on items such as:

• the quality of the junction box and frame
• the packaging and instructions
• cables and other accessories that might be included

(It also depends on how good a deal we can get from the manufacturers!)

Q4. What accessories are included?

Think about what accessories you'll need and how confident/competent you are with the wiring, and whether you would be better getting a kit.

Some panels come with cables already fitted to the junction box (all flexible and rollable panels, smaller Kyocera panels), or with plug-and-go connectors (Sunsei range).

However many will require you to fit cables and sometimes blocking diodes, both of which we can supply.

Consider also whether you need additional cables like a 12V cigarette lighter socket or extension cables, and if these are available for your chosen panel.

Finally, how will you mount the panel? Sunsei panels come with integrated mounting feet, but most rigid panels will have an aluminium frame that needs to be raised above the surface they are fixed to by about 10mm to allow the air to circulate underneath.

We can supply simple mounting brackets, but if you require something more complex (a pole mount, or angled mount) please contact us.

Q5. Does it need to be portable or is it a permanent installation?

Generally speaking if weight and portability is your main concern (perhaps you're a touring cyclist) then you need a folding or rollable panel.

However, if you're not trekking/canoeing/cycling or doing some other extreme sport that requires you to carry all your kit, then you might like to look at our solar kits for Caravans. These are based on a rigid panel, so are less costly, but include a carry bag and stand.

For permanent installations on motorhomes, sheds, boats etc it generally makes sense to use a rigid framed module. However, if you need to walk on the panel (because it is part of your boat deck), or need it to fit on a curved area, you might consider either a rollable Powerfilm panel or a Solara semi-flexible kit.  

You may also want a rollable panel because you want to temporarily fix it in place with bungee cords (for example) on a boat or over a tent.  Again the rollable or flexable panels are very handy.  You can browse our flexible panels by their power rating or in solar kits for particular usage.

If you want to learn more, please visit our website Hongyuan International.

Q6. How do you size a system?

In sizing a system, the aim is to balance the power going into the solar panel with the power going out of the battery over a period of days or weeks (depending on how it is being used).

A 10W panel will give 10W (0.6A @ 16.5V) for each hour under standard test conditions (W/m sq and 25oC).

A quick sunshine hours guide for the UK:

A summer's day will give you the equivalent of 4 hours sunshine in the UK.  A 10W panel will give 40W in that day.

On a winter's day, you'll get the equivalent of 1 solid hour of sunshine and so a 10W panel will give 10W in that day.

These are fairly conservative figures &#; some companies use up to 6 hours in summer. You can do the same calculations with the Amps.

Some simple steps for sizing a 12V system:

1. Find the Wattage of your appliances. List all the 12V electrical appliances you&#;ll use in a typical day, and find out how many Watts they each consume. Usually this is on the appliance or in its handbook. If you can only find a figure for Amps, simply multiply this by 12, to convert it to Watts.

2. Calculate your daily total Watt-hour requirement. Estimate how many hours you would use each appliance for over a typical week, then divide by 7 for a daily rate. Multiply each appliance&#;s wattage by the hours you&#;ll use it for in a day. Then add all the totals together to get the final daily total Watt-hours you require.

3. Next calculate your panel size. Simply divide the daily total Watt-hours you require by the hours of usable light you expect in an average day. This will give you your minimum panel wattage. In the UK, allow 1 hour of light in winter, rising to 4 hours by mid-summer.

4. Then your battery size&#; Multiply your daily Watt-hour requirement by 7 to create a weekly requirement, and divide this by 12 to convert back to Amp Hours, which batteries are rated in. Multiply by two to give the correct battery size.

5. And finally, your charge controller. Size your charge controller according to the Amps produced by your panel. Calculate the Amps produced by dividing the panel wattage by 16.5.

6. A worked example. In one week you want to run a 65W television for 4 hours, and an 8W light for 5 hours. Your daily Watt-hour requirement for the TV is 65 x (4/7) = 37Wh; and for the light you require 8 x (5/7) = 6Wh. Your total daily requirement is thus 43W. You only intend to use the system in summer, so you need a panel that is 43/4 = 11W or more. Your battery size needs to be (43 x 7 x 2)/12 = 50Ah. And you need a charge controller suitable for a solar input of at least 11/16.5 = 0.7A

Solar technicians know that turning the sun's radiation into electricity isn't magic. Solar energy safety takes specific expertise, exacting safety standards, and hard work.

Utility-scale solar installations use rapidly evolving technologies, from photovoltaic (PV) modules and inverters to battery storage and metering. In PV systems, current is "wild" and not limited by electronics. Solar panel safety precautions, control measures, and best practices are different from any other kind of energy generation. Your tools have to be designed to handle the job, because the stakes for solar safety are high.

These are three of the most common electrical hazards with PV systems that you can encounter, along with specific solar PV safety control measures you can take to reduce their risk.

1. Shock or electrocution from energized conductors

Just as with other electric power generation, PV systems present the risk of shock and electrocution when current takes an unintended path through a human body. Current as low as 75 milliamps (mA) across the heart is lethal. The human body has a resistance of about 600 ohms. Per Ohm&#;s law, voltage (V) equals current (I) times resistance (R), so V = IR.

To calculate the amount of current that would course through a person&#;s body if exposed to 120 V, simply divide 120 V by 600 ohms (I = V/R), which totals 0.2 amps or 200 mA. That&#;s more than 2.5 times the lethal limit of 75 mA, so protecting yourself and your workers against such an event is critical.

Electrical shocks are typically caused by a short circuit resulting from corroded cables and connections, loose wiring, and improper grounding. Key places to look for these conditions in a PV system include the combiner box, PV source and output circuit conductors, and the equipment grounding conductor. The grounding conductor bonds all metallic components together&#;and eventually to ground&#;through the grounding electrode conductor and grounding electrode.

Control measures: Rapid shutdown systems

Energy produced from PV string systems varies directly with the sun. To reduce shock hazard for technicians and first responders, we need a way to shut those strings off during a short circuit or power outage. The National Electrical Code (NEC), Section 690.12 requires the &#;rapid shutdown&#; of PV systems both inside and outside the PV array boundary. According to section 690.2 of that code, PV array boundary is a mechanically integrated assembly of modules or panels with a support structure and foundation, tracker, and other components, that form a DC or AC producing unit. This includes controlled conductors located inside the boundary or up to three feet from the point where they penetrate the surface of the building.

As of , the NEC made these requirements more stringent by requiring:

• Modules and exposed conductive parts within the PV array boundary to be reduced to 80 V within 30 seconds.
• Conductors located outside the array boundary to be limited to 30 V within 30 seconds.

Rapid shutdown devices must be located either at the service disconnect or there must be a special rapid shutdown switch. There is an exception for systems that are controlled by module-level power electronics&#;such as micro-inverters and power optimizers&#;that reduce voltage. Arrays with no exposed conductive parts and located more than eight feet from exposed grounded conductive parts, are not required to comply.

In addition, many jurisdictions in the U.S. require that rooftop PV arrays have setbacks that allow firefighters to access the system. For instance, the California Residential Fire Code requires PV modules be at least three feet from the ridge of the roof.

2. Arc faults that spark fires

As with any electrical system, fire is always a potential hazard. Perhaps one of the most common causes is electrical arc faults, which are high power discharges of electricity between two or more conductors. The heat caused by this discharge can cause the wire insulation to deteriorate and thus cause a spark or &#;arc&#; that causes a fire.

PV systems are subject to both series arc faults caused by a disruption in continuity of a conductor, or parallel arc faults caused by unintended current between two conductors, often due to a ground fault.

Control measures: Arc-fault circuit interrupters

An arc fault may lead to a short circuit or ground-fault, but it may not be strong enough to trigger a circuit breaker or a ground fault circuit interrupter (GFCI). To protect against arc faults, you need to install an arc-fault circuit interrupter (AFCI) outlet or an AFCI circuit breaker. AFCIs detect low level hazardous arcing currents and shut off the circuit or outlet to reduce the chances of such an arc fault sparking an electrical fire.

The NEC Section 690.11 mandates that PV systems operating at 80 V DC or greater between any two conductors be protected by a listed PV AFCI or equivalent system component. The protection system needs to be able to detect arc faults resulting from a failure in the intended continuity of a conductor, connection module, or other component in the PV system DC circuits.

3. Arc flash leading to explosions

Large-scale PV arrays with medium and high levels of voltage are susceptible to arc flash. This is especially true when a technician is checking for faults in energized combiner boxes where PV source circuits are combined in parallel to increase current, and when checking medium-to-high voltage switchgear and transformers. An arc flash releases hot gases and concentrated radiant energy up to four times the temperature of the sun&#;s surface&#;as high as 35,000° F (~19,500° C). It occurs when a large amount of energy is available to an arc fault, in both DC and AC conductors.

Arc flash is an issue for systems over 400 V so both residential inverters that typically have a maximum input voltage of 500 V and large-scale inverters that have a maximum of 1,500 V are at risk. Before the advent of large-scale solar energy systems, arc flash was solely considered an AC issue since DC voltage was limited to off-grid applications where batteries of less than 100 V were used. The National Fire Protection Association (NFPA) Standard 70E requires an arc flash hazard risk analysis be conducted and Personal Protective Equipment (PPE) used for DC systems over 100 V.

Control measures: AC and DC side mitigation

Arc flash mitigation in PV systems is divided by DC (before the inverter) and AC (after the inverter). DC-side mitigation for large solar arrays (100 kW +), is especially important at the combiner box where multiple strings of solar panels are combined in parallel to increase the current. To reduce the potential for arc flash, large-scale systems can use multiple string inverters that themselves can connect multiple strings in parallel, instead of using one or two large central inverters that require combiner boxes. AC-side mitigation includes arc resistant switchgear, which redirects arc flash energy through the top of the enclosure, away from personnel and equipment.

Depending on the task, basic PPE for solar PV technicians can include gloves, hard hat and ear protection, safety harness, arc-rated clothing, and a Fluke 87 V Industrial Multimeter.

Choose the correct solar testing equipment

Protecting your workers and your PV system from electrical hazards requires adherence to safe work practices and ensuring that your equipment is rated to withstand these potential hazards. That means multimeters, test leads, and fuses must all be rated for the application you are working on. Here are some basic guidelines:

• CAT-appropriate equipment: Choose a meter rated for the appropriate measurement category (CAT rating) and the voltage level of your application. Your multimeter must be able to withstand average voltage levels and high voltage spikes and transients that can deliver a shock or produce an arc flash. Overvoltage category III V systems are becoming the new normal in solar. The Fluke 393 FC True-RMS Solar Clamp Meter is the only CAT III V/CAT IV 600 V TRMS clamp meter that meets the insulation requirements for CAT III environments like solar installations. It also has Fluke Connect&#; wireless capabilities, so you can monitor measurements from a safe distance on your smart .
• High-altitude considerations: CAT III and IV equipment must be used for PV systems at high altitudes because air becomes less insulating and less dense as you go up, which decreases its cooling ability. This means the breakdown voltage&#;the minimum voltage causing an insulator to become electrically conductive&#;decreases with altitude. For instance, for a 1-centimeter gap between conductors, the breakdown voltage would be 30kV at sea level, 1.2 kV at 50,000 feet, and 300 V at 150,000 feet.
• High-quality test leads: Select test leads that are CAT rated to match or exceed the CAT rating of the digital multimeter.
• High-energy fuse replacements: Always replace high-energy fuses with the same quality part and amperage rating. These fuses are designed to keep energy generated by an electrical short contained within the fuse enclosure. They are life savers and should never be replaced with cheaper generic fuses.
• Probes and probe accessories: Use retractable probes, probe tip covers, or probes with shorter tips to avoid accidentally touching metal to metal and causing a short circuit
• Personal protective equipment: Wear appropriate PPE, including arc-rated clothing, gloves, safety glasses or goggles, hearing protection, and leather footwear as required for the voltage you are working on. The NFPA Standard 70E Table 130.7(C)(15)(c) identifies a complete list of PPE categories and the appropriate arc rated clothing for each rating.

These are just the highlights of how to work more safely with PV systems. Be sure to follow all relevant safety standards and regulations, manufacturers&#; instructions, and your company&#;s safety procedures when testing or servicing any electrical equipment.

See what's on the Fluke Solar Toolbelt

About the expert

Michael Ginsberg is a solar expert, trainer for the U.S. Department of State, author and Doctor of Engineering Science candidate at Columbia University. He is also chief executive officer of Mastering Green, where he has trained nearly a thousand technicians worldwide in solar PV installation, maintenance, and operation.

For more information, please visit Photovoltaic Accessories.

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