DIYers planning to install Solar panels on their RVs, trailers, camper vans, boats, or off-grid cabins have a number of important questions to answer before they flip the switch. Parallel or series wiring is one of the most pressing. 

If you don’t get it right, your system may perform poorly — or not at all. In the worst case scenario, you could cause a fire.

To avoid these negative outcomes and enjoy maximum solar power output, it’s worth your time to figure out exactly how to connect your solar modules. 

Whether your solar panels are arranged in series, in parallel, or in a series-parallel combination, a fully functional, high-performing, and safe solar array is always your goal.

In this article, you’ll learn the basics of series and parallel circuits in electricity as they pertain to solar energy. This discussion will include important information about other necessary components of your solar setup including the solar charge controller and battery pack for solar storage. 

With this knowledge, you’ll be able to determine which wiring design — series vs. parallel — may be better than the other in your unique situation to meet your specific needs. 

Sometimes, a combination of series and parallel wiring among all the solar modules in your system will maximize solar power output. In this case, the connections are more complex, but doable for an experienced and highly motivated solar DIYer. 

There are many factors at play in your decision about series vs. parallel connections in your solar setup, and we’ll consider the major ones here. 

Along with all of this critical information, you’ll find handy sections on the appropriate connectors, wires, and fuses to ensure the safe performance of your DIY solar array. As an added bonus, you’ll discover sample calculations for various wiring plans.

Electrical current, voltage, and power in solar panel systems 101

Whether your solar panels are connected in series or in parallel, there are three fundamental concepts to understand about electricity before you get started. These are electrical current, voltage, and power.

We’ll use all three frequently in this article, so DIY solar newbies should read this section. But if you’re already experienced with solar, feel free to skip this part of the article. (We promise we won’t hold it against you!)

You know that solar cells inside solar modules generate direct current (DC) electricity from the sun. Exactly how does this happen?

First, certain light rays transfer enough energy to some electrons in the photovoltaic (PV) cells in solar panels to cause the electrons to move around randomly. Electrons are negatively charged, tiny particles. 

Due to the presence and arrangement of certain PV materials in solar cells, a separation of charges into positive and negative sides (poles) results when exposed to sunlight. In other words, the sun’s rays create an electrical field inside the solar cells. 

Because of this electrical difference (also called the potential difference), all of the energized electrons move in the same direction toward the positive pole. 

Another word for this potential difference is voltage. 

The electrical energy transmitted from electron to electron as it moves in an orderly fashion in one direction forms a current (flow) of electrical energy.

To summarize: When electrical current moves between a potential difference, an internal electrical circuit (loop) exists inside every solar cell in every solar panel.

When you add an electrical appliance — like a toaster or coffee maker — to the circuit through positive and negative wires leading away from the panels to the appliance, you form an external electrical circuit.

Basically, your solar panels transform the sun’s radiant energy into direct current (DC) electrical energy (a form of electricity). When transmitted through wires, this electricity can perform useful work in your home, cabin, RV, boat, trailer, or camper van. Examples of what it can do include lighting lamps, running a TV, or playing a radio.

Electrical current

Electrical current refers to the flow of energized electrons. It is measured in amps (A) and may be referred to as amperage. 

Current is often symbolized by an upper- or lowercase letter i.

The greater the flow, the higher the amperage. 

A good way to visualize electrical current is to imagine water streaming out of a garden hose or down a cascading river. The moving water is similar to the rapid transmission of electrical energy from one electron to the next in a conducting material such as a copper wire.

Electrical voltage 

The potential difference between two poles that are oppositely charged (positive and negative) is also called voltage. When the difference is large, the voltage is higher. Voltage is measured in volts (V).

A good way to understand voltage is like the water pressure of a garden hose. It’s the force or “push” that moves the water along.

Without this difference between any two points, the water current will slow down or stand still.

To relate this to your solar setup, the voltage of your panels must be significantly greater than the voltage of your battery pack in order to charge it quickly or even to charge it at all.

Electrical power

When a solar panel is exposed to the sun, both electrical voltage and current are created. This means the panels can supply energy to make appliances operate. In other words, the solar panels produce electrical power.

Electrical power is defined as voltage multiplied by current:

P = V x I

Power is measured in watts, noted as W.

All three of these concepts — electrical current, voltage, and power — are central to the following sections on series vs. parallel circuits in solar panel wiring.

Solar panel parallel vs series connection: what’s the difference?

The major practical difference between wiring identical solar panels in series or in parallel is what happens to the output current and voltage in each case:

• Series connection → Total output current of the entire system is equal to the output current of just one panel. The output voltage of the system is additive across all panels.

• Parallel connection → Total output current of the whole system is additive of the output currents of all panels. The total output voltage of the system is equal to just one panel’s output voltage.

To keep it simple, especially if you’re new at DIY solar, we recommend that you use identical solar panels in your system.

This way, you won’t have to deal with significantly varying voltages and currents that require multiple charge controllers and possibly different fuses and wire gauges. Extra components only add to your expense and take up more space. 

Plus, mistakes in placement, confusion with hardware ratings, or incorrect matchups could damage your panels, your batteries, the charge controller, or cause a fire.

This is especially important for an RV, trailer or camper van situation where space is already tight.

Furthermore, wiring panels of different output currents in series means the system current will be equal to that of the poorest-performing one. Consequently, the total power generated will be lowered (since P = VI).

Similarly, connecting modules of different output voltages in parallel will lead to a system voltage equal to that of the panel with the lowest voltage. As a result, the total power output of your system will be less than maximum.

It’s also critical to note that in real life situations, the stated values on the panel spec sheets for output voltage and current are obtained under ideal conditions only. They will most likely be higher than what you actually get from your solar array on a typical day. 

Depending on sun hours, shading, air temperature, angle and direction of the panels and a host of other factors, solar panel efficiency will vary constantly. This is true whether your panels are connected in series or in parallel.

Because of this, as a good rule of thumb, assume that your panels will reach only 70-80% of their stated output voltage and current. So, when determining how much total power you need for a certain time frame or wish to store, be sure to figure this into your calculations.

To avoid other potential power losses, make sure none of your panels are partially shaded and all are working properly.

Of course, when you are actively installing whatever solar panel configuration you choose on your vehicle or structure, make sure the panels are covered to avoid electric shocks or worse. Ouch!

What is parallel solar panel wiring?

When solar panels are wired in parallel, each panel functions more or less independently of all the others. Current flows through multiple paths compared to just one possibility in a series connection. 

It also means that what happens to one panel (such as shading or storm damage) does not affect the other panels’ performance. This is different from a series connection where whatever happens to a single panel affects the others in terms of total power production from the array.

To wire solar panels in parallel, you collect all the positive terminals of all the panels in one group, bringing them together to form just one positive terminal using a branch connector (see below for more on connectors).

Likewise, you do the same for the negative terminals of all the panels, collecting them from all the panels to terminate in just one negative end, again using a branch connector. 

Then, once you’ve formed one positive and one negative grouping from all the panels, you attach them to the charge controller in your system. 

In most cases, you’ll need long cabling to extend the positive and negative leads from the roof of your vehicle or structure, and passing through to the charge controller, and again to connect to the battery bank. (See below for more on wiring.)

Sample calculation for parallel solar panel connection: volts and amps

For simplicity’s sake, we’ll use whole numbers when possible.

If you have two PV panels rated at 100W each that you wish to connect in parallel, you add the output currents together then multiply the sum by the open circuit voltage (V_oc) of one panel to determine the estimated power output.

Assume the V_oc is 20V and the output current is 5A.

P= (5A + 5A) x 20V = 200W

What is series solar panel wiring?

When solar panels are connected in series, there is only one path for the current to flow.

You achieve this by connecting the positive terminal of one panel to the negative terminal of the panel next to it, forming what’s called a daisy chain or a string. You could have any number of panels connected in this manner although 2-8 panels are common for small solar DIY projects. 

Essentially, you’re making one large solar panel with a combined voltage equivalent to the sum of the voltages of all the panels in the daisy chain. The current flowing through them all is equal to the output current of just one panel.

Such an arrangement leaves an unconnected positive terminal on one end panel and an unconnected negative terminal of the panel at the other end of the panel string. Those two unconnected wire leads go into your charge controller through single-contact MC4 connectors and solar extension cabling (see below for more on connectors and wiring).  

Sample calculation for series solar panel connection: volts and amps

To keep the calculation simple for illustration purposes only, we’ll use whole numbers as much as possible.

If you have two 100W PV modules, use the open circuit voltage (V_oc ) and the output current to calculate the power of your solar setup. Assume those values are 20V and 5A, respectively.

So, add the voltages (20V) of the two panels together. Then multiply their sum by the current (5A) to get the estimated power from the array. 

P = (20V + 20V) x 5A = 200W

What is series-parallel solar panel wiring?

In series-parallel wiring, two or more identical solar panels are strung together in series alongside two or more identical modules in a separate daisy chain series configuration. For small projects, up to 16 panels, with groups of 2, 4, 6, or 8 in series, is feasible. 

It all depends on the physical dimensions of the panels in relation to the space you have allotted for them.

However, it’s also recommended to make sure each of your series loops contains the same number of identical panels if you want to set them up in parallel. This is to prevent power loss. 

For example, each series grouping has either 2, 4, 6, or 8 identical panels. Then, a separate series of 2, 4, 6, or 8 identical panels each are put in a parallel arrangement with all other groups, also in series and containing the same number of identical panels.

To wire multiple series of panels in a parallel manner, the unconnected positive leads from each series configuration are combined into one group through a branch connector. In the same way, all the unconnected negative leads from each series loop are brought together into another branch connector.

Finally, the combined positive terminals from all the panels in parallel go as a unit into the positive entry port of the charge controller. Likewise, the group of unconnected negative wires from all the panels in parallel come together into one negative end that is connected to the negative entry port of the charge controller.

Of course, there are extension cables involved from one connection point to another. 

Sample calculation for series-parallel solar panel connection: volts and amps

In a series-parallel connection, you put in parallel two or more strings of panels, each of which is in series.

To keep it simple, we’ll use whole numbers.

Following the pattern of the previous two sample calculations in this article, we’ll use panels rated at 100W each with a V_oc equal to 20V and output current of 5A.

As an example of paralleling two series strings with four modules each:

For each series string: 

P = (20V + 20V + 20V +  20V) x 5A = 400W

Next, we put the two series strings in parallel with each other to get the total estimated power output of this series-parallel PV system:

P =  (5A + 5A) x 80V = 800W

Note: Putting all eight panels in series is possible only if your charge controller is rated for approximately 200V, which is not typical. (In reality, V_oc  is higher than what we used in this example for 100W panels, meaning the total output voltage from a series connection would be more than V_oc x 8 = 20V x 8 = 160V.) Similarly, designing an array with these eight panels in parallel would create an amperage that’s quite high (5A x 8 = 40A). You’d need a charge controller that could comfortably handle slightly more than that. 

When is series-parallel solar panel wiring a good idea?

Sometimes you may have sets of solar panels with different output voltages and currents. Possibly you purchased them at different times but would like to use them together on your camper van, RV, trailer, boat, or off-grid cabin.

Sometimes, connecting them in series-parallel fashion will allow you to have the best of both worlds — plenty of solar power using all of your resources.

If you go this route, you must be sure that the final maximum power, voltage, and current outputs from the series-parallel configuration do not exceed the maximum respective input values of your charge controller. 

Of course, your wire gauges and fuses must be adequate to handle the maximum load, too, and be appropriate for whatever currents you have.

You may also wish to include a second charge controller to manage widely differing voltages and currents (if you have them in your mix-n-match solar array) along with a second battery pack.

Connectors for series and parallel wiring of solar panels

There are several different connectors used in solar panel wiring. Here’s a brief overview of the major types.

MC4 connector

The most common type of connector for solar panels is the single-contact MC4 connector. MC is the abbreviation of the name for the original manufacturer of this piece of solar hardware. The number 4 refers to the size of its entry portal (4 mm). 

Most PV modules you purchase today come with MC4 connectors already attached to the negative and positive lead wires on the back of the panels.

The positive wire is usually attached to what’s commonly referred to as the female end of the MC4. The negative lead is equipped with the opposing male MC4 end. (No crude jokes allowed here!)

However, you’ll need to extend these short lead wires so that they will join up with the charge controller which could be over 10 ft. away from the panels. 

To do this, you’ll need to crimp complementary male and female MC4s onto the opposing MC4 parts on the panel leads by using a crimping tool. Then you can connect a wire of the precise length you need to complete your solar array.

Result: a custom-made solar array that exactly fits your vehicle or structure!

Branch connectors

Typically used in parallel connections between PV modules, branch connectors are another type of MC4 connector. They allow you to group multiple positive or negative leads into one end that eventually plugs into the charge controller.

Y connectors

Shaped like the letter Y, these parallel branch connectors are sold in pairs. Y connectors are meant to work with two panels only. They are rated for both voltage and current.

T connectors

In the form of the letter T, these parallel connectors allow you to join three positive or negative lead wires into one. Similar to Y connectors, they are sold in pairs and rated for current and voltage.

Wires for solar panels in series or in parallel

In a solar array, electrical current flows from the PV modules through a charge controller and into your battery pack via wires. 

Copper wire is preferable to aluminum wire if you want to ensure smooth and rapid power transmission with minimal loss. It’s also more expensive.

The electrically conducting metal is encased in a non-conducting plastic sheath for safety and efficiency in electrical conduction.

The amount of current flowing through your solar system and how long that distance is determines what the wire thickness and length should be. It’s critically important to get the sizing right to avoid power losses and prevent fires.

The American Wire Gauge (AWG) scale lets you choose the wire size that’s best for your solar array. The lower the gauge number, the more amperage it can tolerate.

As a general rule, PV modules rated at over 50W need 10 AWG cabling. 

For very large parallel configurations, you may need to use a thicker combiner wire to handle the amperage and prevent huge power loss.

Generally, the wires between the controller and the battery bank can be the same gauge as those leading off of the panels to the controller. 

However, in systems where the controller performs a huge voltage drop before going to the battery pack, you’ll need a thicker wire to handle the increased amperage. 

Here’s an abbreviated AWG table with maximum allowed amperages.

In all cases, you want to keep all the cabling in your entire solar system as short as possible for safety, power loss prevention, and cost considerations.

• For simple series wiring, the cabling will pass through a vehicle entry gland to your charge controller. 

• For parallel wiring of more than two panels or series-parallel configurations, you’ll need a combiner box (in place of branch connectors) located between the PV panels and the charge controller.

Fuses for solar panels in series or in parallel

It’s essential to place fuses in your solar system to prevent the wiring from overheating and causing a fire or damage to your PV modules, charge controller, or battery bank.

At the bare minimum, it’s a good idea to place at least two fuses in your off-grid solar system: 

1. Between the panels and the controller
2. Between the controller and the battery pack

Note: Some simple, low-power series arrays do not absolutely require a fuse per the electrical codes for PV systems, but it’s not wrong to include one. Just added safety. 😉

Fuses are typically placed on the positive leads in your solar system.

To figure out the fuse size for your solar array designed in series, in parallel, or in series-parallel, you need to determine the amperage of the entire system before it enters the controller.

As a general rule, increase that value by 25% to be sure your fuse can handle it.

For example, if your array amperage is 10A, your fuse size should be 10 x 1.25 = 12.5A.

To choose the correct fuse to place between the charge controller and battery bank, look at the controller’s rating and go with that.

In cases where you need a fuse that can handle a value that falls between two fuse sizes available on the market, purchase the one that is larger.

Solar charge controllers for series vs. parallel solar panel connections: PWM vs. MPPT

It is critical to place a solar charge controller between your PV modules and your battery bank in both series and parallel connections. The controller prevents the batteries from overcharging, which shortens their useful life. 

Furthermore, today’s batteries cannot accept the high voltages that solar panels can put out. Some controllers knock the voltage down to an acceptable level for your batteries.

Currently, there are two major types of charge controllers for solar arrays: PWM or MPPT.

MPPT controllers are considerably more efficient than PWM controllers and can perform advanced functions, so they cost significantly more.

Each type has its own amperage and voltage rating. 

To avoid damage to your controller, you must be sure that your solar array, no matter the wiring arrangement, never exceeds those ratings.

Be sure to use the open circuit voltage (V_oc) and not the nominal voltage when choosing the appropriate charge controller.

In fact, it’s a good idea to stay under the voltage and amp ratings of the controller by about 25%.

Specs on the charge controller will also list the wire thickness and battery types that are compatible with it. If you already have this equipment, it’s cost-effective to shop around for a controller that will integrate nicely with what you already have.

Similarly, if you already own a PWM controller and don’t wish to upgrade to a more expensive MPPT, a parallel connection for your panels may be worth considering as a way to meet your energy needs with the controller you have. The lower voltage more typical for parallel wiring is usually fine with a PWM controller. (But watch the amperage rating!)

Note, however, that PWM controllers often create a lot of noisy interference with radio and TV signals. That could ruin your vacation. Fortunately, it’s possible to get around that.

On the other hand, you’ll definitely need a MPPT controller for the higher voltages generated through a series connection. 

Although solar charge controllers are wonderful gizmos that do amazing things in terms of storing power for you, today’s devices cannot boost an incoming voltage to make it push through and charge a higher-voltage battery pack. 

Battery bank size considerations for series vs. parallel solar panel wiring

It’s common to have 12V, 24V, or 48V battery banks for small, off-grid solar projects whether they’re wired in series, in parallel, or in a series-parallel combination.

For all types of solar panel wiring, keep in mind that you will need higher voltages than the battery bank’s total voltage rating to charge them in a reasonable amount of time and keep them topped off.

Here’s a table with approximate charging voltages for various battery bank sizes. Note that values may vary slightly depending on the type of battery you have.

On top of this, the charge controller itself needs 5V to begin charging then another 1V to continue charging.

Tip: Make sure you account for the “extra” volts needed for your controller + battery pack to perform at peak levels when you plan out your series or parallel solar panel configuration. 

Is a series or parallel connection better for solar panels?

Generally speaking, a series connection is preferable for most smaller solar projects. Usually, this includes RV, boat, trailer, and camper van trips. 

There is a fourth reason why series wiring is preferable over parallel connection in solar panels. It may matter to people wanting to maximize power output for longer times. 

A series connection allows you to get higher voltages compared to parallel wiring. It’s possible to get a difference of at least 20V-30V all the time, (except at night, of course), between the voltage at your panels and the required voltage for battery charging. (See previous section for more on this.)

In other words, with a series connection, you’ll be able to do some early morning and evening charging of your battery bank when there’s some daylight available but no direct sun. This is less likely with a parallel configuration. 

Of course, a parallel setup may be better in your specific case. This is true especially if you can’t escape partial shading or are prone to storm damage. Both of these situations greatly impact power production in series connections but not in parallel arrangements.

RV, boat, trailer, or camper van travelers using solar have less of a problem with choosing a series connection than people living off-grid do. Moving out of the shade and staying away from stormy areas are possible solutions that aren’t available to solar cabin dwellers.

In all cases, doing some basic calculations to figure out which solar wiring scenario is better for you is essential.

Above all, you want to keep the voltage of your system high enough so that your charge controller will work properly and permit charging of your battery bank. 

At the same time, you need a design that will allow you to use thinner (cheaper) wire, meaning amperage cannot be very high.

Keeping everything close together to prevent voltage drops is also an important consideration. If you don’t factor in power loss due to the distance the current must travel from the panels to your batteries, charging will go slowly or not at all.

Key takeaways on series vs. parallel connections of solar panels

Solar array DIYers need to figure out the best way to wire their solar panels together to maximize their solar power output. The two major ways to accomplish this are series or parallel connections. 

For most small solar projects dealing with fairly minor energy needs of a few hundred watts per day, a series connection is better. Arrays with modules placed in series require fewer parts and use thinner (less expensive) wire than systems with panels arranged in parallel.

However, if you’re solar electrifying a large cabin off-grid, requiring thousands watts per day, or if you have an enormous battery bank that you want to keep topped off, a parallel connection could be the better way to go.      

Depending on your level of technical savvy, a series-parallel setup could be the best option of all. This is especially true if you’d like to maintain a higher voltage for continual battery bank charging during the day — even in the early morning and evening hours — but keep the current low enough to allow you to use cheaper wire.

As a general rule of thumb, you’d like to configure your solar array to be about 20V-30V higher most of the time during the day than the minimum voltage needed to charge up your battery bank. 

At the same time, you’d like to keep the current flowing through your system low enough so that you can use 10 awg (or thinner) wire if cost is a big factor for you.

With a little bit of planning ahead of time, your solar array placed in series, in parallel, or in series-parallel can provide you with the maximum power output allowable under realistic conditions.