300 Watt Solar Panel Wiring Diagram & Kit List

Graham Bogie

This is a 300 Watt Solar Panel Wiring Diagram with a complete list of DIY parts needed and kits available.

Use this as a guide to fitting your RV, campervan, motorhome or caravan solar set up.

We’ve designed the diagram so it’s simple to understand for newbies to campervan electrics.

Campervan electrical systems can be dangerous so if you’re not sure what you’re doing or don’t feel confident to complete a solar set up yourself, seek the help of an electrician.

This post is one part in our DIY campervan solar system series which includes 12v solar panel wiring diagrams for other sizes too.

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What Can A 300 Watt Solar Panel Power?

How much a 300 watt solar panel can power depends on a number of variables including the time of year, the weather, your location and the type of charge controller you install.

But here’s an idea of what you might expect to get on an average day of 4 peak hours of sun with an MPPT controller.

A 300 watt panel will provide about 90-96 amp hours per day. 

That’s about enough to run:

  • an inverter for occasional use,
  • a medium, energy efficient fridge, 
  • recharge a few small devices on a USB like phones, tablets and kindles,
  • run your roof vent during day and night to stop condensation, 
  • keep a composting toilet’s fan running 24/7,
  • run a diesel heater for a few hours and
  • switch on a few LED lights inside the campervan.

Bear in mind this is based on an average sun light day. 

You may not be able to replenish the batteries everyday so consider keeping something back as contingency to avoid cycling the batteries more than necessary.

A 300w systems gives you a day or 2 in reserve, depending on your usage and without other sources of recharging.

Who Will Suit A 300 Watt Solar Panel System?

A 300 watt solar panel set up is a good size for a couple in a medium sized RV.

It can support wild camping or boondocking in your RV for relatively long periods from spring through to autumn with careful use and monitoring of the battery levels.

If your needs are critical, definitely consider the ability to charge your batteries from shore power and the alternator too.

We have 320w on our LWB Sprinter camper and find it perfect from Spring to Autumn. 

We’re confident staying off grid with no shore power or driving for about 5 days. 

We need to be much more careful with our use and monitoring during cold, winter months though. 

If your ideal solar system is larger than 300w, build the system with scaleability in mind.

You can add panels and batteries in the future as your needs grow and budget allows.

You could consider extra juice from a portable solar panel system but that’s for another post.

300 Watt Solar Panel Wiring Diagram

There’s a few points worth clarifying about this wiring diagram before you get into the detail:

  • The wiring diagram shows only the supply side installation of a 300 watt solar panel system. It goes as far as charging the battery. For the load side detail, check out our post on campervan wiring
  • It excludes charging the battery from shore power or the alternator. 
  • For information on how to fit the battery, see our post on campervan batteries.
  • The diagram also excludes wiring an inverter – it sits on the load side of the battery.
  • The 300 watt solar panel wiring diagram assumes 3 x 100w panels are being fitted. If you happen to be fitting 2 x 150w panels instead, see our 200 watt solar panel wiring diagram.
  • We’ve included 2 diagrams below. The first shows a 300w solar panel set up wired in series. In the 2nd diagram, they’re wired in parallel.
  • Read our complete guide to wiring your solar panels in series or parallel if you’re not sure how best to wire your array. It covers mixed panels too and includes an interactive calculator to find the most efficient setup for your panels.

300 Watt Solar Panel Wiring Diagram in Series

300 watt solar panel wiring diagram series

300 Watt Solar Panel Wiring Diagram in Parallel

300 watt solar panel wiring diagram parallel

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DIY 300 Watt Solar Panel Parts List

This is a list of every component you need for a 300 watt solar panel set up on your RV or campervan.

Solar Panels

Solar panels harvest sunlight, converting it to electricity.

There are different types of solar panels but we recommend using monocrystalline as they’re the most efficient. 

3 x 100w Rigid Monocrystalline

100w Rigid Monocrystalline

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Solar Charge Controller

Solar charge controllers regulate the current from the panels to a safe level so it can charge the batteries.

A 30A controller is adequate for a 300 watt solar panel set up.

30a PWM Controller (budget option)

30A PWM Solar Charge Controller

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30a MPPT Solar Charge Controller

30A MPPT Solar Charge Controller

Enter coupon code MowgliAdventures for 10% off at Renogy.com

If you think you may want to scale up your solar capacity in the future, consider buying a higher rated MPPT solar charge controller because it’ll be more cost effective in the long term. 

  • 400 watt solar system | 40a
  • 600 watt solar system | 60a (50a will be fine if you can find one)
  • 800 watt solar system | 100a (70a will be fine if you can find one)

Check out our complete guide on how to select the right solar charge controller for your setup.

And if you need to know what size to get, use our interactive solar charge controller calculator.

How Many Batteries Do I Need For A 300w Solar Panel?

How large a battery bank you need depends largely on how much power you use and even when you use it. Use our solar battery bank calculator to work out how much energy you use each day, and what size battery bank you need to meet that level.

That said, a battery bank stores the power generated from your solar panels. If you don’t have a large enough battery bank, you could potentially waste the energy you’ve worked so hard to harvest.

A 300W solar panel setup could generate at most, around 25ah of power per peak hour. However, how many peak hours of sun you can reasonably expect to receive differs by location, time of year and the weather.

In the US, you’re unlikely to get much more than 5 or 6 hours of peak sun conditions in mid summer. Therefore, a 300w solar panel system will generate a maximum of 125-150ah per day.

Although you may use some of that energy as you generate it, it’s more simple and prudent to err on the side of caution. So the minimum size battery bank you need for a 300w set up is 150ah – more if you don’t install Lithiums.

You may have more than one charging source or want some contingency so you can of course increase that size as much as you like.

There are 3 types of campervan batteries. We recommend choosing Gel or Lithium-ion, though AGM batteries are also popular.

Lithium Ion Battery

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If space isn’t a major issue, you’ll never be unhappy of a bigger battery so always go up a size rather than down.


Solar Panel Mounting Brackets

Solar Panel Mounting Brackets

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These allow you to mount the solar panel to your van without needing to drill holes in the roof. You need one pack for every panel.

Always resist any urge to breach your roof.

MC4 Connectors

MC4 Connectors

Solar panels come with about a metre of cable so not normally long enough to reach your battery bank. 

MC4 connectors are rated to cope with the current and weather proof so you can extend the cable.

For wiring 3 panels in series, you need 1 single pair of MC4 cable connectors.

For wiring 3 panels in parallel, you need 1 pair of 3 pronged MC4 T-branch connectors and 1 single pair of MC4 cable connectors.

Solar Panel Cable

Solar Panel Cable

The point of using different colours is to easily identify the negative and positive wires. 

If you can’t get red, use the black cable and mark it with red electrical tape or heat shrink.

Measure how much extension cable you need before you buy. 

For both the positive and negative sides, you need enough to run from the end of the existing solar panel cables to the battery, via the solar charge controller and kill switch.

Solar Panel Gland Seal

Solar Panel Gland Seal

Sadly, the only way to run the solar cable into the van is by drilling a hole. Plug it up and make it water tight with a gland seal.

Fuse Holder

Fuse Holder

This sits between the solar charge controller and the battery cut off switch.

ANL fuse holders are easier to mount than in-line holders. However, the fuses needed for these are more difficult to find than the standard blade fuses

Bear this in mind if you’re travelling for long periods to areas where online shopping may not be an option.

The fuse between the solar panel and the solar charge controller should be 1.3 times the size of the Optimum Operating Current of the panel (see the back of the panel for its specification).

For a single 100w panel a 10A fuse is normally adequate.

If installing more than one panel, check our wiring diagram for 200w to see how this differs for wiring in series vs parallel.

The fuse between the solar charge controller and the battery, should be the same rating as the solar charge controller.

2 x 1250 Amp Battery Cut Off Switches

1250 Amp Battery Cut Off Switches

Cut off switches on the battery supply line and the circuit supply lines allow the battery to be isolated.

The battery cut off switch must be greater than the total capacity of the battery bank.

If you want to upscale in the future, it’s worth fitting a larger one now to save changing it later.

Battery Terminal Eyes / Wire Lugs

Battery Terminal Eyes / Wire Lugs

These allow you to connect the cable to the battery. 

Choose a lug size rated greater than your wire gauge and with an eye large enough to fit on the battery terminal.

1/4” Red & 1/4” Black Heat Shrink

1/4” Red & 1/4” Black Heat Shrink

Heat shrink seal electrical joints to cover the bare wires.

On this solar install, you’ll need it to connect the inline or ANL fuse and the battery terminal connectors.

Sikaflex 221 (Panel Adhesive)

Sikaflex 221 (Panel Adhesive)

Use this adhesive to secure the solar panel mounts to the van roof.

It’s a bit messy, so wear latex gloves when applying it.

You need about 4 or 5mm of sealant between the bracket and the panel for a robust bond. It takes about 24 hours on a dry day to cure.

Fuses and wires must be rated for the system’s load and cable run length. We’ve covered everything you need to know in our campervan wiring post.

300 Watt Solar Panel Kit

If you’d prefer to avoid shopping around, you can buy pre-configured solar panel kits

They usually include the solar panel, solar charge controller and mounting brackets. Some include extension wires.

If you opt to buy a solar kit, check what’s included and what additional things you’ll need to buy. 

And make sure it makes economical sense.

This is a 300 Watt Solar Panel Kit complete with:

  • 3 x 100W 12V Monocrystalline Solar Panel
  • 40A MPPT Charge Controller (so space to scale up)
  • all cables and connectors
  • mounting brackets
  • fuse holders & fuses.

You need to buy the cable entry housing and battery separately.

How To Install 300 Watt Solar Panel System

Our guide details everything you need to install a campervan solar panel system.

It’s written to suit any size of system so it makes reference to how to install multiple panels.

It may come in useful when considering upscaling your set up though.

There’s also a section about the tools you need to install your campervan solar panels. 

Some of these tools can be quite expensive so unless you expect to get a lot of use out of them in the future, try to borrow them where possible.

We do encourage you to invest in a multimeter though and keep it in your RV toolkit while travelling and living in your van.

Check out our complete guide for how to use a digital multimeter in your camper.

It’s on our van life essentials list for good reason!

Graham Bogie

Graham Bogie - Expert in RV & Campervan Electrics - Mowgli Adventures

Graham is a seasoned marine electrical engineer with two decades of experience designing customized electrical systems for plant machinery and converting campers and overland vehicles. His expertise has led him to author the reputable Campervan Electrics Handbook and become the chief designer of our Campervan Electrical Design Service. As a knowledgeable figure in the field, his YouTube channel, blog, Facebook group, and newsletter, offering electrical advice and product reviews, reach more than a million users each year.

4 thoughts on “300 Watt Solar Panel Wiring Diagram & Kit List”

    • Hi David, The inverter size is determined by how much AC load you need to power from the battery bank, rather than how much solar you have. There’s loads of background info in our post about inverters for more detail or go straight to the inverter calculator. Either way, you need to assess what AC load you want to run at any one time, then size the inverter to delivery that. Then make sure you have enough battery and you have the capability to recharge the batteries. I hope this helps. Graham


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