Solar Panels & Charge Controllers

The Basics

The solar panel collects power from the sun and sends it out to the charge controller.
The charge controller confirms the power from the panel is the correct voltage for the battery, and if not, converts it, before sending it out to the battery.
The battery stores the collected power.

Solar Panels

First, let’s cover some basic information about most solar panels then we will talk about the different types.

General Panel Basics

Solar panels normally have cells that collect the power, a junction box where the MC4 connections (wires) come out of, and a frame or film. Solar panels are typically named for the watts and the voltage of the system they are intended for (also known as “nominal” voltage).

Solar panels normally have a sticker on the back which gives the panel’s specification information as they can vary from manufacturer to manufacturer. Because the stickers can vary on information provided, here are the basics to look for:

Solar Panel Sticker Breakdown

Nominal Voltage: The voltage of the system the panel is made for & typically named after.
Power (Pmax): The highest watts typically made by the panel & typically named after.
Optimum Operating Voltage (Vmp): The maximum volts output aka the typical voltage sent to the charge controller.
Open Circuit Voltage (Voc): Voltage with no load aka the voltage if you just tested the solar panel with no load.
Optimum Operating Current (Imp): Maximum amps output aka the amps it should output when connected to a charge controller.
Short Circuit Current (Isc): Most amps the panel can handle aka use this for rating wire and fuses.
Weight: What that panel weighs.
Dimensions: The measurements of the panel.

Typical Solar Panel Voltages

Nominal Voltage: Actual Voltage the Panel Makes
12 Volt Panels: Vmp 17v (Vmp) to 22v (Voc)
24 Volt Panels: Vmp 32v (Vmp) to 44v (Voc)
36 Volt Panels: Vmp 48v (Vmp) to 66v (Voc)

As you can see solar panels actually make more volts than they are named for. For example, a typical “100 watt 12 volt” solar panel will actually make between 17 & 22 volts when in the sun. That is because batteries aren’t just 12 volts right? The voltage varies on how full they are!

Another way to think about it is… the solar panels are making more voltage than the batteries can take allowing them to push the power collected down into the battery. Since electricity follows the path of least resistance, the 12v battery is lower voltage than the panel, so the power flows down into the battery.

Mono VS Poly VS Flexible

Solar Panels come in a few different types. The main 3 used in the RV world are:

Monocrystalline (mono)
Polycrystalline (poly)
Thin Film (flexible)

Monocrystalline (mono)

Renogy 12v 100w mono compact (42.2 x 19.5 x 1.38 in)

Description: Monocrystalline (mono) solar panels tend to have rounded edges on the individual cells and the older ones might have circular cells. They are normally a dark blue or black color, and they are a framed panel which are sometimes referred to as a “glass panel”.
Efficiency: Mono panels are the most efficient per surface area.
Cost: They are typically more than poly but less than flexible panels.

Polycrystalline (poly)

Renogy 12v 100w polycrystalline (39.7 X 26.7 X 1.4 In)

Description: Polycrystalline (poly) have square cells and are typically a slightly lighter color than mono, and if you look at them up close you can see what looks like flakes that sorta resemble particle board. Just like the mono panels, they have a metal frame and are referred to as a “glass panel”.
Efficiency: Poly panels are typically a little larger than mono for the same amount of power generated.
Cost: They tend to be the cheapest solar panel option.

Thin Film (flexible)

Renogy 12v 100 mono flexible (48.0 x 21.6 x 0.08 In)

Description: Thin film (flexible) solar panels don’t have a frame like the others do and are typically able to be flexed or bent slightly. They come in both Mono or Poly.
Efficiency: Depends on if the panel is mono or poly.
Cost: They are normally the most expensive solar panel option of the three.

Charge Controllers

Just like with panels, let’s start with general info and then talk about each one.

General Charge Controller Basics

Essentially the solar charge controllers job is to protect the batteries. It basically takes in the higher voltage from the panels and sends out the lower voltage to the battery bank. It also acts like an off switch for when the panels aren’t making any power (aka night time).

Charge controllers are typically rated by the amount of amps they can handle. For example, a 10 amp 12v/24v charge controller can handle 10 amps at either 12 or 24 volts. Just remember that the voltage typically shown in the name is the voltage of the system (aka the battery) it is intended for. Sorta like how a “100w 12v” solar panel is made for a 12 volt system.

Be sure to check the specifications of the charge controller you are thinking of purchasing to confirm it can handle the amount of amps, watts, and volts you plan to use it for.

Solar charge controllers are typically very simple and have a spot for the solar panels (PV) to be connected and a spot for the battery to be connected. Some have a load connection with a light bulb symbol to show that. Most load outputs are rated the same as the charge controller is rated for, and (if you connect something to that load) can be used to power anything when the battery voltage is high enough or to turn off that connection when it is low. Most RVers don’t attach anything to the load of a charge controller as it is essentially an option.

Remember to ALWAYS connect to your battery first. This allows the charge controller to see the voltage needs of the battery. Then you can connect the solar panels. Unless for some reason the directions tell you otherwise, of course.

MPPT vs PWM

Solar charge controllers come in two types:

Pulse Width Modulation (PWM)
Maximum Power Point Tracking (MPPT)

Pulse Width Modulation (PWM)

Renogy 30 amp 12 volt PWM

PWM is the oldest technology and reduces the voltage from the panel down to what the battery needs. They also act like an off switch when the panels aren’t making any power or when the battery is full.

PROS:

Much cheaper than MPPT
Great for small solar systems

CONS:

Solar panel voltage must be same as battery voltage (parallel only)
Possible wire price increase due to limited lower voltage
Less efficient than MPPT
60 amp is largest PWM available

Maximum Power Point Tracking (MPPT)

Victron 30 amp 12/24 volt MPPT with Bluetooth

MPPT is the newer technology. It takes the extra volts it receives from the panels, and converts those volts to extra amps for the batteries. They are typically more programmable, and sometimes offer a networking ability for remote monitoring.

PROS:

More efficient than PWM
Panel voltage does not have to match battery voltage which means you can use higher voltage panels and or wire solar panels in series and or parallel
Possible cheaper wiring cost due to ability to use higher voltage solar array (either higher voltage panels and or panels wired in series).

CONS:

Cost more for the charge controller

What Size Charge Controller?

Remember this is a general & easy way to figure it out. Some controllers are able to handle more watts than others even though they may be rated at the same amps. So be sure to check the specifications of the charge controller you plan to use.

Figure up how many watts of panels you have. Example: 2 panels X 100 watts = 200w
Divide total watts by battery bank voltage. Example: 200w / 12v = 16.6 amps
Take the amps and add 20% (safety margin). Example: 16.6 + 20% = 19.92
Round up. Example: 19.92 round up to 20 amps

Recommendations & Tips

Be sure to do an energy audit first to know how many panels you need for the battery bank you plan to recharge. However, before you buy anything make sure to measure your roof so you have a good idea of what panels you can fit and where they can fit. I say that because you may do an audit and find that 400 watts is enough, but you have roof space for 600 watts. In that case, be sure to keep in mind that you may expand later as you might use more than you think. If you decide you want to run the fridge on the inverter like I did, it will be easier & cheaper in the long run to go ahead and buy the next size up charge controller, then add panels later if you need to expand. If you buy a charge controller too small in the beginning, you may find yourself with more expenses down the road because you’ll have to not only buy a new charge controller, but also run new wires. I know this from personal experience!

Which Charge Controller Should You Go With? MPPT or PWM?

To keep it easy, if you have a small system and you’re on a budget then a modern PWM is a great option.

If you plan to go with a system larger than 400 watts then you will probably want to go with a MPPT. That will give you way more options for panels (and wiring) up on the roof.

PWM Recommendation (400w & Under)

PWM charge controllers have come a long way and are a great budget option, especially for smaller systems. Just keep in mind that your panels will all have to be 12 volts and connected in parallel. That means you will need more and bigger wire than if you went with an MPPT which gives the options of higher voltage panels and wiring in series. Again, they are a great charge controller. If you are limited to 400 watts on your roof anyway and you want a good charge controller at a great cost, then PWM is perfect.

Renogy Wanderer Li 30A 12V PWM

General Information:

Nominal Voltage: 12 volt systems only
Max PV Input Power (watts of panels): 400 Watts
Max PV Input Voltage (panel voltage): 25 volts dc
Rated Charge Current: 30 amps dc
Max Gauge Wire: AWG 8 (8 mm2)

The Renogy Wanderer PWM is a great charge controller at an excellent price point. It’s perfect for any battery type (check your specific battery specifications and compare to Renogy specifications to confirm). You can use it in the most basic form or customize it a little. It gives you the option to add a temperature sensor and/or a Bluetooth dongle to be used with an app for additional customization. Now it isn’t as great as an MPPT, but for a small system it is a great choice. If you plan to have a 12 volt battery bank with a 400 watt or less solar panel array then do yourself the favor and get the 30 amp option. I say that because the 10 amp option and the other “cheaper” options out there will limit you. You are going to spend $15-$20 on a cheap one, so you might as well spend the extra few bucks and get the good 30 amp one listed above. Furthermore, the 10 amp PWM is only really good for one panel. So why not have the extra amps available for a few extra dollars. That way if you decide to add an additional panel or 3 later you’ll have the room. After designing and installing a few systems, I know that having options on the back end is always nice. I have also learned that if you buy the cheapest available, it sometimes works out.. but oftentimes it doesn’t. Which could cost you more in money and time if it doesn’t work out.

MPPT Recommendations

I prefer the MPPT mainly due to the fact that you will have a larger variety of solar panel and wiring options. For example, the Victron 100/30 MPPT means it can handle 30 amps and the panels’ voltage can be all the way up to 100 volts! So you can buy just about any panel out there and not have to worry about voltage. Plus you can wire your solar panels in series, parallel, or series/parallel, so long as they don’t exceed the units specifications. Plus they tend to work with multiple nominal (battery bank) voltages. Let’s say you start with a 12 volt system and 400 watt solar array, but decide to step up to a 24 volt battery bank later. You will still have room for 400+ more watts with the same MPPT you already have.

I personally recommend the Victron MPPT charge controllers due to the quality build, compact design, networking capability, AND their user friendly interface via the free “Victron Connect” app. If you are going with a Victron battery monitor, then a Victron charge controller is a no brainer. The two will be able to talk to each other via the built in network, and you won’t have to buy or run a data cable. Nor will you have to use some funky computer software, or buy an additional item just to customize them. It can all be done via Bluetooth on your phone or tablet. Plus you don’t have to go to the screen to see it. You can check your system from a decent distance (about 30-50ft or so) even outside your RV while you’re relaxing. I love this feature because I can stop the RV, pull up the app on my phone, and see how many watts the panels are bringing in. That way if there is a shady area in the camping spot, I can position our motorhome with some shade on the RV but not on the panels before we even set up. All this can be done while sitting in the drivers seat!

Victron 30 amp 12/24 volt MPPT with Bluetooth

General Information:

Nominal Voltage: 12 or 24 volt systems
Max PV Input Power (watts of panels): 440 Watts (12v) / 880 watts (24v)
Max PV Input Voltage (panel voltage): 100 volts dc
Rated Charge Current: 30 amps dc
Max Gauge Wire: AWG 6 (13 mm2)

12 Volt Battery Bank: If you are doing a 12 volt battery bank and your solar array will be 440 watts or less, and you want the best MPPT, then this is definitely the best option. Yes there are cheaper options out there. With a 400 watt or smaller system if you want to go cheaper, just go with a PWM above and save the money. For the few of you that want the best even with a small system, then look no further. If you plan to go with 400 watts but have more room on the roof then you may want to step it up to the 50 amp version so you can add more panels later if you want.

24 Volt Battery Bank: If you have a 24 volt battery bank and your solar array will be 880 watts or less then get the 30 amp option. If you will be going with 800 watts and have room for more on your roof then you may want to consider going up to the 50 amp model so that you will have more room for expansion if you end up wanting to add more panels later.

Victron 50 amp 12/24 volt MPPT with Bluetooth

General Information:

Nominal Voltage: 12 or 24 volt systems
Max PV Input Power(watts of panels): 700 Watts (12v) / 1400 watts (24v)
Max PV Input Voltage(panel voltage): 100 volts dc
Rated Charge Current: 50 amps dc
Max Gauge Wire: AWG 6 (13 mm2)

12 Volt Battery Bank: If you plan to have up to 700 watts on your roof then the above charge controller is the one to go with. The next sizes up get pretty pricey, but remember you can always add a second charge controller. I say that because this 50 amp version is about 1/3 the cost of the 100 amp version. So buying two of them will give you 100 amps at a lower cost. If you are going with a very large system then you may want to consider going up to a 24 volt battery bank then adding a step down converter for your RV’s 12v dc fuse box.

24 Volt Battery Bank: If you have a 24 volt battery bank then this charge controller can handle up to 1400 watts! That is a ton of power for sure! If you need more than that, adding a second 50 amp is cheaper than buying the 100 amp version.

What Solar Panel Should You Buy? Mon or Poly?

In my personal opinion it depends on your roof space. If you are limited on space then mono panels are the way to go since they are more efficient per surface area. For example, a Renogy 300 watt mono panel is the same size as a Renogy 270 watt poly panel.

If budget is an issue, but roof space isn’t, then grab some poly panels. After all, if you have plenty of roof space and can only afford 300 watts of mono but you can afford 400 watts of poly panels.. well 400 is better than 300 so you’ll get more power for the money you have.

You can typically find “residential” solar panels locally at a great deal. Just be sure to know which charge controller you plan to go with, PWM or MPPT, as residential panels come in many voltages and might not work if you have a PWM charge controller.

Monocrystaline Panel Recommendations

Hightec Solar Inc 200 Watt 12 Volt Panel (Rectangle)

If I would have known about the Hightec Solar Inc options before I built our system then I would have saved myself some money and purchased them instead. Continuous Resources is a locally owned place that offers great customer service and products. The Hightec Solar Inc panels come with a 25 year warranty AND the dimensions make them great for most RV roofs. Plus buying a 200 watt panel instead of two 100 watt panels means less wires to deal with and less mounting brackets or holes to deal with.

Click To View/Purchase: www.continousresources.com/200wHighTecREC

Nominal Voltage: 12 volts
Power (Pmax): 200 watts
Optimum Operating Voltage (Vmp): 21.052 volts
Open Circuit Voltage (Voc): 24.335 volts
Optimum Operating Current (Imp): 9.5 amps
Short Circuit Current (Isc): 9.87 amps
Weight: 32 lbs
Dimensions: H 59 x W 26.6 inches

Hightec Solar Inc 200w 12 Volt Panel (Square)

Click To View/Purchase: www.continuousresources.com/200wHighTecSQ

Nominal Voltage: 12 volts
Power (Pmax): 200 watts
Optimum Operating Voltage (Vmp): 21.052 volts
Open Circuit Voltage (Voc): 24.335 volts
Optimum Operating Current (Imp): 9.5 amps
Short Circuit Current (Isc): 9.87 amps
Weight: 32 lbs
Dimensions: H 39.37 x W 39.37 inches

HQST 100 Watt 12 Volt Compact Mono Panel

If you only have room for 100w panels then the HQST are identical to the Renogy panels and have an awesome warranty at a lower cost. If you just want to go with Renogy then look above as they are listed up by the mono section.

Nominal Voltage: 12 volts
Power (Pmax): 100 watts
Optimum Operating Voltage (Vmp): 17.9 volts
Open Circuit Voltage (Voc): 21.6 volts
Optimum Operating Current (Imp): 6.72 amps
Short Circuit Current (Isc): 6.24 amps
Weight: 16.5 lbs
Dimensions: H 42.2 X 19.6 inches

Polycrystalline Panel Recommendations

As I stated earlier, if you want to save money and plan to go with poly panels then take a look at your local markets as you may find some for cheaper. If not, here are a few options below. Just keep an eye out on what voltage the panels are.

Renogy 270 Watt 24 Volt Poly Panel

Nominal Voltage: 24 volts
Power (Pmax): 270 watts
Optimum Operating Voltage (Vmp): 31.40 volts
Open Circuit Voltage (Voc): 38.60 volts
Optimum Operating Current (Imp): 8.60 amps
Short Circuit Current (Isc): 9.03 amps
Weight: 42.8 lbs
Dimensions: H 64.96 X 39.25 inches

HQST 150 Watt 12 Volt Poly Panel

Nominal Voltage: 12 volts
Power (Pmax): 150 watts
Optimum Operating Voltage (Vmp): 19.1 volts
Open Circuit Voltage (Voc): 22.7 volts
Optimum Operating Current (Imp): 7.89 amps
Short Circuit Current (Isc): 8.09 amps
Weight: 22.1 lbs
Dimensions: H 51.3 X 25.9 inches

HQST 100 Watt 12 Volt Poly Panel

Nominal Voltage: 12 volts
Power (Pmax): 100 watts
Optimum Operating Voltage (Vmp): 17.8 volts
Open Circuit Voltage (Voc): 22.4 volts
Optimum Operating Current (Imp): 5.62 amps
Short Circuit Current (Isc): 5.92 amps
Weight: 16.5 lbs
Dimensions: H 39.7 X 26.7 inches

Disclaimers

Although I do try and keep up with the current options, and I will keep updating this page over time, you are responsible for verifying any information as it may change & might not be up to date.
The links to products are affiliate links, therefore we will receive money for directing you to the website for purchase.