How To's and Why Not's, Rv Solar, RV Upgrades & Mods

Sizing a Solar Panel Array to Fit Your Needs

When designing an off-grid energy system that includes solar, properly sizing the solar panel array is vital to overall system performance and satisfaction. Ensuring the proper design, layout, and size will allow for the best operation of the off-grid system. Understanding the components and wiring options is fundamental to being able to be successful. No two systems are necessarily the same, with a lot of differences coming from system utilization and expected goals.

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Reading Solar Panel Specifications

Understanding the data on this label will help you size your solar charge controller.

When reading a panel label there are some key specifications that, when calculated based on how the solar panels are configured, will help steer the solar charge controller decision. I will go down the list and explain each data point and why it is important.

Maximum Power(Pmax)

This is the panel’s maximum wattage output. The maximum wattage is found on a curve between the open-circuit voltage and the short circuit current. It is the optimum power output of the panel. When sizing a solar charge controller, this number is always additive when calculating maximum wattage, regardless of the wiring layout. In some situations, it could be beneficial to exceed the solar charge controller’s maximum wattage, ensuring adequate charger output in low light conditions.

Open Circuit Voltage(Voc)

This voltage is the maximum no-load voltage output of the panel. This number will fluctuate with temperature. When considering the size of a solar charge controller, this voltage needs to be calculated based on the wiring plan. MPPT solar charge controllers will have a maximum voltage that the solar array should not exceed. This is found by multiplying the open-circuit voltage by the number of panels in series.

Voltage at Pmax(Vmpp)

The max voltage is the voltage that the panel will produce its maximum wattage at. When evaluating your panel performance, you will take this number and multiply it by the number of panels in series. This will give you your working voltage going into your solar charge controller.

Short Circuit Current(Isc)

The short circuit current represents the maximum amperage that the panel will produce. However, it is the maximum amperage at zero volts. This current comes into play when wiring panels in parallel. Solar charge controllers will list a max current that the charger can handle from the panel array. Therefore, to find this number, multiply the short circuit current by the number of panels in parallel.

Current at Pmax(Impp)

Current at Pmax is the working current of the panel. This is the current that one would expect to see when the panel is putting out its max power. When evaluating your panel performance, you will take this number and multiply it by the number of panels in parallel. This will give you your working current being going into your solar charge controller.

Solar Panel Array Wiring

Before deciding what size solar charge controller you need, there needs to be some thought dealing with solar panel layout and wiring. How the panels are mounted to the roof, solar panel layout, and panel wiring can influence the number of solar charge controllers and wiring configuration. Therefore, let’s take a look at the various wiring methods.

Parallel Wiring Solar Panels

Example of three 36 cell solar panels wired in parallel configuration for the solar panel array
3 panels wired in parallel

Wiring panels in parallel at the beginning was the only option. Panels were built to produce voltages around 18 volts. When using a PWM solar charge controller, they match the panel voltage to the battery voltage. As the battery state of charge rises, the voltage will increase. If one panel is in shade, the others in parallel will continue to provide output. This configuration will also result in the wires to the solar charge controller carrying high current levels.

The use of an MPPT charge controller allows for the more efficient harvest of solar power when panels are in parallel. They will allow the panel to produce energy at its maximum Power Point. This also allows the use of higher voltage panels such as 60 and 72 cell configurations.

When wiring panels in parallel, panels should be the same voltage. The solar charge controller is going to regulate them all to a common voltage. When calculating potential output during the design phase, remember that voltage will stay constant but amperage will be additive. For example, the panels above have a Pmax of 200 watts, VOC of 22 volts, an ISC of 9.7 amps, and Vmpp of 18 volts. Three panels in parallel will produce 600 watts total. The VOC for three panels will be 22 volts. These three panels will have an ISC of 29.1 amps. The working voltage of this array will be close to 18 volts Vmpp.

Series Wiring Solar Panels

Image show typical wire layout for three panels wired in series for the solar panel array.
3 panels wired in series

Wiring solar panels in series should only be considered with MPPT solar charge controllers or PWM controllers for 24 volt or 48-volt battery banks. When the panels are wired in series, the voltage becomes additive and the amperage stays the same. This can reduce the size requirements and ultimately the cost of the required conductors going to the controller.

Using the same example as above, the panels have a Pmax of 200 watts, Voc of 22 volts, an ISC of 9.7 amps, and Vmpp of 18 volts. Three panels in parallel will produce 600 watts total. The VOC for three panels will be 66 volts. These three panels will have an ISC of 9.7 amps. The working voltage of this array will be close to 54 volts Vmpp.

Series/Parallel Wiring Solar Panels

Common 3S2P solar panel wiring configuration for the solar panel array.
6 panels in a 3S2P wiring configuration

The last method of wiring solar panels is a combination of the first two. When done correctly you can get the benefits of both methods. This will allow you to work with a layout that will fit your needs. There are a few practices that need to be kept in mind when creating your solar array design. When assembling strings of panels, each string must be the same voltage. As you are planning the layout of the roof, try to keep panels in strings aligned in the same direction. Do not think that you have to get all of the panels onto a single controller.

Determining the Sizing of the Solar Panel Array

There are a few different ways to approach sizing a solar panel array. System expectations, space availability and mounting options, and budget are key points to evaluate when sizing the solar panel array. But in the end, it will be a personal decision on what will take priority.

Expectations When Sizing a Solar Panel Array

At some point in the designing process, a decision on the overall role of solar panel array is going to play into the design process of the energy system. Is it going to be a primary means of providing energy or just a method to slow the draw on the battery bank? A general goal is to have approximately 200 watts of solar panels for every 100 usable amp-hours of battery. As the cost of solar panels has dropped considerably in the last few years, people are starting to exceed that estimate or to fill the roof to capacity.

Space Availability and Mounting Options for a Solar Panel Array

When evaluating a solar panel array, panel layout and mounting techniques can be a key factor in overall performance. The available roof space of an RV can dictate a solar panel array size and limitations. Flat mounting solar panels on the roof will result in performance loss because of the low angle of sunlight contacting the solar panel face. Tilting brackets allow for the proper angling of panels, however, most will require manually tilting which will involve time on the roof.

There has been some pretty creative racking systems custom designed and built by owners to help raise the panels in order to avoid roof obstacles. Attention to the mounting hardware, panel orientation, and the height and weight of the mounts and panels is just as important. A standard 60 cell panel can weigh between 40 to 50lbs each. They all will add to the total weight of the RV.

In order to make up for deficiencies in the mounted solar panel array, the use of a ground deploy solar panel array can be a viable option. These could consist of either an assemble suitcase set of solar panels or a home built, combination of panels, and a quality solar charge controller. Planning the system to be adequate in less ideal conditions will generally result in better performance overall.

Budgeting for a Solar Panel Array

The cost of solar panels has been becoming cheaper over the last few years. As technology has been improving, higher wattage panels are becoming cheaper to manufacture. Along with that, the availability of used or excess panels leftover from installations are also options to lower the costs. Bundled kits from some of the big-name manufactures tend to be some of the more expensive routes to take.

On the other hand, cheap equipment and panels should also not be the sole factor in gear selection. Sometimes you do get what you pay for. There are deals that are too good to be true. Knowing how to tell the difference will help save a lot of extra work down the road. Finding equipment with a good reputation and a long warranty period is a good indicator of quality equipment.

The cost of solar panels for the array can seem to be all over the spectrum. For a new standard size aluminum frame 36 cell solar panel, expect prices around $1 per watt. For quality flexible panels of this size, prices start at $1.50 to $2 per watt. Finding quality used or leftover solar panels from local solar installers can produce prices of $.30 to $.50 range for 60 and 72 cell residential panels. However, with the larger panels, layout or regular handling becomes more difficult.

Still looking for some assistance?

If you would like a consultation to discuss your specific needs, CLICK HERE to get signed up for Justin’s solar consultation. We can get a meeting scheduled. I do have a list of recommended vendors that do pay an affiliate fee. I could get credit based on people following links in my blogs or working with those vendors to quote packages for consultation projects. If you would like to source your own parts and equipment, consulting fees may apply depending on complexity and involvement. 

A complete list of parts and equipment that we utilize in our system can be seen at Kit.co/optingoutofnormal

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Check out our blog “How to size a battery bank to fit your needs“. We show how we get the most benefit out of our Battle Born battery setup. If you are boondocking in the National Forest or plugged into an outlet, you can harness the great features of the batteries. This allows you to maintain your energy loads while still living normally in an RV.

Check out some of our other blogs to see how we use our system. 

How to size an inverter to fit your needs
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Are you in search of a like-minded group of solar enthusiasts? Maybe you are looking to find the latest tips and tricks for your install? Come check out my group on Facebook. 

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Our current off-grid capable power system.
Boondocking / Dry Camping, How To's and Why Not's, Rv Solar, RV Tips, Tricks & Lessons, RV Upgrades & Mods

How to size an inverter to fit your needs

Are you trying to calculate the size of an inverter that will fit your needs? Maybe you are only looking to run a TV or instant pot. Perhaps you are wanting to power the air conditioners full time with the power you collect from the sun. In the following post, I will show you how we have come up with our inverter sizing to fit our needs. I will also show our system wiring.

Identifying an inverter based on your expected loads

There are several key factors that one needs to address when trying to properly size an inverter. What load you want the inverter to cover, are there other available sources of power available, and how long you want the inverter to power the load.

Knowing what your loads are and how they receive power is key to sizing your inverter to fit your needs.

The first key factor that you need to consider is the size of your expected load. To help calculate your needs check out our blog RV Appliance Power Consumption. It also helps to know what leg the power is fed through on a 50 amp power system. In some situations, a power outlet on a kitchen island may be different than an outlet on an external wall. This could be important if you are deciding which electrical circuits you would like to power.

We knew that we wanted to have the 5th wheel powered completely so that we would not need to change our habits. We wanted to function the same if we were off-grid or plugged in somewhere. Our desires are not to run a lot of high loads at once. We just did not want to compensate for our incoming power. Most of our loads, we found, are between 1100-1500 watts each. Calculating the max combined load was as easy as counting how many loads we wanted to run at the same time.

Are there going to be other power sources available that could change the size of your inverter to fit your needs?

Check out my post from last year dealing with inverters and some of their options. In that post, I write about the technology that most higher-end inverter/charges are coming with. It allows them to supplement other sources of AC power. It can be referred to as a hybrid boost or power assist by some manufactures. This feature may allow you to reduce the size of your inverter needs when running higher loads. Some of the other benefits of some inverters include the ability to pass through 50 amps of power on two separate legs.

In our first setup, we chose to go with a GoPower 2000 watt hybrid inverter. The benefit of that inverter was that it allowed easy wiring to our existing AC system. It would automatically power both legs of our 50 amp service when inverting. But it did not come without its flaws. You can see them in our YouTube Video here. Overall the GoPower worked well for us. However, in the end, it was not quite what we needed to fit our needs.

In our current setup, we wanted to be able to power both legs of service. We wanted to do it with the same power that we would see from an RV pedestal. Our loads are not consistently large for long durations. At the time of installation, that meant going with the only manufacture that could accomplish that. We chose to go with products from Victron Energy.

The third consideration for trying to size an inverter to fit your needs is duration of loads.

With knowing the size of your load, and any assistance that may be available to supplement, the final piece is how long is the load going to be carried. When sizing an inverter to fit your needs you do not want to expect your inverter to continuously carry its maximum load. Inverting 12v DC energy to 120v AC energy will create heat. There will also be some inefficiencies in the inverting process. Some ways to help counter these limitations are to use a larger inverter or change the battery voltage of the bank.

Victron Energy makes several choices of inverters. They rate their inverters by the measurement of Volt-Amps. The actual continuous wattage output is a little lower. A 3000VA unit rating on a Victron Energy inverter will continuously supply 2400 watts of power for 30 minutes. Each product specification sheet will define inverter capabilities. Victron Energy also makes inverters in 5000VA ratings if your intentions were to carry larger loads for longer durations.

The second option of changing the battery bank option will also help reduce heat. The increase of battery bank voltages also allows for the use of smaller sized wire. However, consideration needs to be given to the voltage requirement of the system already in the RV. For our loads, there was not enough benefit of choosing to go with a system other than 12 volts.

How did we wire our inverters to fit our needs?

Our dual Victron Inverter wiring to fit our needs.

Our current system consists of two Victron Energy 12/3000/120 50 inverter chargers that are the perfect size to fit our needs. Each inverter is capable of charging the battery bank at 120 amps per hour. They will provide 2400 watts of continuous power for 30 minutes. They are capable of surging to 6000 watts. The inverters are wired in split-phase to allow them to safely provide service to each leg of power going to my main circuit breakers.

Helping to feed split phase power to the inverters is a 100 amp autotransformer. Its purpose is to take a single phase of power and provide the second 180-degree leg of split-phase output. Single-phase could be from a generator of 30 amp or less shore power connection. The available current is split between the two inverters. The benefit is that each inverter will also pass-through power and charge the battery bank.

A 50 amp transfer switch is in use to switch between the standard 50 amp shore power connection and autotransformer outputs. This eliminates any kind of manual switching that will be a requirement to safely make the change. It will also ensure that no damage occurs if the wrong connection is made.

Still looking for some assistance?

If you would like a consultation to discuss your specific needs, send me a message at Justin@OptingOutofNormal.com. We can get a meeting scheduled. I do have a list of recommended vendors that do pay an affiliate fee. I could get credit based on people following links in my blogs or working with those vendors to quote packages for consultation projects. If you would like to source your own parts and equipment, consulting fees may apply depending on complexity and involvement. 

A complete list of parts and equipment that we utilize in our system can be seen at Kit.co/optingoutofnormal.

Check out our blog “How to size a battery bank to fit your needs“. We show how we get the most benefit out of our Battle Born battery setup. If you are boondocking in the National Forest or plugged into an outlet, you can harness the great features of the batteries. This allows you to maintain your energy loads while still living normally in an RV.

Check out some of our other blogs to see how we use our system.

Our Top 7 Free Camping Spots and Boondocking Tips
Bisti Badlands ~ New Mexico’s Most Hidden Gem
Living Full-Time in an RV for 3 years. Our 15 Tips

Are you in search of a like-minded group of solar enthusiasts? Maybe you are looking to find the latest tips and tricks for your install? Come check out my group on Facebook.

Our current off-grid capable power system.
Boondocking / Dry Camping, How To's and Why Not's, Rv Solar, RV Tips, Tricks & Lessons, RV Upgrades & Mods

How to build a battery bank to fit your needs.

Are you trying to build a battery bank to fit your needs? Do you still find it confusing when thinking about what your RV power system needs may be? I am going to give a detailed walkthrough of our system showing what we have in use for our system. I will also point out what works well for us. The material works best by breaking it into subgroups. This allows me to better explain how they work in relation to all the other equipment to build the system starting with the battery bank. 

The most important part of the system really comes down to the battery bank. It is a big reason why you should build it specifically to fit your needs. It provides energy storage for the entire system. When calculating your battery bank sizing, you should evaluate several factors. Some factors being expected size of loads, duration of loads, and what available inputs.

How did we choose to build our battery bank so that it would fit our needs?

When designing our system, I wanted to make sure to build it to fit our needs. I knew that I would be powering my rig with split-phase service from two inverters. It is designed to be used in off-grid mainly boondocking situations. My loads were not going to be large for the two inverters at the same time. Most of my continuous loads were on DC circuits. Such as the Norcold propane absorption refrigerator, led lighting, and propane water heater. My AC current loads expected to be two Apple Laptops, occasional microwave or Instant Pot use for cooking, hairdryers and curling irons, and emergency use of the bedroom air conditioner for my senior dog for high temperatures. 

Assessing the largest loads helps guide you to a minimum size battery bank to fit your needs.

Each of the Victron inverters recommended a battery bank capable of outputting 300 amps to supply its energy. I do not expect both of my inverters to draw heavily on the battery bank at the same time. My minimum battery bank calls for 300 amps output by following the recommendation for the minimum for a single inverter to fit my needs.

Calculating your daily battery bank needs is important.

If you do not know your individual usage yet, there are tools available to help you learn. You can check out my post on calculating your needs here. I recommend that you consider installing a BMV-712 Battery Monitor or a Smart Shunt. Also, consider purchasing a Kill-o-watt energy meter. Both will allow you to learn what your usage currently is. I knew from my first system that my average continuous usage was going to be 9-16 amps or 100-200 watts. So when looking at my bank size I wanted to be able to maintain my loads 24 hours a day. When I was not bringing in solar I would expect to see a usage of 1200-2400 watts of power overnight. Each of the Battle Born GC2 batteries has a rating of 1200 watts. This would mean I would discharge up to two batteries worth of stored energy overnight.

Knowing how long you want to go without running a generator or plugging in will allow you build a battery bank multiplier to find a good fit.

I also knew that I wanted to build my battery bank to last for at least three days before I needed to find a charging source if my solar panel array was not sufficient due to inclement weather or shading. So after two or three nights with inclement weather during the day with solar harvests sustaining daily usage, I would then have to consider using an alternative source of power. For those times I carry two 2000 watt Champion inverter generators that fill my off-grid AC power requirements in inclement weather. Individually I am not able to get full use out of the charger in the Victron inverter/charger due to current limitations. I am able to get higher charging rates when the generators are in their parallel configuration. 

How did we wire everything together?

Our battery bank built for our specific needs.
Our battery bank layout and wiring built for our needs.

Additional components shown include 400amp BlueSea t-fuses. They are a good choice for inverter loads that have continuous high currents as well as the high surges. A common mistake that I see DIY installers make is that they try to save a few dollars on the small required components that turn out to be large weaknesses in the system. Fuses are some of those components. Most gold plated ANL audio fuses will cause trouble in a system due to their high resistance that they cause. 

The inverter battery disconnect switches are after the fuses. They will disconnect the battery power in case there is a reason to isolate the inverter. Such as to change the fuse or perform maintenance to the system. For long service life, do not switch under load. Ensure the inverters are off at their power switches prior to switching the battery disconnect switches to on or off.

The last two components shown are a 120 amp circuit breaker and a pair of busbars. These components often have cheap products in their place that can cause problems in the system. The circuit breaker can be a source of high resistance and erratic tripping issues.  I recommend BlueSea or Bussmann circuit breakers for reliable service. Do not utilize circuit breakers in place of catastrophic fusses for inverters. The busbars are used to make better connections. For the busbars, I recommend either the BlueSea or BEP Marine busbars. Pay careful attention to current limits and try to make connections that will aid in the least amount of resistance.

Still looking for some assistance?

If you would like a consultation to discuss your specific needs, send me a message at Justin@OptingOutofNormal.com. We can get a meeting scheduled. I do have a list of recommended vendors that do pay an affiliate fee. I could get credit based on people following links in my blogs or working with those vendors to quote packages for consultation projects. If you would like to source your own parts and equipment, consulting fees may apply depending on complexity and involvement. 

A complete list of parts and equipment that we utilize in our system can be seen at Kit.co/optingoutofnormal.

Check out our blog “How to size an inverter to fit your needs“. We show how we get the most benefit out of our Victron Inverter setup. If you are boondocking in the National Forest or plugged into an outlet, you can harness the great features of the inverters. This allows you to safely manage incoming loads while still living normally in an RV.

Check out some of our other blogs to see how we use our system. Boondocking ~ How We Spent $48 in Camping Fees For The Whole Year of 2018
Free Camping in Idaho ~ 5 Epic Boondocking Areas!
Finding Resources While Boondocking

Are you in search of a like-minded group of solar enthusiasts? Maybe you are looking to find the latest tips and tricks for your install? Come check out my group on Facebook.