RV Solar Basics

I started my RV solar research literally YEARS ago. I wasn’t focusing on it continuously but I didn’t know how to proceed because I wasn’t sure of the questions or things I needed to know. And I’m the type of person who needs to know EVERYTHING, like why this, what are the other options?

So I collected all my knowledge and I want to share my learnings with you. I’ve tried to organize it in a way that makes clear and simple sense focusing on different aspects of a solar set up. 1) The panels and charge controllers 2) Batteries 3) Inverters, Converters, AC/DC and 4) How to determine your personal and specific electric usage to see what set up size you need.

Basic Solar Set Up

Let’s start with the basics:
A solar set up consists of solar panels, batteries, charge controller, and inverter.

Some engineers and solar enthusiasts would make it clear that this isn’t a solar system, which to most of us would immediate allude to planets rotating around the sun, but instead would call it a PV system. PV stands for photovoltaic. It’s even common to call the solar panels, PV panels. Solar panels to some would strictly refer to solar thermal panels used to heat water in a home. The word photovoltaic refers to the production of electric current from the sun, as opposed to the heat from the sun.

PANELS

Let’s talk about types of panels. There used to be a big difference between mono-crystalline and poly-crystalline.

Mono was and still is more energy efficient and because of the way it’s cut, it’s more efficient in physical size as well. Mono-crystalline cells are usually or maybe always square. Poly-crystalline panels are made with rectangular cells.

However, nowadays the difference in efficiency is very little but the cost of mono still remains a fraction higher.

A good price on PV panels might be about 1$ per watt on panels. It used to be closer to $5/W 10-20 years ago. You can get even cheaper panels especially for used ones. However, do your research before buying used ones. Perhaps the original owner of these panels removed them from their house because they were recalled.

Some people say to buy your panels based on your charge controller and to get your charge controller based on your need. When determining the size of panel to get you may want to just base it on wattage desired or voltage desired depending on your charge controller.

There’s a reason to use certain amount of watts or cells in a panel and once you choose one you must maintain the same specifications on current or voltage depending if they’re hooked up in series or in parallel, respectively.

Series vs Parallel

So what is series versus parallel? Panels can be hooked up to raise or maintain levels of power or current. The same is true for battery connections. Series connections increase volts while parallel connections increases amps.

Mixing Panel Specs

So let’s talk about what happens when you mix panels of differing watts, volts and amps.

Dealing with current, or amps, wired together in series, the current will be reduced to the lower panel’s amps. So if you connect a 3A panel to a 4A panel, the overall current is dragged down to 3A.

The same thing happens to the voltage when PV panels are wired together in parallel. If you connect a 15V panel to a 24V panel, the overall voltage is dragged down to 15 volts.

Mixing panels with differing watts isn’t as big of a deal, especially if they are close in wattage. If you mix panels with differing watts but same current, or amps, when connected in series, or with the same volts when connected in parallel there is minimal loss in power but there is loss. You will lose 50% of difference between them if they’re in the same array. So if you have a 100W panel connected to a 150W panel, the difference is 50W. You’ll lose 25W in that array. 100W + 150W = 250W – 25W = 225W power output in that array.

Though, if you mix panels with different wattages while using an MPPT controller, the different wattages confuse the controller a bit since it changes up the current and voltage some.

Another thing to keep in mind when preparing a PV system for your RV is that your RV or trailer might not have all the real estate space on the roof for all that you need or want. So keep that in consideration.

PV panels get hot. Not only because the sun beams down on them all day, but because they’re producing electricity which heats the solar cells up even quicker than just a pice of metal in the sun. When they’re hot, they work less efficiently. You can wash them off in the middle of day with water to cool them down to assist in the efficiency of the electricity production. However, depending on the type and thickness of glass, just like with pouring hot drinks into glass cups, a significant and fast enough thermal transfer can potentially crack the glass face of the panel.

Washing your panels and keeping the glass face clean will also help in the efficiency of electrical production.

CHARGE CONTROLLERS

You’ll want a charge controller in your RV’s PV system set up. A charge controller regulates the power coming into the batteries so that you don’t get too much power at once and end up frying your batteries.

MPPT vs PWM

There are two types of charge controllers, MPPT and PWM. If you have a PV panel array output higher than 200W, you’ll want to get an MPPT or if you have an array that will be producing more power than you’ll use right away but will want to use for later (like at night) you’ll want to get an MPPT. If you’re building a backyard set up to power something small or continuous like a pump, you could get away with a PWM controller. With a PWM controller your PV array voltage output should match the battery voltage but it can be higher when using an MPPT controller. MPPT controllers can converter higher coltages like 24V down to 12V.

Basically the PWN cannot store extra power that the panels create. The MPPT does a better job at regulating unused power to save for later versus dumping it like the PWM. However, the cost difference is noticeably higher to buy an MPPT controller.

With an MPPT you can potentially gain 10-20% more power using an MPPT controller and with a PWM you could potentially lose 10-20% of power.
So buy your charge controller based on your electricity need and then buy the panels based on charge controller that you choose.

From the series: