June 24, 2024

How To Safely Install an Inverter for AC Power Off-The-Grid

Adding a power inverter to simulate a shore power connection when off-the-grid is one way to provide AC power to your entire RV with minimal additional wiring. This approach is affective, but potentially dangerous if you have an onboard battery charger.

For an inverter and battery charger to coexist in an RV’s electrical system, you’ll need to ensure that both will NOT be operating at the same time. Allowing your battery charger to recharge the batteries while the inverter is running creates a potentially dangerous situation.

There are three ways to avoid this.

First let’s get a better understanding of the potential problem.

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Why Is This a Problem?

A power inverter draws its power from your DC battery bank and generates AC power. It may seem like a good idea to just plug your shore power cord to your inverter and power all AC circuits in your RV, but this creates one big problem. Your converter/charger (wired to run only when on shore power or generator) will start charging your batteries. In other words, your batteries will be recharging themselves. Not good. Right?

NOTE: This does NOT apply to RV systems with a hybrid inverter/charger (like a Victron Multiplus). Combination units like this include the charger, inverter, and necessary switching components in one device.

Locate the batteries, inverter and converter/charger in this crude diagram of my RV electrical system. Notice how the inverter is powered from the batteries and the converter/charger is then powered by the inverter.

Diagram of my electrical system with the inverter hooked up without the automatic switch
Components of my off-grid electrical setup (not including solar)

In order to avoid this problem of the battery recharging itself, we need to prevent the charger (or converter/charger in my case) from running while the inverter is also on.

OPTION #1 – Install a Dedicated AC Circuit For Your Inverter

Keeping your inverter and associated electrical outlets completely separate from the rest of your RV’s electrical system will eliminate the possibility of inadvertently charging your batteries.

You’ll simply extending the inverter AC output to a handful of new outlets inside your RV which will only be powered from the inverter. It’s like running an extension cord from your inverter inside your RV (which is also an option). If you need multiple inverter-only circuits, a small electrical panel can be installed to distribute the AC power.

While this is a common approach, it’s not optimal. Your access to AC power off-the-grid is limited to the new outlets you install. The rest of your AC equipment or receptacles will NOT have power and you’ll feel like you’re camping off-the-grid.

So wouldn’t it be great if everything worked as if you were on shore power?

OPTION #2 – Install a Relay to Automatically Disconnect the Battery Charger

With this approach, you connect your inverter directly to your main AC electrical line and power all of your AC circuits.

It’s like being on shore power, but you still have to manage your power usage to stay within the capabilities of your inverter and battery bank.

EXAMPLE: Don’t turn on your microwave or air conditioner unless your battery bank and power inverter can handle it.

You will have to switch off your converter/charger each time you switch over to inverter power as it will also be powered by your inverter.

This can be done manually if your charger is on its own circuit breaker, but you can’t forget to do it. The best approach is to automate the process.

Adding a relay to switch off your converter/charger automatically is an inexpensive and effective way to ensure your battery charger and inverter are never on at the same time.

You’ll need an electronically controlled switch called a contactor that will control power to the charger (or converter/charger).

120 VAC 30 Amp Contactor

Electrical Contactor : The contactor will automatically switch when a 120 VAC switching voltage is applied to the A1 and A2 contacts (from the inverter output). The output of the converter’s 15 amp circuit breaker is wired to the normally closed contacts (NC). So under normal conditions, power is allowed to go through. Read more on Amazon

Here’s how it works. Under normal operating mode (when the inverter is not running) power to the converter/charger would be allowed through (normally closed – NC). When the inverter turns on, 120 volt AC power from the inverter would activate the switch and disconnect power from converter/charger.

I installed a relay to automatically shut-off my converter/charger when power is applied to my inverter. I explain how I set it up and why in this video.

Video: How to Automatically Disconnect a Converter Charger when using an inverter

While this configuration has worked well for several years, I have now implemented option #3.

Here are some parts you’ll need to install the contactor

DIY Electrical Enclosure
Electrical Enclosure

DIY Electrical Enclosure : The contactor was mounted in this enclosure on a DIN rail and access holes were drilled for the wires. Read more on Amazon

6" DIN Rail Kit
Din Rail – Contactor Mount

6” DIN Rail Kit : The contactor is designed to mount on DIN rails like these. Hot glue was used to attach a single DIN rail to the bottom of the enclosure before attaching the contactor to it. Read more on Amazon

Wago Wire Connectors : These are my favorite type of wire connectors. They are strong, accommodate multiple gauge wire (up to #12 AWG), and completely reusable. They are a little more expensive than standard twist connectors, but they are worth it. I get the multi-pack and now use them for everything. Read more on Amazon

Here’s are some other materials and tools I use for electrical wiring.

OPTION #3 – Wire Your Battery Charger or Converter/Charger to Run Only On Shore Power

Wiring your converter/charger directly to the shore power inputs will guarantee that it will only charge your batteries when on shore power.

One option is to wire the charger AC power input to the shore power side of an automatic transfer switch. Your shore power cable typically goes directly into a transfer switch. So making the connection there eliminates the need for a sub-panel. There are automatic transfer switches pre-wired for this purpose like this one from Go Power.

I installed one in my RV wired for 30 amp service. The installation was pretty straight forward. Check it out in this video.

Video: Install a transfer switch to automatically switch from shore power to inverter power

If your RV has 50 amp service you will need to install a 50 amp transfer switch like this one from Go Power.

Here’s a video demonstrating how to install the 50 amp transfer switch in a fifth wheel.

Video: Install a transfer switch for 50A RV

NOTE: Once installed, you’ll need to switch the power input to the converter/charger. Move the hot, neutral and ground wires from the AC junction box and reconnect them inside the transfer switch on the shore power side of the switch.

If either of these options work for your situation, you can also install an electrical sub-panel off of the shore power input.