Know Your RV Inverter’s Limits

There are two devices that you may have on your rig that sound similar but have different functions. Every RV has a converter charger which is what takes the 120 volt AC power from the park and converts some of it to 12 volt DC power to run 12 volt devices and charge your batteries. Some RVs come with an inverter as well. An inverter allows you to take stored DC power from the battery and turn it into AC power to run your 120 volt appliances. These are great devices for camping off the grid or when boondocking in a parking lot because you can use many of the appliances on your RV without having to fire up a generator. Like everything, they come with some limitations.

Watts

Wattage is a measurement of energy transfer. Inverters are rated for a maximum wattage that they can put out. It is good to know what your inverter is rated for as well as how many watts you are attempting to use. For example, I have a 1000 watt inverter. I could plug ten 100 watt light bulbs into it. I’m not sure why I would do that in my RV, but I could. I unfortunately can’t plug my one 1500 watt heater into the inverter as it would draw to much wattage.
When we are dry camping and using our inverter, we avoid overloading it by reserving the inverter for certain tasks that we know it can handle. We know we can watch TV, charge our devices, and run one or two small devices as needed. For any job that is bigger than that, we will use the generator.

Related Read: To Invert or Not Invert: RV Inverters Explained

If you are unsure or don’t want to remember how much wattage each device uses, most devices have their wattage listed somewhere on them. When we first started, we would put a piece of masking tape on our appliances and write their wattage and amperage rating on the tape in order to quickly tally what we had running.

Modified VS Pure Sine Wave Inverters

Alternating Current or AC for short is called that because the current alternates direction back and forth. DC or Direct Current, which is the type that your batteries put out, flows in one direction. Many devices that are meant for AC power will not run off of a DC source no matter how much voltage is added. For this reason, an inverter must take the DC power from your batteries and mimic the alternating sine wave that a real AC source would make.


SINE WAVE

There are two was an inverter can do this. The less expensive way makes a square step pattern. This works for many electronics. For some, it can cause them to malfunction. Most motors such as those in fans are going to run a lot hotter which over time will shorten the life of the appliance. Digital clocks and some other digital electronic devices may not function properly. If you have to run sophisticated medical equipment off of an inverter such as an oxygen mask for sleeping, then a modified sine wave inverter is not for you.

Related Read: What Size Inverter Do I Need To Run My CPAP

MODIFIED SINE WAVE

That’s not to say you can’t run things off of a modified sine wave inverter. Most laptop chargers and cell phone chargers actually convert the AC current from the inverter back to DC to charge the device. It actually is more energy efficient to get a 12 volt version of these chargers in order to eliminate the conversion step.
A pure sine wave inverter most perfectly mimics that of true AC power from the electrical grid. It is the more expensive of the two, but it is the best choice for running electronics off of the inverter frequently. If you are unsure about which inverter suits your needs, there is a great article here that goes into much more detail.

Battery Drain

The battery powers the inverter; this of course uses up the power stored in the battery. Increasing the load on the inverter (i.e. plugging more stuff in) will drain the batteries faster. Having an idea of how long your batteries will last before needing to be recharged on a typical load you put on your inverter is important.

Most batteries are rated in amp hours abbreviated as an A.H. rating. This, for many batteries, is stated for a 20-hour rate of discharge. In other words, a 100 A.H. battery lasted for 20 hours at a constant rate of discharge. In order to figure out what load was used to determine the A.H. rating, we divide 100 by 20 hours to get a discharge rate of 5 amps per hour. So for a typical 100 A.H. battery, we can put a 5 amp load on it for 20 hours. If the load is greater than 5 amps the battery will discharge faster.

The formula for amps is A = WATTS / VOLTS. Let’s say we are running an appliance that takes 100 watts.

100 watts /12 volts (Your battery’s voltage) = 8.33 amps

The result is slightly higher than the drain of 5 amps this battery was tested with. I’m sure there is a complicated way to calculate the exact drain but I usually divide my resulting amp calculation into my A.H. rating to get a rough estimate.

Example: 100 A.H. / 8 amps = 12.5 hours of run time.

This method is approximate so I give myself a margin of a couple hours. With a 100 A.H. battery, I could expect to get about 10 hours of continuous operation from a 100 watt device.
Many inverter systems have monitoring panels that allow you to keep track of amps being used by the system and battery power. So you will most likely not have a lot of math homework while using your inverter. The main take away from the section above should be that just because your inverter is rated for 1000 watts or even 2000 watts, that doesn’t mean you are going to be able to run a 1000 watt appliance for a long time if your battery storage capacity isn’t large enough.

If you would like to learn more about how to calculate how many batteries you would need to suit your inverter needs, this site has some great info.