Inverters

A small inverter.

Inverters are an electronic device that takes one type of electrical power (direct current or "DC"), and turns it into another type of power (alternating current or "AC"). In the context of this wiki, many nomads use inverters in their vehicles to change the electrical power generated by the vehicle while driving, or power stored in batteries, to the type of power commonly used by household appliances and other devices that are designed to plug into a wall outlet. Inverters convert DC to AC, but somewhat confusingly, a different device called a converter is used to handle power needs while your vehicle's electrical system is plugged in to the power grid (shore power).

Most nomads will want an inverter of some sort in their vehicles, but it's not a requirement. If you don't have any appliances or devices designed to plug into the power grid (sometimes called mains), then you don't need an inverter. In many cases, you can find a DC-powered version of a product that is more typically powered by AC. However, large appliances and other products that use a lot of power generally require AC.

Inverter losses

No inverter is 100% efficient (thanks physics!) and a not-insignificant amount of power is lost (converted to heat) in the process of converting the DC to AC. The amount of the loss depends on the inverter, how much electricity you're using at the moment, the voltage of your system, and other factors, but you can assume around 10% losses for a typical nomad setup. Inverters also use a bit of power of their own whenever they are powered on, even if they're not actually "sending" AC power to anything. This baseline power usage may be referred to as stand-by losses, quiescent draw, or parasitic draw. If you have an always-on inverter in your rig, this can be a significant drain on your battery.

Because of these losses, it's usually better to power something directly with DC power instead of AC, if you have the option to choose. Also, many electronics come with a household power cord, but internally use DC power, so they do their own conversion from AC to DC. This means that if you use a separate inverter you're just doubling up on the losses. For example, if you are using an inverter that is drawing power from your batteries, and plug a laptop into that, the electricity path is DC (battery) to AC (inverter) and back to DC (laptop's "wall wart" power supply). If you can keep DC power the whole way, you'll save some valuable electricity and generate less waste heat.

Inverter size

Inverters are available in many different sizes, so you can choose one that matches the amount of AC power you need. Inverters that provide more power (watts) are more expensive and physically larger, and the inverter losses are higher. Because of these factors, it's a good idea to get the smallest inverter that meets your needs.

Using a huge inverter with a small battery doesn't make much sense either: ... people will put a 2kW inverter ... on a single 12v battery, but this can be compared to a 5-second drag car which uses 5 gallons of gas to run the 1/4 mile, but then only having a 2 gallon gas tank.

Types of inverters

There are two different types of inverter: Modified Sine Wave (MSW, also called "square wave") and Pure Sine Wave (PSW). The distinction refers to how closely the power output from the inverter matches the electrical "look" of normal power-grid-produced AC power. MSW inverters are cheaper than PSW inverters, but the AC power they produce isn't as "clean". Some devices will not work with MSW power, and some devices can even be permanently damaged by MSW power. However, there are many devices and appliances which work perfectly with MSW power.

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Modified Sine Wave

Originally called "square wave" inverters, MSW inverters are inexpensive and ubiquitous. If your inverter doesn't say it is MSW or PSW then it is probably MSW.

Pro:

• inexpensive
• widely available
• works for many/most AC items

Con:

• can cause motorized items to run warmer
• can cause some electronics to malfunction
• some reports of damage to electronics; see below

MSW and electronics

Probably 80% of the stuff you plug in will work fine [on MSW], 10% will fail pretty quickly, and the other 10% it will be difficult to tell.

While MSW powers resistive loads like crockpots and coil heaters perfectly, there is anecdotal evidence that MSW can damage some electronics. The most commonly repeated stories are Dewalt cordless tool battery chargers and laptop chargers. Milwaukee battery chargers may also require PSW.

Laptop chargers that work on MSW will usually run somewhat warmer and may buzz. Laptop chargers that will not work on MSW tend to get hot, not power on at all (no light), or cycle on/off. Laptop chargers and other power supplies might be tested thusly:

One way that I used to check computer/electronic power supplies ... take a Kill-a-Watt meter and measure the power factor. If it was >0.9, it probably would work fine. (power factor is the PF button on the kill-a-watt)

Rule of thumb:

Everything that has electronics and use a power supply or an integrated power supply to lower voltage will be fine [on MSW]. (Radio, charger, Screens etc.) Everything that uses a power motor (often without electronics) will not (hairdryer kitchen appliances etc.)

Other devices which do not like MSW:

• 433MHz remote radio controlled plugs
• asynchronous motors with a capacitor to build up the 90° phase do not run well on MSW
• hair clippers
• In an apples-to-apples comparison knurlgnar24 found a microwave to run 10% less efficiently and at 20% less power on MSW.

Pure Sine Wave

PSW inverters are as good as (or better than) than AC mains power. All AC items will run on a properly sized PSW inverter.

Pro:

• very clean power
• works with everything

Con:

• Typically at least twice as expensive as MSW for the same wattage rating.
• Not as widely available as MSW; you may not find them at truck stops, big box stores, etc.

Not all PSW are prohibitively expensive. Bestek makes a 300w PSW for about $50. See the top-rated review for oscilloscope testing and limitations.

Inverter chargers

Inverter chargers are combination devices that provide the function of an inverter (direct current to alternating current) and a converter (charge the batteries) along with other functions. They are especially useful in campers and other vehicles that are connected to shore power regularly and run heavy AC loads like air conditioners.  Learn more …

Sizing your inverter

Once you have made your MSW vs. PSW decision, the next step is sizing the inverter (picking the right capacity).

Estimating your needs

You will need to know the largest draws you will need to meet at the same time. If your TV is 40w, and your your laptop is 100w you would need (40w + 100w) * 1.10 (inverter losses), or an inverter that can power at least 154w.

Motorized equipment like drills likely have greater current requirements for startup and for running. Be sure to measure both startup and running (see below). There are tricks you can use to minimize the start load, like starting a sewing machine at a low-load position, or easing into the trigger of a variable-speed power tool.

Note that to actually power the inverter without voltage sag or dipping so much into battery capacity you may want need something like a 1:1 ratio of panel:inverter on flooded lead and 1:2 on AGM or lithium. This would mean 400w of solar to run a 400w inverter (dipping into the battery as needed) or 400w of solar for 800w of inverter on AGM/Lithium, which have greater throughput.

Leading the label

If you are lucky, the device will show power consumption in watts; this is useful because watts are watts here you are talking about 120vac wall power or 12vdc automotive power.

Some devices will only show Volts and Amps. Since Watts = Volts * Amps we can do the math to get watts:

• 110v x 1A = 110W
• 110v x 0.5A = 55W
• 110v x 10A = 1,100W (possibly more than your camper's power system can provide)

A standard wall plug in the US is 110v x 15A, or 1,650w.

Kill A Watt

Kill A Watt is a power monitor which you can use to determine the actual power your appliances and other devices are using, since the usage rarely matches the nominal value given on the label. This is helpful when determining your power needs to correctly size your electrical system. The Kill-A-Watt is manufactured by P3 International.  Continue reading …

Understanding inverter specs

Inverters are generally rated in watts in terms of continuous resistive loads (heating elements, etc.). Inductive loads (things with motors, some electric blankets, etc.) can draw more than their rated wattage and overpower the inverter. Plan accordingly.

While there are both peak and continuous duty ratings it is common for marketers to advertise only the peak rating. The peak rating is only for short bursts of heavy draw.

Stand-by power (parasitic draw when no loads are present) may be a factor when an inverter is run for many hours. The inverter drops to a power-saving mode then checks regularly to see if any load has been added.

• an explanation of Victron inverter power save modes
• Renogy inverters >700w appear to have a power saving mode

Why not buy the biggest one available?

The immediate temptation might be to buy a massive inverter and call it good. There are factors arguing against this approach:

price - an oversized inverter wastes money

efficiency - an oversized inverter wastes power. All inverters have a parasitic draw, a level of baseline power consumption no matter what it is doing. Even sitting idle. Larger inverters generally have larger parasitic draws, which tax your house batteries or solar setup.

Imagine a 3000w PSW with a 1.6A parasitic draw which you use to charge a cellphone (0.3A). The inverter will still consume 1.6A even though you only need 0.3A. Even worse, it will consume 1.6A while idling. If you are disciplined you can remember to turn the inverter off when not in use. Higher-end (and more $$$) inverters sometimes have a powersaving mode that allows an idling inverter to sleep. The model linked above sleeps at 0.35A. It would still jump to 1.6A when powering the example cellphone charger.

If all you need is small amounts of AC the same maker above makes a 150W PSW model that idles at 0.15A although it lacks the sleep function of its larger stablemate.

noise - smaller inverters are usually cooled with a heatsink, which is silent. Larger inverters may have a fan that spins constantly or, preferably, as needed under load.

space - larger capacity inverters are physically larger, eating into limited RV space

The case for inverters

Even though inverters cost money directly (purchase price) and indirectly (~10% inefficiencies increase charging and battery costs) there are cases where an inverter may be more useful than 12v:

• with devices which only exist in grid power (110vac, etc.) models
• with devices which are much cheaper in grid power configuration. This includes items which are rare in 12vdc but commonplace on the used market (thrift stores): coffee grinders, small cooking appliances like rice cookers, crock pots, etc.
• with devices that have strange DC input voltages provided by wall-warts: 3v, 7v, 9v, etc.
• when you need long wiring runs. This can be useful for donating power to another vehicle: inverter -> extension cord -> other camper

The case for multiple inverters

> "One can get a small PSW for the delicates and get a Beast of a MSW for those less delicate electronics..." - Sternwake

> "I use two for redundancy and efficiency. Large loads need big inverters to power them. Most inverters will scale down their power use to the load that is being placed on them, but they all have a minimum amount of power that they use just when they are powered on. Generally, the larger the inverter, the larger this parasitic power draw is. Most items that need large inverters are not used constantly. You will only use a microwave a few minutes per day probably. There is no need to waste all the extra power required to keep the big inverter on constantly for that few minutes of big power draw each day. I use a much smaller inverter for the majority of things that get powered in the van. The television, dvd player, and antenna amplifier all draw less than 200 watts combined. The refrigerator runs at about 150 watts intermittently. My 1000 watt inverter will easily handle all of that and have enough surge capacity to handle the refrigerator compressor when it turns on. Because its smaller, it is more efficient and doesn't waste as much power just by being on." - Coultergeist

Overvoltage and undervoltage

Most inverters will shut off if they get too hot, or if the input voltage is beyond certain points.

The low voltage cutoff is to protect your batteries from over-discharge. The high voltage cutoff is to protect the inverter itself. In addition, the current consumed by the inverter may increase as bank voltage drops.

HandyBob reminds us that "Properly adjusted solar systems with temperature compensation will regularly exceed the 15 volts many inverters use as a high voltage shut down."

Sharing power

Because higher voltage power has less losses over long wires, it may be most efficient to share power between RVs as 120v. The donor vehicle would invert to 120v, plug in an extension cord to their inverter and run it to the recipient. The recipient could plug it cord into their shore power inlet or use it as 120v.

Differences from shore power 120vac

Inverters make 120vac in different way than shore power does. Alan Georges explains:

> ... inverter outputs are usually two 60 VAC lines driven 180 degrees out of phase, so that there is a net 120 VAC potential across them.

>That's all fine, as long as what's plugged in is electrically "floating," that is, not connected to any external ground.

> The situation gets more complicated when there's a 3-prong 120 VAC plug. The third prong is a safety ground which is assumed to be tied to a ground rod.

> You can see that the neutral and ground wires [in shore power] are tied together at the panel. That works, because the hot is 120 VAC. You can also see how that is very different than two "half-hot" 60 VAC lines, and how that kind of tie-in would short out one leg of an inverter.

> In fact, the situation with an inverter is very similar to [a] 240 VAC circuit... which has 2 120 VAC lines driven 180 degrees out of phase, i.e., "push-pull."

Resources

	Resource	Description
	Adding a large inverter	How to add a large (2000+ watts) inverter to an electrical system
	Adding an inverter	Learn how to add an inverter to an existing electrical system.
	Adding an inverter-charger	Learn how to add an inverter/charger to an existing electrical system.
	Comparison of inverter-charger and inverter	Learn the difference between an inverter-charger and an inverter.
	Testing inverter losses	Tests on multiple inverters to see their real-world inefficiencies compared to their rated values.
	Search Amazon	Search Amazon for related products.
	Wikipedia	Wikipedia article on this topic.
	Search forums and groups	Search van life discussion groups for "inverters"
	Search related sites	Search van life sites for "inverters"
	Search NomadLife.wiki	Search other pages on this wiki for "inverters"

Some or all of the content on this page was originally sourced from this page on RVWiki