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Words of Wisdom: “Inverters are excellent battery depleters.” – sternwake1)

Words of Wisdom: “One can only put so much inverter on so much battery… A lot of people will put a 2KW inverter, or larger, 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.” – sternwake2)


Inverters take 12v DC and turn it into 110v AC. For shore power charging see Converters.

It is more efficient to use DC power directly whenever possible. In particular it is more efficient than running a laptop of phone charger off an inverter.3)

Note: only AC loads will require an inverter. Devices that run off a "wall wart" adapter that output DC almost never require an inverter. Things that charge off USB (phones, fans, tablets, etc) can use a 12v-to-USB port. Laptops often have car chargers that run off 12v (example). Search for “[specific make/model] car charger”


Inverting from DC to your local shore power spec will necessarily incur losses. These might be 10-30%, depending on the inverter, the inverter's efficiency spec, etc. In the absence of hard information you might use 20% losses as a rule of thumb. This means it will require 20% more power to run your load than the rating spec.

50w / 0.8 efficiency = 60w demanded from the system

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.


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


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

MSW and electronics

In the end, 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. – BB4)

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.5)

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 – BB6)7)

Appliances with motors tend to run poorly and hotter than usual.

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.) - WombatCombat20208)

Other devices which do not like MSW:

  • 433MHz remote radio controlled plugs9)
  • asynchronous motors with a capacitor to build up the 90° phase do not run well on MSW10)
  • hair clippers
  • In an apples-to-apples comparison knurlgnar24 found a microwave to run 10% less efficiently and at 20% less power on MSW.

Also see this technical post on the topic by therealtimewarren.

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.


  • very clean power
  • works with everything


  • $$$. Typically at least 2x more expensive than 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 most often found on rigs that are on shore power much of the time and run heavy AC loads

It provides several functions:

  1. invert 12v battery power to 120vac
  2. pass-through shore power when available, with auto-switching
  3. some can bond shore and battery power to support heavy 120vac loads
  4. charge battery banks like a converter
  5. some also handle solar charging

They tend to be expensive, some over $1000.

Note: the Victron Multiplus line has outputs for maintaining the starter battery:

The main DC terminal can supply the full output current. The second output, intended for charging starter battery, is limited to 4A and has a slightly lower output voltage.11)

Sizing your inverter

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

special case: tiny loads

For ~120w loads you can use a small inverter intended to be powered by the cigarette lighter port. The ports are typically fused at 10A, hence the ~120w wattage limit (12v x 10A = 120w).12)

Note: Ciggy ports are already prone to failure and poor connections. An inverter that hangs out of the socket will make matters worse. To avoid stressing the socket you can use an inverter with a built in power cord (as pictured above). If you already own one designed to live in the socket13) consider using a soft wire extension that will allow you to relocate the inverter.

In any case, find out what kind of fuse your ciggy port uses and have some on hand. Mistakes are easy to make and easy to fix (if you have the spare fuse waiting).

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) * 1.1014), 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 position15), 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.16) 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.

reading 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 The best way to understand how much your 120vac equipment needs during startup/running is to use a power monitor like the Kill A Watt. You may be pleased to find that some appliances run at much lower power than their labeling would suggest.17) It is rare to find a device that pulls more than the label states.

Kill A Watts may be available for loan from your public library18), and sometimes used at thrift stores.

The buttons on the traditional Kill-a-Watt have two functions each. You press the button repeatedly to see the information:

  • Volt - Not directly useful for load testing, Later when you have an inverter and plug the meter into the outlet you will be able to see your inverter's output voltage. It is generally quite close to your grid power voltage.
  • Amp - Amps at 110vac. Not particularly useful for load testing.
  • Watt / VA
    • Watt - this reading is the most important thing for load assessment. Add 10%-20%19) to tell how much power the appliance will actually consume.
    • Volt-Amperes is apparent power, normally not directly useful. If you have a device that is unexpectedly tripping an inverter (200w load tripping 300w inverter when powering on) you might watch VA to see if apparent power is >300w; this can happen with some loads. For this reason some inverters are rated in VA rather than W. See Power Factor below.
  • Hz / PF
    • Hertz - as with Volts, you can check your inverters AC frequency with this. In the US it is 60Hz.
    • Power Factor - this reading can be indirectly useful. It is a relationship between Watts and VA (see above). One rule of thumb is that devices with a PF of >=.90 are good candidates for MSW inverters. A resistance heater may have a perfect 1.0 power factor and works great on MSW. A laptop charger or other wall wart might have a PF of .040 - .60, which suggests it might overheat on MSW. Also see this video about power factor.
  • kWh / Hours
    • kWh are 1,000w-hours. This is the actual amount of power used over time
    • Hours are clock hours. Dividing kWh by Hours will give you your hourly consumption. Multiply by 24hrs for daily consumption. Run over multiple days to get better averages.

understanding the specs

Inverters are generally rated in watts in terms of continuous resistive loads (heatihg 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 advertize only the peak rating.20) The peak rating is only for short bursts of heavy draw.

Stand-by power 21) 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 directly22) and indirectly23) there are cases where an inverter may be more useful than 12v:

  • with devices which only exist in grid power24) models
  • with devices which are much cheaper in grid power configuration. This includes items which are rare in 12vdc but commonplace on the used market25): 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…” - Sternwake26)
“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.” - Coultergeist27)

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.

HandyBob28) reminds us that “Properly adjusted solar systems with temperature compensation [for lead batts] will regularly exceed the 15 volts many inverters use as a high voltage shut down.”29)


Most inverters have fans to pull cooler air through them. Some do this only when they are too warm, or at a given power output regardless of temp, and some run 100% of the time.

If your fan runs only when hot, do not turn off the inverter until the fan has cooled the insides and stopped running. With continuous running fans put your hand on the inverter after the load is stopped to feel when it is safe to power off the inverter.

running big loads at lower power

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 cord30) to their inverter and run it to the recipient. The recipient could plug it cord into their shore power inlet or use the end of the extension cord directly.

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.31)
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.”32)
power factor is the PF button on the kill-a-watt
technically you could use any size inverter as long as you remembered not to draw >10A
don't buy this!
inverter losses
the label likely has to list the highest-possible wattage draw under any conditions
depending on your inverter's efficiency
parasitic draw when no loads are present
purchase price
~10% inefficiencies increase charging and battery costs
110vac, etc
thrift stores
outdoor rated and approprate gauge
electrical/inverter.txt · Last modified: 2024/06/15 22:00 by frater_secessus