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electrical:inverter

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)

AC-only items will require an inverter.

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
  • generally more efficient under no load (?)

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

there is anecdotal evidence that MSW can damage some electronics. The most commonly repeated stories are Dewalt cordless tool battery chargers and laptop chargers.

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:

  • $$$. Many times more expensive than MSW.
  • Not as widely available as MSW; you may not find them at truck stops, big box stores, etc.

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 10A, your refrigerator is 10A, and your corded drill is 20A then the most you would need is 30A, since you would not (I hope!) run the drill and watch TV at the same time.

Note that motorized equipment like the drill likely have different current requirements for startup and for running. Getting that motor spinning takes more power than running it. There are tricks you can use to minimize the start load, like starting a sewing machine at a low-load position.4)

The best way to understand how much your 110v equipment needs during startup/running is to use a power monitor like the Kill A Watt. You may find that even non-motorized 110v items run at much lower power than their labeling would suggest. Kill A Watts may be available for loan from your public library.5)

understanding the specs

Inverters are generally rated in watts.

While there are both peak and continuous duty ratings it is common for marketers to advertize only the peak rating.6) The peak rating is only for short bursts of heavy draw.

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 multiple inverters

SternWake says: “One can get a small PSW for the delicates and get a Beast of a MSW for those less delicate electronics…”7)

Coultergeist says: “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.” 8)

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.

HandyBob9) 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.”10)

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.

electrical/inverter.txt · Last modified: 2018/06/16 20:41 by frater_secessus