Inverters, High & Low Frequency

Inverters are a deep subject and things can get really complicated really fast. Let me
see if I can break it down into basic bite sized chunks. (this is where I tell you how to
build an inverter :wink:)

There are two basic types of inverters: High Frequency and Low Frequency.

Low Frequency Inverters

Are physically large, have a very heavy transformer, can withstand heavy power surges,
usually triple the inverter’s rating, for up to 20 seconds.

They are designed to handle large induction motor starting currents, (like when the
lights dim when the air conditioner starts up), for things like air conditioners, table saws,
water pumps (well pumps and/or swimming pool motors) & etc. Devices with induction
motors have a large surge of current to start them (3 to 5 times the running current
listed on their power label).

As an example a refrigerator has a compressor that is rated at 10 amps. It might pull 35
amps for 5 seconds to get it up to speed and then drop back to its rated 10 amps for
normal running. Low frequency inverters are what you want if you are going to run
things with induction motors - like air conditioners, and power tools (table saw?) in a
mobile workshop.

Low frequency inverters can often cost 1.5 time to 2 times the cost of a high frequency
inverter. They are also taller to accommodate the large cube shaped transformer.

High Frequency Inverters

Are usually smaller (at least flatter or thinner), are about half the weight of the low
frequency inverter, and can withstand power surges of double the inverter’s rating for 1
second or less - often less than half a second.

These inverters don’t handle heavy induction motor starting currents well. They work
fine for most appliances like microwave ovens, regular or induction hot plates, small
dorm style refrigerators, brushed motors like vacuum cleaners, fans, lights - normal
household items.

You can overcome some of the induction motor starting current issues with what is
called a “soft start” circuit. This works for things like air conditioners but not so much for
power tools.

High frequency inverters are a good choice if you don’t plan to run items with large
induction motors or if you expect to run the occasional induction motor, then over size
the inverter by 50% to 100% and/or see if a soft start option is available for the
appliance you wish to run.

Other factors

All inverters have a quiescent power draw. This is the power that it takes to run just the
inverter when nothing is plugged into it. The larger the wattage of your inverter the
larger the quiescent power draw will be.

As an example, my 1,500 watt high frequency inverter pulled 0.4 amps when turned on
but nothing plugged in. My new 3,500 watt high frequency inverter pulls 2.9 amps
under the same conditions. This is why it is important to be able to turn the inverter off
when not in use. Most inverters offer a remote control panel as an optional accessory so
you can easily turn it off when not in use.

All inverters are rated at a certain temperature. As the temperature rises the output of
the inverter falls. My 3500 watt inverter will put out 3500 watts until it’s internal
temperature reaches 100F. Then it’s power output will drop as temperature rises to
140F when it will only produce 2400 watts. Above 160F the inverter will shut down
completely to protect itself. Remember that it’s easy to reach over 100F+ ambient air
temperatures in the summer so the power derating curve of the inverter can be

Many of the inverters, especially the low frequency types with a large heavy
transformer, will also have a battery charger integrated into them. This charger will be
of older technology and most likely won’t be useful for a lithium battery setup.

Low Frequency Inverters:
AIMS Low Frequency, Magnum MS series, Outback M series
Expect to pay $1,500 to $2,000 for a 2,000 watt LF inverter.

If you are not sure whether the inverter you are looking at is low frequency or not, check
how much it weights (it should be much heavier than the hf inverters of the same
wattage), then check it’s surge rating (it should be about 3 times it’s rated wattage for
15-20 seconds)

High Frequency Inverters
I think there are only a few manufacturers and the same inverters are rebranded over
and over. Some good brands seem to be,
Samlex PST series, Go Power, Aims, Cotek SD series.
Expect to pay $900 to $1200 for a good 2,000 watt HF inverter.

All my inverters have been high frequency Cotek and I’ve been happy with them. My
current inverter is an SD-3500-112. This is a high frequency inverter but way oversized
so should run about anything I’m likely to have. I looked at Low Frequency inverters but
I only have 5 1/2 inches between the mounting panel and my rear door so no
room. Most of the LF inverters are 7"+ high and I just didn’t have the space.

For a van conversion/motor home I wouldn’t go less than a 2,000 watt high frequency
inverter. Of course a nice low frequency inverter would be wonderful but most likely
overkill for what is needed unless you want to try to run an air conditioner off your

batteries without shore power. I’d also get one of the major brands as listed above. If
you get a good one it should last many years.

The reason I wouldn’t go less than 2,000 watts is that in warm weather it could be de-
rated well below 2,000 watts. You also want to look ahead. Right now you may not see
yourself needing more than a few hundred watts to run an Instant Pot. I assure you,
your world and ideas will change and one day you may want a microwave oven or an
induction hot plate. The small premium you pay for an inverter that will easily run these
will be well worth it.

I originally had a Cotek pure sine wave 1500 watt inverter. Had a devil of a time finding
an induction hob that would work. Most were 1600 - 1800 watts. Same goes for the
microwave oven. What finally drove me to upgrade to the 3500 watt inverter was
testing an Incinolet incinerating toilet. These things are power hogs but it would sure be
nice to use the potty for a couple of weeks and have only a cup of white powdered
ash. Alas it just wasn’t practical, but I’m glad I upgraded the inverter anyway. Without
the power hungry toilet I don’t need 3500 watts but 2000 to 2500 is the sweet spot.

Take a serious look at the Cotek SD2500-112. And whatever you do make sure to get
a Pure Sine Wave inverter. Modified sine wave inverters are obsolete and a waste of
money regardless of their price. Modified sine wave inverters don’t run electronic
devices well and often not at all. I’m sure you’ll want to be able to charge your laptop,
cell phone, or other electronic gear. Even the Instant Pot has an electronic processor
so Pure Sine Wave is a must.

Hope this helps,