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What inverters do

Inverters are used to make AC power from a DC source.  With a battery bank of 12 or 24 volts an inverter can make AC power of 120 volts at 60 Hz.  They are often used in boats, trucks and cars to provide AC power for electronic equipment and appliances but the applications are endless and one of the most common uses these days is in solar energy installations where inverters can be used to power an entire house from a battery bank charged by solar panels.

How inverters work

Inverters work by making alternating current from the DC power source and then passing it through a transformer to change the output voltage. Pure sinewave inverters produce a waveform to match that of shore power sources.  Other inverters merely provide an approximation of a sine wave output, sometimes called a "modified sinewave".

How big a battery bank is needed

A quick rule of thumb for 12 volt DC inverters providing 120 volt output is that the amp draw on the DC side will be 10 times the amp output on the AC side.  So to provide 120 volt AC power to an appliance that draws 8 amps the inverter will draw 80 amps from the 12 volt battery bank. It readily becomes apparent that large battery banks are needed to provide real power for any length of time.  Batteries last longer if they are not drawn down greater than 50%.  So for example to run our appliance for two hours would need 160 amp hours from the battery bank (80 amps x 2 hours)  To avoid running our battery down more than 50% it would need to have a capacity of at least 320 amp hours to run the appliance for two hours

Sinewave vs Modified Sinewave

True sinewave inverters supply power with an output waveform that matches that of conventional shore power.  All appliances running on this time of inverter should run in a completely normal manner.  Modified sinewave inverters actually use a square wave output where the duration of the pulses is adjusted to match the demand of the load. They  work well for most applications but can cause motors to run hot and so may not be suitable for extended duration running of air conditioning or refrigeration motors.  They can also cause small battery chargers and converters such as laptop power supplies to run hot, and there are sometimes problems with speed controllers, timers and other equipment.  True sinewave inverters tend to be more expensive but dont suffer the same problems described above.

Seperation of power supplies

Inverter output must not be allowed to connect to any other power source (eg shore power or generator) so distribution systems for inverter output must be either separate or switched via a transfer switch.  The transfer switch may be manual or automatic.  The relavant section of ABYC A-31 is shown below: If an inverter (inverter/charger), and any other source(s) of AC can supply a branch circuit or receptacle, then the transfer from one power source circuit to another shall be made by a means that opens one source circuit before closing the alternate source circuit, preventing arc-over or feedback between
sources. The transfer switch (e.g., switch gear) shall be protected against overcurrent. The current rating of the transfer switch (e.g. switch gear) shall be at least equivalent to the ampacity of the branch circuit feeding the transfer
switch. Inverter (inverter/charger) integral switching shall switch: all ungrounded conductors, and the grounded (neutral) conductor from ground

Other relevant standards

On boats inverters are covered by the ABYC standard A-31 which defines an inverter as: Inverter – A device, powered by batteries, designed  primarily to provide AC current at a required voltage and frequency powered by batteries.
Some additional standards from ABYC A-31: All marine power inverters shall meet the applicable requirements of UL 458, Power Converters/Inverters and Power Converter/InverterSystems for Land Vehicles and Marine Crafts and Supplement SA, Marine Power Converters/Inverters and Power Converter/Inverter Systems. The frequency and voltage regulation shall comply with section 27 of UL 1248, Engine- Generator Assembly for Use in Recreational Vehicles Power inverters shall provide isolation of the AC output from the DC supply circuit. Integral inverter receptacle(s) shall be protected by an integral GFCI device in accordance ABYC E-11, AC and DC Electrical Systems On Boats The receptacle is to be used only with cord connected loads. A visible means (e.g., voltmeter or lamp) of determining that the inverter is “on line” and/or in “standby” mode shall be provided at the AC main electrical distribution panel. A warning label shall be provided at the AC main electrical distribution panel to indicate that the electrical system includes an inverter.

Further Reading

Inverter Specifications: The difference between VA and Watts