Power Inverter Questions and Tips

DC to AC Power Inverter for emergencies, disasters, and grid-downPower inverters convert DC power into AC power. For this conversation I will only refer to 12vDC to 120vAC power conversion. I believe it is important to have the ability to have 120vAC power available during emergencies, disasters, and grid-down situations. There will be needs such as running electronic equipment, chargers, medical equipment, etc. While diesel, gas, or propane generators are one option, they tend to be noticeable as well as requiring fuel. Solar units with one or more deep cycle batteries can often meet all the power requirements. However, you may have to convert the 12vDC power to 120vAC power. That is the job of a power inverter.

Questions –

What is the difference between modified and pure sine?

Modified is a blocky sine wave and a pure sine is a cleaner rounded sine wave.   A pure sine wave is as close to city power as it gets via inverter.  Pure sine inverters are intended for sensitive electronics and some motors.  Modified sine wave inverters should work with the majority of devices.

How long will my inverter last with “X” number of batteries?

When choosing batteries for your battery bank, you should consider two things; what you are running and how long you want to run your system? 

1) Take the output AC amps of the load you will be running and multiply them by the volts output (vAC) to figure watts. 

2) Then, watts divided by input volts (vDC) gives the DC amps consumed per hour. 

3) Once you have your DC amps per hour, you need to include the power loss through the inversion, which is about 10%.  Generally you should just multiply your DC amps per hour by 1.1 and this will give you the right amount in your final DC amps required total.

Amps AC (your load) x Volts AC = Watts

Watts/Voltage DC = Amps DC per hour required

Amps DC per hour  x  1.1 = actual Amps DC

What size inverter do I need?

Inverter sizing depends on the type of device being powered.  Device start-up or surge should always be considered.  For example, motors and compressors have start-up surges.  It’s generally a good idea to oversize an inverter.

What type of batteries should I use?

Type of battery being used primarily depends on how often and the amount of time you will be running your system. In general, deep cycle marine/RV AGM batteries and sealed batteries and are the best.

How much power can I get from my cigarette lighter?  

DC cigarette lighter adapters commonly used in vehicles average 150 watts to 180 watts, UL recommended rating for a cigarette outlet is 80 watts max; This is because the wiring within the port is small, as well as, fused at 15 amps DC.

How do I wire batteries together?  What is series vs. parallel?

Correct wiring of your battery bank is important.  When your batteries are not wired correctly, your system may not work and/or permanent damage could occur.  So, always remember that series increases the voltage, and parallel increases the amps. Series is achieved by connecting positive of battery one to negative of battery two.  Parallel is achieved by connecting batteries positive to positive & negative to negative.

What size cables should I use?

Cable sizing is determined by the size of your inverter.  National Electric Code standards are strongly suggested when wiring a system.  Recommended sizing is as follows; #4 gauge is not to exceed 157 amps DC (approximately 1500 watts), 1/0 is not to exceed 291 amps (approximately 3500 watts), and 4/0 is not to exceed 454 amps (approximately 5000 watts.)  These ratings are based on a 12 volt system.

How far should inverter be from my batteries?

Inverters should be safely mounted near to the batteries powering the unit as possible; therefore, 10-12 ft. should not be exceeded without sizing the gauge of your cables to the next available size up.

Will my car/truck electrical system handle an inverter?  If so, what size?

Vehicle electrical systems are typically designed to be maintained by an alternator, only using the battery for short bursts of power to get the motor started.  So, small systems will work fine via the electrical system in your vehicle. Anything that exceeds the amount of power that the vehicle can produce should have an auxiliary system which is charged separately.

Should inverters have high and low voltage shutdown?

Absolutely! Low voltage shutdown should be 10.5vDC on all inverters.  And hi voltage shutdown varies between 15-17 volts depending on the specific inverter. 

Tips –

  • Never leave an inverter connected to a line where other power (vAC) may feed into inverter.  It doesn’t matter if inverter is on or off.  Unless your inverter is a grid tie inverter.
  • Using inverters in moist areas is a common cause for their drivers to burn out.  Even an outdoor extension cord that was lying in a puddle for a day may have enough moisture to burn out the drivers of the inverter.
  • If allow cords that may have gotten wet to dry a few days before using with the inverter again
  • Always prevent foreign objects from entering inverter through the vent or fan openings
  • Keep cables between inverter and batteries as short as possible.  This will help your batteries perform their best.
  • Always use sealed batteries if you want to keep the inverter close to the battery bank.
  • Don’t over tighten nuts to battery or inverter, but do check them occasionally to ensure they are snug.
  • If the cables between your battery and inverter get hot while under heavy load, then you should consider using next size larger cables.
  • Both “hot” and “neutral” lines are hot on many inverters.  Do not ground the neutral line.  Do not connect neutral to panels that may have neutral grounded unless you are sure that the input and outputs of inverter are isolated.
  • Using a 12vDC inverter, your input Voltage should never exceed 15vDC (17vDC for some models) or the inverter may shut down with Over Temp and you should hear an audible beep.  This may cause permanent damage to an inverter.
  • When selecting an inverter, try to buy one that will stay in the continuous operating range and do not rely on advertised surges.  This will help your inverter last longer and operate more efficiently.
  • Standard 110vAC outlets are typically rated at 1500W or 15Amps.  This is for heat dissipation through the outlet and is typically not a limitation of the inverter.
  • If cables need to be run, it is best to keep inverter as close as possible to your sealed deep cycle batteries.  Use extension cords on the output side (AC output) rather than extending the DC cables.
  • Ham operators or other preppers often need to mount the inverter in the back of their trucks.  In a case like this it would be best to mount a secondary battery near the inverter and wire it to the primary truck battery.
  • An inline fuse is recommended in any inverter system, to protect your batteries from a ground fault fire.
  • AC plug testers do not work with most inverters, you will see an open ground.
  • Be aware of lightning storms.  If struck, inverter could go to a permanent overload state and may even smoke.
  • AC extension cords should not exceed 100 feet or you will have a voltage drop and not push enough power down the line.
  • Inverters emit RF and may cause interference.  This is noticeable in AM radio and often monitors and computer mice.
  • Never parallel multiple inverter outputs, this will generally burn the AC drivers out.
  • If the inverter does not turn on, or has a constant beeping sound, verify you have the proper input voltage supplied to it.  Also ensure the battery connections are clean and secure.
  • Never start a vehicle with inverter turned on, this will cause eventual and permanent damage to inverters.
  • If you do run cables from battery to inverter that have any chance of shorting to a chassis or ground, it is best to fuse the + battery cable nearest to the battery as possible.

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