Inverters convert power from a DC source to AC output. The input can be from a 12 volt, 24 volt, or 48 volt battery bank, and output can be 110-120 VAC, or 220-240 VAC. There are modified sine and pure sine (true sine) versions in various wattages. A modified sine unit approximates the smooth wave form of household AC (alternating current) with a stepped form. This works for a lot of applications, though lower efficiency or higher heat generation may result. Modified sine inverters are widely available, and considerably less expensive than pure sine units of the same wattage. Pure sine units produce a precise output wave, which can even be more ideal than utility grid power, since grid power can fluctuate in frequency and voltage, sometimes considerably. Pure sine units operate equipment more efficiently, motors run cooler, etc. Electronic applications, computers, and particularly video applications are prime applications for pure sine inverters.

Sizing the Inverter

  Inverters are rated by running wattage, and surge wattage. Surge wattage is rated for a given time period (in seconds), and the inverter will shut down, if this is exceeded. A realistic assessment must be made of the total running wattage, as well as surge wattage of those devices which may affect the total. Motors, for example, can have an amp surge up to 3 or 4 times their rated running load upon startup. See the chart that follows. Most devices have an amp draw or wattage on the name plate or in the owner's manual. The inefficiency of inverters (about 10 to 20%) must also be considered, both in the sizing of the inverter, and the battery bank.

Type of Device Surge Factor for Determining the or Appliance Continuous *Wattage of the Inverter (No. of times the running power rating of the device/appliance)
Refrigerator / Freezer5
Air Compressors4
Automatic Washer3
Sump Pump3
Furnace Fan3
Industrial Motors3
Kerosene/diesel fuel heater2
Circular Saw3

Sizing the Battery Bank

An inverter usually will require deep cycle lead acid batteries of appropriate capacity. Battery bank sizing will be based on running wattage over a length of time, before battery recharging can be effected. In solar or wind applications, power generation may be occurring during power consumption, resulting in a net gain or loss to the battery pack at any given time. The unit of battery storage capacity is the amp hour (ah), and the AC wattage must be converted to DC amps and multiplied by required hours to arrive at the number of amp hours of battery capacity required.

An electrical formula frequently used for DC system calculations:

V x A = W (Volts times Amps equals Watts)  or, W / V = A (Watts divided by Volts equals Amps)

An example:
You have a 500 watt AC running load, and you need it for 4 hours a day. Dividing the 500 watts by 12 volts of the DC side equals 41.6 DC amps of current draw. Allowing for the inverter inefficiency (let's use 20%), gives 41.6 x 1.2 = 49.99 amps. A simpler method is to divide the AC watts by 10 (incorporating the inefficiency). In this case, 500 watts divided by 10 equals 50 amps. Multiplying the 50 amps by the desired 4 hour run time gives 200 amp hours discharged from the battery pack. For a 50% discharge cycle of the battery pack, you would use a 400 amp battery pack (actually, you would need more than 400 amp hours, because of the accelerated discharge rate). For longer battery life, a 25 % discharge of the pack would be helpful, in this case indicating an 800 amp hour pack (once again, actually more than 800). See the tutorial on batteries for information on depth of discharge, cycle life, temperature effects on capacity, rate of discharge, etc.

Below is a chart of some typical battery sizes applicable for powering inverters:

  BCI*Group Battery Voltage, V           Battery AH 31 12 105 4D 12 200 8D                   12 245 GC2 (Golf Cart) 6 220   * Battery Council International

Inverters in vehicle applications
Excepting small electronic applications in autos, it is recommended that for powering the inverter, one or more auxiliary deep cycle batteries should be used that are separate from the SLI (Starting/Lighting/Ignition) batteries. The inverter should be powered from the deep cycle batteries. For charging the SLI and the auxiliary deep cycle batteries, the output from the alternator should be fed to these two sets of batteries through a battery isolator of appropriate capacity. The battery isolator is a solid state electronic circuit that will allow the alternator to charge the two sets of batteries when the engine is running. The isolator will allow the inverter to be operated from the auxiliary batteries and also prevents any draw on the auxiliary batteries from seeing the SLI batteries. Battery isolators are available from auto / RV / marine parts suppliers.

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