How To Size An RV Power Inverter

General Information on Starting Power and Running Power

When sizing an inverter, you need to take into consideration both the Running Power and the additional Starting Surge Power requirements of the AC side loads.

Some loads require much higher Starting Surge Power for the first few seconds before settling down at the Running Power. The inverter should be sized on the basis of the Starting Surge Power of the AC side load. The total starting Surge Power is equal to the normal Running Power plus the Additional Starting Power. The following symbols and equation will be used for explanation purposes:

  • Running Power, Watts - Wr
  • Additional Starting Power or Additional Continuous Power, Watts - Wa
  • Total Starting Surge Power, Watts = Running Power (Wr) + Additional Starting Power / Additional Continuous Power (Wa)

The specifications of inverters may include a rating termed Surge Power. Normally, the duration of this Surge Power rating is not quantified in the specifications and is assumed to last for less than 1 second. Unless the Surge Power Rating of the inverter is specified to last for > 5 seconds, this Surge Power specification of the inverter should not be considered for purposes of sizing loads with high and prolonged starting surge power. Only the Continuous Power rating of the inverter should be considered in such cases.

Electric motor driven compressor / pump based loads like refrigerators / freezers, air conditioners, air compressors, sump and well pumps etc require a very high Starting Surge Power of around 5 to 7 times the Running Power for the initial start up. This Starting Surge Power may last for 2 to 5 seconds. This Starting Surge Power is required to provide for the Locked Rotor Current of the motor and the extra effort required to move the piston of the compressor /pump if it was positioned just below the Top Dead Center or just above the Bottom Dead Center and also to over come the back pressure in the condenser / discharge line. The Additional Starting Power Wa should be taken as 4 times the Running Power Wr.

Switched Mode Power Supplies (SMPS) that do not have Power Factor correction draw a non linear, pulsing current with a high Crest Factor (Ratio of instantaneous value and the RMS value) that may reach a value of 4. Also, the effective Power Factor of these types of power supplies is very low around 0.6. If the inverter is sized based on the Running Power of these units, the inverter will be continuously overloaded from instant to instant due to the higher peaks of the pulsing current. The inverter for these units should be sized at 3 times the Running Power of the power supply. Alternatively, for sizing purposes, the Additional Continuous Power Wa should be taken as 2 times the Running Power Wr. Consumer electronics like computers, TV, VCR, audio / video devices etc use this type of power supply. (Please note that this sizing is not applicable if the SMPS has Power Factor correction and the Power Factor is around 0.9).

Motors used in washing machines have to start under load due to the weight of the clothes, water and the drum. Hence, the Additional Starting Power Wa should be taken as 2 times the Running Power Wr. This starting power normally lasts from 2 to 5 seconds.

The Watt rating of the microwaves normally indicates output cooking power in Watts. As the microwave is not an efficient device, the AC input power in Watts should be taken as 2 times the output cooking power in Watts i.e. the Additional Continuous Power Wa should be taken as 1 time the Running Power Wr. Hence, a microwave with 900 Watt cooking power will require an 1800 Watt inverter. (If the power rating of the inverter is indicated as the AC input power in Watts, the factor of 2 is not applicable).

Using the Inverter Sizing Tool

The Inverter Sizing Tool given below helps in sizing the appropriate capacity of the inverter based on the loads and also provides recommendation for an appropriate Kit consisting of an Inverter, Solar Panel and Charge Controller. The following symbols and equations are used:

  • Total Running Power of the items selected, Watts - Wrt
  • Highest Additional Starting Power out of the items selected, Watts - Wah
  • *Highest power for sizing purpose, Watts - Wrt + Wah

* It is assumed that only one out of the selected items starts at one time

Since the AC loads rarely start at the same time, the load with the highest Additional Starting Power Wah has been factored in.

The Tool will indicate recommended Inverter Kit based on the following data entry and logical sizing calculations:

  1. From the list of typical loads provided, select and enter the quantity of loads you wish to power at the same time.
  2. The logic will first add the Running Power Wr of all the selected items  let it be denoted as Wrt. It will then pick the highest Additional Starting Power out of the selected items  let it be denoted as Wah. Finally, it will add Wrt and Wah to arrive at the highest power to enable selection of the closest inverter. The kit will be based on the Model of the inverter that has been selected.

Calculation Example

Device / Appliance Running Power, Wr, Watts Additional
Starting Power or Additional Continuous Power *, Wa, Watts
Quantity Extended
Running Power, Wt, Watts
Refrigerator - Electric Compressor based 200 800 1 200
Color TV 225 450 1 225
Lap Top Adapter 120 240 1 120
Incandescent Lamps 60 0 6 360

* Additional Continuous Power is applicable to SMPS based loads that have no Power Factor Correction.

Total Extended Running Power, Wrt = 200 + 225 + 120 + 360 = 905 Watts

Highest Additional Starting Power, Wah = 800 Watts

Highest Power for sizing the inverter = Total Running Power, Wrt(905 W) + Highest Additional Starting Power, Wah (800 W) = 1705 Watts