Understanding Inverter Output Power Ratings

Inverter output ratings are traditionally defined in terms of Apparent Power in VA (Volt Amps) & Power Factor. Apparent Power is the product of the maximum output volts & amps available under the most favourable operating conditions for the inverter. This maximum operating point is defined by the Power Factor; the Real Power available in watts is the maximum VA multiplied by this said Power Factor. Real power is the effective power value as if the V & A were in phase & the PF =1 as would be the case if the load was an electric fire bar or a fully power factor corrected power supply.

Probably one of the most basic definitions of power factor is that it equals the cosine of the phase angle between the voltage & current. This is difficult to conceptualize in terms of the input to a non PF corrected power supply because besides the current phase leading the voltage its whole waveshape changes from a sine wave to a series of current peaks. Both effects are due to the large storage capacitance following the input rectifier. This greatly complicates the issue but plays a large part in the reason for the traditional rating method for an inverter.

In many older non corrected power supplies the power factor may be considerably less than 1, say down to 0.6. Even today there are many smaller power computer/server power supplies which only have passive PF correction giving power factors in the region of 0.8-0.9.

It is very important to ensure the inverter is sufficiently rated to drive the load. Consider an inverter having a VA rating of 1000 & power factor of 0.8 giving a real power limit of 800W. Neither the VA rating nor the real power watts rating can be exceeded. If the load PF is greater than 0.8 then the power limits remain as previously defined, however, if the load PF is less than 0.8 the real power limit will be further reduced.

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