With an off-grid system, if an appliance supports powering via both 12V DC and 240V AC, is there any difference with the power efficiency of either method excluding the draw of an inverter used to power via 240V AC?
And how does the power efficiency compare when the draw of an inverter is taken into account?
First question regarding the power consumption of some arbitrary device that accepts 12VDC input or 240VAC input cannot be known without making measurements of the current draw from the source and making the necessary calculations. There is no way to know how a black box device converts either input to its required operating point and how the efficiency of the methods will vary.
The use of an external inverter to convert 12VDC to 240VAC will not be a 100% efficient process. Once again it would not be possible to really know the inverter efficiency without measurements and calculations.
Once you knew the efficiency of the inverter you could factor that into the differences of power usage by the device that you have measured. But it can be said that if the device itself consumes less total power when operating at 240VAC versus when operating at 12VDC then there may be the possibility that using the external inverter could be more efficient overall.
Many small Appliances run their internals of a low DC voltage .Sometimes this voltage is 12.The appliance has an internal Power supply ,usually switchmode these days .This means that there are inverter losses and powersupply losses to factor in .Small flyback SMPS can be only 70% efficient .Large invertors often have poor partload efficiency and reduced efficiency when operating into poor power factors .It is possible that your overall losses will exceed 50% .
