HP has not conducted a side by side comparison of energy use simplex to duplex.  The estimate is that it may take marginally more electricity to print duplex, while importantly saving a sheet of paper.

HP studies have shown that the primary use phase impact of printing is paper use (and all the resource consumption and energy consumption that goes into producing paper). Therefore, duplexing would be our suggestion if you are sensitive to environmental impacts.

The energy consumption may vary depending on the duplex efficiency of the device.

For most printers, enabling duplex printing generally reduces the printing speed. As a result, although the duplex motor itself may slightly increase power consumption, the slower print speed can offset this effect, and in some cases the overall energy consumption during duplex printing may actually be similar or even lower.

Note:    The following information in red is for the reader’s use only and is not intended for proactive use

However, there are also devices with 100% duplex efficiency. For example, the A3 MX7 Series (Jasper/Moonstone) has a default duplex efficiency of 100%, which means there is no reduction in print speed when printing in duplex mode. 

In this case, the energy consumption may increase slightly due to the additional operation of the duplex motor.

For reference, the MX4 Series (Citrine/Pearl) has a default duplex efficiency of approximately 70–80%. To achieve 100% duplex efficiency, an accessory called the “2nd Exit” needs to be installed.