These are just my thoughts on an "energy efficient" gaming rig:
Look for a CPU with a relatively lower TDP but enough performance to avoid bottlenecking the GPU of your choice. No overclocking the CPU. Get a low-current CPU cooler, lower amp draw than the stock cooler. Get a low-voltage RAM of ample speed and capacity. Get low-current/amp case fans. Hunt for a GPU with lower TDP, lesser supplemental PCIE power connectors (better if none), but will still benefit from the monitor resolution you would be gaming. Avoid LED lights in any of your components. Purchase a high-quality and high-efficiency PSU that is double the wattage of your calculated rig power draw (at 50% usage 220VAC = highest efficiency curve). Get a monitor with low power consumption or those with "power-saving features". Scrap the HDD; get an SSD instead (if you can do away with just 1 drive, the lower the power consumption). An mATX or mini-ITX board also features less connections/ports (if ATX, avoid plugging in other accessories). Disable all unnecessary background programs.