windows 10 desktop - does sleep mode use a lot of energy?

Sleep mode will keep the RAM and usually all of your USB devices powered. The USB devices will either go into a sleep mode as well, or continue operating. For example, my USB mouse will turn its lights off when my computer sleeps, but I can still press a mouse button to wake the PC.

All the power hungry parts of your PC will stop functioning under sleep, and they also do so in hibernation. Hibernation will also shut down the RAM and depending on the computer, your USB devices may receive power.

I tested my computer for sleep power consumption - 0.047A at 251VAC which means about 12 watts. For comparison, that's probably two phone chargers worth of power. If you pay 16 cents per kWh, that's 15 dollars per year (if you left your PC in sleep for the whole year).

However, I put my computer into hibernate mode and the power consumption dropped by 1 watt (compared to sleep). Having the RAM powered in sleep state is not expensive, as the RAM is not in active use - only the memory has to be retained.