100 watts to kWh per day and month

2.40 kWh/day

for 100 W running 24/7 · 72 kWh/month · $11.88/month at 16.5¢

100 W by hours of use

Use	Per day	Per month	Cost/month
1 h/day	0.10 kWh	3.0 kWh	$0.49
2 h/day	0.20 kWh	6.0 kWh	$0.99
4 h/day	0.40 kWh	12.0 kWh	$1.98
8 h/day	0.80 kWh	24.0 kWh	$3.96
12 h/day	1.20 kWh	36.0 kWh	$5.94
24 h/day	2.40 kWh	72.0 kWh	$11.88

kWh = W × hours ÷ 1000. Devices with thermostats or compressors cycle, so actual use is often lower than the rated wattage × hours.

2.40 kWh running 24 hours, 0.80 kWh at 8 hours.