300 watts to kWh per day and month

7.20 kWh/day

for 300 W running 24/7 · 216 kWh/month · $35.64/month at 16.5¢

300 W by hours of use

Use	Per day	Per month	Cost/month
1 h/day	0.30 kWh	9.0 kWh	$1.49
2 h/day	0.60 kWh	18.0 kWh	$2.97
4 h/day	1.20 kWh	36.0 kWh	$5.94
8 h/day	2.40 kWh	72.0 kWh	$11.88
12 h/day	3.60 kWh	108.0 kWh	$17.82
24 h/day	7.20 kWh	216.0 kWh	$35.64

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

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