Watt is a unit for the rate of energy consumed (in the case of appliance).
If a 100-watt device is on for 10 hours, it will consume (100) X (10) = 1000 watt-hours of energy.
This is more commonly denoted as 1 kilowatt-hour (kWh). Your electric meter, like a cash register, measures the number of kilowatt-hours you consume each month. And you pay your electric supplier a rate based on $/kWh. The national average may be on the order of $0.10/kWh.
So for every 10 hours your 100-Watt appliance is running, it will cost you a whopping 10 cents.
An electrician would think more in terms of 'amps', which is a unit of current.
An appliance using 100wattts would be powered by 120 Volts; and the current draw would be 100/120, or 0.8 amps.