I recently read an article in a well-known consumer-facing technical hobby magazine, where the author used the phrase, “watts per day.” This is an erroneous concept (the technically-trained author should have known better), which illustrates a matter that is often confusing to the average person.
“Watts per day” is not a meaningful unit of measurement.
The watt (W) is a unit of power, which is defined as the rate at which energy is transferred or used. One watt is equal to one joule per second (J/s).
The unit of energy is typically measured in joules (J), but it is also commonly measured in watt-hours (Wh) or kilowatt-hours (kWh). One watt-hour is equal to one watt of power used for one hour, and one kilowatt-hour is equal to one kilowatt (1000 watts) of power used for one hour.
In other words, the “watt” conveys the notion of the rate of energy consumption. When we pay for energy consumed, we pay for kilowatt-hours.
Therefore, if you see the phrase “watts per day,” it is likely that the author was trying to convey the notion of power used over a period of one day, and the correct usage would be to express it in terms of watt-hours (or kilowatt-hours) per day.
As an example: if a piece of equipment is rated at 100 watts, and the equipment is used for 8 hours every day, then we can say that the equipment consumes 800 watt-hours of energy (not power) per day.
Additional reading: why “RMS watts” makes no sense!