The size of a panel is referred to by its watts (W), which is a measurement of how much power it can produce.
The size of a system is generally referred to in kilowatts (kW), meaning 1,000 watts.
Your utility company bills you for your kilowatt-hours usage. This is a way to measure energy over time. Your solar system will produce a certain number of kilowatt-hours per day or per year. The kWh figure we refer to most often is your annual use. Talking in yearly terms includes and averages out the changes over the course of the year. In general, we try to get a solar system’s annual production to match your annual use.
During the year the energy you produce is worth exactly what you pay for it, but when your utility pays for the annual excess, they pay you based on a lower wholesale rate.