2015-01-27 14:44:03 -0500
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First of all, let's make clear the difference between watts (or kilowatts) and watt-hours (or kilowatt-hours). Generally, when you're talking about watts (W) in the context of electrical power, you're talking about instantaneous power generated or consumed by a given device: How much power is being generated or used at a given moment? Electrical equipment is usually rated in watts or kilowatts (kW: 1kW = 1,000W).
The total amount of electricity consumed or produced over time is measured using watt-hours (Wh) or kilowatt-hours (kWh: 1kWh = 1,000Wh). When you get your electricity bill, you'll see that you are billed for each kWh of electricity that you've consumed, or credited for each kWh of solar electricity that you send into the electrical grid.
For example, a 4,000 watt (or 4 kilowatt, 4kW) vacuum cleaner uses 4kW of electricity when you have it turned on. If you run it for an hour, you will have used 4,000 watt-hours (or 4 kilowatt-hours, 4kWh) by the end of that hour. Similarly, if a 300W (0.3kW) solar panel generates power in full sunshine for an hour, by the end of that hour it will have generated 300 watt-hours (or 0.3 kilowatt-hours, 0.3kWh) of electricity.
But solar panels don't generate the same amount of electricity all throughout the day - in the mornings and evenings (when the sun is low in the sky) they will generate less power than in the afternoon (when the sun is shining on them directly). This is why you'll sometimes hear 'watts peak' (Wp) used instead of watts when someone is talking about a solar panel's capacity. 300 watts (300W or 300Wp) refers to the panel's peak capacity: how much electricity will the panel produce in perfect weather conditions while pointed directly at the sun? (Read more about solar panel tilt here.)
In order to determine how much an individual panel (or more likely, a whole solar energy system) will generate in a day, you have to look at the total amount of sunshine that you can expect for the area you live in and the season that you're in. Your panels will generate more power in summer than in winter, simply because there is more sun during the summer; the numbers in the table below (5th column, 'Solar Radiation') are annual averages of available sunshine for different cities.
As we mentioned, sunshine is stronger in afternoon than in the morning or evening. So when we talk about solar irradiation, we 'compress' the total amount of sunshine into easy-to-understand units. Your 300W panel may only generate 150W in the morning, but this number will rise steadily as the sun gets higher in the sky. The panel's output will get up to around 300W for 2-3 hours around noontime, and then will start to drop off again as the sun gets lower in the sky. The figures in the 'solar radiation' column below take this fact into account.
So if your have your 300W solar panel in New York (where solar irradiation = 4.49kWh on average annually according to the table), it will generate (4.49kWh x 0.3kW =) 1.347kWh for the day.
Of course, this math also applies to a whole solar panel system: a 6kW solar system in New York, for example, would generate (4.49kWh x 6kW =) 26.94kWh on average daily. But you'll also need to keep in mind that there are always certain inefficiencies in a larger system (inverter efficiency, etc), meaning that the total power produced will be slightly less than this figure (usually by around 80%).
We hope this clears things up for you.
You may also want to check out another article we put together about the impact of electricity prices on solar system payback and return on investment: "Impact of electricity rates on the economics of a solar panel system".