What is the Meaning of a 250 Watt Panel

[Prince]

When a solar panel says it produces 250 watts what does that mean?

250w per hour, per day? What?

[J.P.M.]

That's the output of the panel when its connected to a circuit, its temp. is 25 deg. C., and it is pointed directly at a radiant source of energy whose intensity is 1,000 W/m^2 which loosely approximates the radiant intensity of a full sun under clear skies at mid day. It is used as a comparison among different types of panels, sort of the way EPA fuel economy #'s can be used to compare gas mileage of different vehicles.

For real life estimates of panel output or system performance, see PVWatts on the net. It's a bit more involved but manageable once you spend a few minutes w/the help/info screens and do a few practice runs.

[Prince]

Perfect!

Thank you

[J.P.M.]

Perfect!

Thank you

You're welcome.

On further thought, it is probably a better, but still somewhat loose analogy to compare the S.T.C. (Standard Test Conditions) output of a solar panel to the horsepower rating of an engine in a vehicle, as both are an approximation or a measure of the max. output of either an internal combustion engine or a solar panel.

[DanKegel]

On further thought, it is probably a better, but still somewhat loose analogy to compare the S.T.C. (Standard Test Conditions) output of a solar panel to the horsepower rating of an engine in a vehicle, as both are an approximation or a measure of the max. output of either an internal combustion engine or a solar panel.

It's a good analogy.

Very roughly and ignoring inefficiencies:
Three "250W" solar panels generate about 600 watts at noon, or a little less than one horsepower
Three "250W" solar panels generate about 3.5kWh/day, or enough to drive a Leaf about 10 miles, or about an tenth of a gallon of gasoline

(assuming
1 horsepower = 746 watts = 0.746 kW
1 gallon gasoline = 33,400 watt-hours = 33.4 kWh
kWh per solar panel kW on a sunny day: ~5
Nissan Leaf EPA mileage = 30 kWh per 100 miles )

[J.P.M.]

It's a good analogy.

Very roughly and ignoring inefficiencies:
Three "250W" solar panels generate about 600 watts at noon, or a little less than one horsepower
Three "250W" solar panels generate about 3.5kWh/day, or enough to drive a Leaf about 10 miles, or about an tenth of a gallon of gasoline

(assuming
1 horsepower = 746 watts = 0.746 kW
1 gallon gasoline = 33,400 watt-hours = 33.4 kWh
kWh per solar panel kW on a sunny day: ~5
Nissan Leaf EPA mileage = 30 kWh per 100 miles )

???

[inetdog]

When a solar panel says it produces 250 watts what does that mean?

250w per hour, per day? What?

Just to nail this definition down:

Power is the rate at which energy is delivered. The amount of energy is measured in watt-hours, the amount of energy a one watt load or source consumes or provides if delivering at that rate for one hour.
The classic analogy of water flow compares watts to gallons per hour and watt-hours to total gallons over some time period.

You can then talk about how many watt-hours a panel will produce over the course of one day, and that will depend on how much total light the panel gets during the day.
Since the morning and evening sun shines less light on the panels and at a less useful angle, the number of "sun hours" (obtained by dividing the total watt hours be the watt rating of the panel) in a day will be much less than the number of hours of daylight.
In middle latitude US you will get 4-5 sun hours each summer day. Much less in winter.

[DanKegel]

Seems like this ought to be in an FAQ / sticky somewhere if it isn't already!