# PV Student

Solar Education Resource

Power v. Energy,

what is the difference?

Where is the W? The W is in Watts and poWer, so they are the same.

A power company does not sell power, it sells energy, which is power x time & time is money.

Let me clarify watts & kilowatt hours.

To use a light-bulb analogy, you can buy a 100 watt light-bulb.

If you leave it on for 10 hours, that is a kilowatt hour (kWh).

If you have a dimmer switch and have it consuming 50 watts, it would take 20 hours to consume a kWh.

PV works like a dimmer switch with the solar energy rarely giving you 100% of the PV rated watts.

That is why you can have the sun in the sky for 12 hours and only get 4.2 peak sun hours per day of energy.

Power is measured in watts.

1 amp x 1 volt = 1 watt

Energy is measured in watt hours or kilowatt hours, (this is where it is temporal).

The utility sells energy which is measured in kWhs

Another analogy is speed and miles.

watt : speed

kilowatt hours: distance

(speed x time = distance)

Another thing is that you can buy a car that is rated at 100 mph, but you will probably average a slower speed.

A lot of people like to use the water analogy.

Pressure is like volts.

Current is like the hose diameter.

You can have a certain pressure and hose diameter and that would be your maximum flow-rate, which is analogous to watts.

If you open a valve, it can never exceed this flow rate, but you can open the valve half-way and you will have half the flow-rate.

If you open the valve for an hour, you will have released a certain volume of water & that volume would be analogous to energy or kWhs.

1 joule per sec. would be a rate, which would convert to watts, which is a rate.

Rate times time = quantity.

1 Joule = .0002778 watt hours = quantity of energy

1000 watt hours = 1 kWh