What is a kWh - especially when solar panels are involved?

To understand what a kWh is, you first need to understand what a Watt is. Basically, a “watt” (W) is a measurement of power. Mathematically: 1 watt = 1 volt * 1 ampere

A Kilowatt (kW) is a standard unit of electrical power equal to 1,000 watts, or to the energy consumption at a rate of 1,000 joules per second. A megawatt (MW) is one million watts.



Interesting, but what does it really mean? Let's use the analogy of water. Voltage is the pressure (e.g., water pressure), and amperage is the flow. Imagine a faucet in your house: usually the water pressure is low * and the pipe size is fairly small = normal amount of power. Now imagine a fire hose: typically the water is under high pressure * larger pipe size = much more power.

So what is a Killowatt hour (kWh)? Actually, it doesn't make much sense technically - since a watt is already a measurement of joules per second … but since nobody remembers what a joule is, the utility company decided to use the term kilowatt hours.

The best way to think about a kWh is that it's a measure of how much electrical power (as measured by an electrical meter) is used in one hour. For example, if you had ten 100 watt light bulbs turned on for one hour - you would use up 1 kWh.

In any case your electrical company puts in a meter (which is not always accurate by the way) and send you a bill based on how many kWh you use up during their billing period, plus whatever extras they tack on depending on where you are.

Reducing Your Electrical Bill

There are several ways to reduce your electrical bill, which are (or will be) covered in other sections of this web site, but let me just summarize these briefly for you now. Basically, there are basically two approaches … reduce the amount you spend, and generate more money to offset your electrical bill. More specifically,

1. Location: Electrical rates vary according to where you live. It's kind of like gasoline prices - which are a function of the cost of oil, refinery costs, transportation, distribution, taxes, etc. Electricity costs are a function of how the electricity is produced (e.g., electricity from fossil fuels continues to increase because oil, gas, and coal prices increase), applicable regulations, government support / taxes, and a whole bunch of other costs I get into in the section on Grids. I doubt if you chose where to live based on electrical rates -- but if you are a big power user (like a producer of silicon panels), utility rates are an important factor in locating where to build your plant.

However, for most people you usually have only two choices for electricity: buy it from the utility provider that has a monopoly to sell electricity you live in, or produce it yourself.

2. Long Term Conservation: By this I mean … choosing a home / office / building that is energy efficient, or that at least uses energy efficient appliances. According to the National Board of Realtors in the USA … energy efficient houses sell for a premium over homes with higher utility bills. In fact, a study done indicates that the difference is around 20 times the amount of the energy savings (another good reason to consider going solar).

3. Daily maintenance: Besides turning off lights - there are lots of things you can do to lower your electrical bill. For example … did you know that 75% of the power used by consumer electronics happens while the equipment is "turned off" -- which means in reality it's in the standby mode. These "phantom loads" occur in televisions, stereos, computers, DVD's, and kitchen appliances. To find phantom loads turn off all lights at night and look for any LEDs or other “glows” in the house. Keep in mind that any device that requires resetting after a blackout or power surge generates a phantom load.

Here are 65 other great tips on how to save on your electrical bill that were researched and prepared by the Long Island Power Authority.

4. Generate Electricity: This can be done a number of ways. Since this site is dedicated to information about solar energy - I'm going to limit my discussion to solar solutions. Basically, as we've covered (or will cover) elsewhere in this site - solar energy can be used to produce heat or electricity. Cutting your heating bill will reduce your electrical bill (especially if you have an electric hot water heater installed).

Photovoltaic cells can be used in a variety of ways to produce electricity. Basically, the more photoelectric cells you use the more electricity you will generate. One of the advantages of solar energy is that solar energy products are scalable and in some cases portable (e.g., solar power generators, solar battery chargers, etc.).

Estimating kWh from a Solar Panel

To estimate how many kWh of electricity can be produced from a solar panel, you first need to know some basic information:

1. kWp: What is the kWp (sometimes called Peak Power or Pmax) of the panel(s) you plan to use?

2. Adjustment: Under the standard testing conditions, the sunlight is assumed to be perpendicular to the panel. Typically, panels are mounted at an angle to optimize the amount of sunlight striking the panels. However, if the angle is not optimized, this can lower the amount of electricity produced. Similarily, shade falling on the panels will also reduce the amount of electricity produced.

You should also anticipate that there will be some electricity lost between the panel and when it is delivered to the grid. For example, some electricity will be lost when the DC is converted to AC by the inverter (inverters are around 95% efficient). additionally if the panels are exposed to a lot of heat (e.g., in a very sunny area this can decrease the amount of electricity produced by 3 to 4% at temperatures over 35˚ C and up to 12% to 14% for temperatures above 50˚ C. Also as the panels age they will generally lose around 0.5% efficiency per year.

3. Insolation: How much sunshine does your area receive per year (assuming no shade)? Please note, in the equation below we use the annual insolation level. After considering the above variables, the basic guideline to calculate kWh that a panel will produce over a full years is as follows:

estimated kWh = kWp * adjustment * insolation/1000

Where 1,000 is the standard insolation used when calculating Peak Power produced by the panel under standard testing conditions.

Click on the appropriate link to return to the top of this page about kWH or to return to our overview about Solar Cells.