How much electricity does a solar panel
produce? How many panels do you need? Find out how to calculate solar production and your true energy needs.
Solar power is produced by photovoltaic panels that convert sunlight to electricity. The power that a solar panel can produce is based on its size and material, on its orientation to the sun and on the amount of sunlight it receives.
How much energy does a solar panel produce?
When a solar panel is manufactured, it is tested under Standard Test Conditions (STC) which are equivalent to full sun at sea level at 25 degrees Celsius. The panel is then rated for the number of watts it produces under those test conditions. For example, a typical solar panel for a home system might produce 200 Watts. Five of these panels would then produce 1000 Watt, or one kiloWatt. When the sun shines on the one kW array for one hour, it will have produced about one kWh of electricity. If you have the equivalent of six hours of full sun in a day the array will have produced six kWh. This is the amount of energy you now have available to operate your electrical appliances.
The number of kWh you can expect the solar array to produce in a typical day, month or year in your area depends on the amount of sunshine you receive. Days vary in length with the seasons. Some days are cloudy. On overcast days the panels will only produce about 20 - 60% of their full rating, depending on the thickness of the cloud cover. These variations are taken into account by using climate data that provides the average solar radiation per day throughout the year. Just multiply the solar radiation by the panel rating to determine how much energy you can expect.
How much electricity do you use?
The number of solar panels that you need depends on the amount of electricity that you use. Just like solar production, this is measured in kWh. If you are already connected to an electrical utility provider you will be able to find this information on the utility bill. Look for the kWh delivered between actual readings and calculate your monthly usage from the dates given.
If you are planning to use a standalone (off-grid) solar power system you will have to calculate the power you will need. Here is how it works. Multiply the power (in kWh) used by one of your appliances by the number of hours per day that you use it - this gives you the kWh for that appliance for one day. Do this for everything that uses electricity (including pumps and mechanical equipment) and add them all up. This will give you the total number of kWh per day. This calculation is called a load analysis.
How large of a system do you need?
Let's say that you are using 400 kWh per month. If a one-kW solar array in your climate conditions would produce an average of about 100 kWh per month, you would need a four-kW array. Converting light to electricity is not 100% efficient - the direct current electricity produced by the panels must be converted to the alternating current used by household appliances. This is done by an inverter. Most inverters are about 90% efficient, so your solar array should be large enough to offset this 10% loss.
How much space does this take? Each kW of solar panels requires roughly 100 square feet of roof space. For best production, the panels should be tilted towards the equator at an angle approximately equal to your latitude. If you don't have enough suitable roof space, you can use pole mounts, ground mounts or trackers.
Do you really need to use all that power?
Solar is expensive. Wouldn't it make sense to use less energy and buy fewer panels? This doesn't mean giving up something that is important to you. It is surprising how much energy is wasted on things you are not using and wouldn't miss if they were shut off.
A 60-Watt light bulb, left on for 24 hours uses 60 W x 24 hrs = 1.44 kWh per day
An energy efficient fridge uses about 400 kWh/year = 1.1 kWh per day
One light bulb uses more energy in a day than the fridge! This really brings home the advantage of using energy efficient bulbs and shutting off lights and other appliances when they are not in use.
There are also 'phantom loads' that are not readily apparent but whose power use will add up over the day. For example, anything that has a remote control (like the TV or stereo) draws some power at all times so that the remote will be ready to turn it on at any time. As we have seen, even a small amount of power, if it is continuous for 24 hours a day, can really add up. These loads can be mostly eliminated by installing a switch or using a switched power bar that turns off the appliances when not in use.
How can I get more value for my solar energy dollar?
A solar power system can offset all of the energy that you use but it is much less costly to save energy first. Use energy efficient lighting and appliances. Shut off the lights, the computer and the TV when you are not using them. These simple measures can drastically cut down the size - and cost - of the solar power system that you need.
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