This article will help you figure out the answers to your questions about solar panels if you are interested in installing them on your property.
Determining if you should buy solar panels depends on these three questions:
- How much do you currently pay for your electricity?
- How much energy would solar panels produce on your property?
- Does your power company buy unused solar energy?
We will help answer these questions to determine if solar power is the best option for you.
Solar panels are made of many solar cells (photovoltaic cells), most often made from crystalline silicon. These cells take in energy from the sun's rays, converted through the semiconductor, creating an electric field that transfers voltage and current.
The Chemistry Behind Solar Cells
The crystalline silicon is sandwiched between the conductive layers. A solar cell uses two different silicon layers: an N-Type silicon (negative charge with an extra electron) silicon and a P-Type silicon (positive charge with extra spaces for electrons).
These layers creates a P-N Junction, a boundary between those two types of silicon's negative and positive charges. The positive charge on the bottom allows the electrical current to pass through the junction in only one direction.
Each silicon atom is connected to nearby atoms by four bonds which keep the electrons in place.
Electrons are the only moving part in a solar cell, and they go back to their original position. Solar panels can last for decades because of the lack of moving parts.
The electrons flow through an external circuit and convert it to direct current (DC) electricity.
How Much Energy Does a Solar Cell Produce?
One solar cell only produces around 0.5-0.6 volts and about 2-4 watts. You can connect each solar cell in modules to achieve more power.
For example, 12 solar cells are enough to charge a cellphone. You will need many solar panels (solar cell modules) to have enough to power a house.
How Many Solar Cells Are in a Solar Panel?
Common configurations of solar panels include 32, 36, 48, 60, 72, or 96 solar cells.
How Much Power Comes From the Sun?
Each hour, the Earth receives 1,000 watts of energy per square meter (10.76 square feet) from the Sun, in the best conditions. However, it can vary depending on the location, weather, and time of day.
Multiply these amounts to get the kilowatt-hour energy output of a solar panel:
- Square Meters (total solar panel area)
- Solar panel efficiency (%) - Typically 15–20%
- Annual solar radiation (kWh/m2/year) - Typically 1200–2600 kWh/m2/year in the US
- Performance ratio (%) - Typically 75%
The performance ratio is the relationship between the actual and theoretical energy output from the entire system.
Amount of Power Needed
First, determine how much power you're using in your home currently by looking at your electricity bill. It will be listed as Kilowatt Hours (kWh) Used (or similar), which is the amount of power you used in a month or per billing period.
Total the amount of electricity used for all months, then divide by 12 to get the average amount of power you'll need per month. This average is the best indicator for calculating power usage during both warmer and colder months.
Expected Hours of Sunlight
Determine the amount of direct sunlight that will be shining on your solar panels.
The total amount of direct sunlight is determined by:
- Where you live (latitude).
- Weather (sun/clouds).
- Obstructions (things blocking direct light to the solar panels).
You will want to get the average throughout the year since solar panels produce less energy during months with less sunlight and more clouds.
If you're located in the US, you can find the total amount of sunlight hours for your area on Google Project Sunroof. After you enter your location, it'll give you the number of hours of usable sunlight per year. Divide that number by 365 to find the daily amount of sunlight on average.
Total Amount of Solar Panels Needed
We assume up to 25% of the energy produced will be lost due to inefficiency from energy conversion after it is output from the solar panel. For example, if you get 5 hours of peak sunlight per day with a single 320-watt solar panel, that solar panel would generate around 1.2 kWh per day (1,200 watts) or 36 kWh per month.
You will need to adjust accordingly for your specific usage.
Here's how to calculate our example to find out how much one solar panel will produce in one month:
320 watts x 5 hours of sunlight per day ÷ 1,000 watts per killowatt x 0.75 performance ratio x 30 days per month = 36 kWh per month
The performance ratio is the relationship between the actual and theoretical energy output from the entire system. The typical performance ratio for a new system is 75%.
You will divide your total amount of power needed by the amount of energy your solar panel will produce.
For example, if we usually use 800 kWh per month and our solar panels produce 36 kWh per month, we will need 22 solar panels to meet our needs.
The calculation for the total number of solar panels needed for our example:
800 kWh used per month ÷ 36 kWh per month per panel = 22 solar panels
Here is a rough estimate of how many 320-watt solar panels you might need if your get 5 hours of sunlight per day:
|Average Energy Used Per Month||Number of Solar Panels Needed|
Amount of Energy You Want to Offset
How much energy do you want to offset with solar power?
We're assuming you want 100% of your energy to be produced by solar panels for these calculations. If you don't want your house to be fully powered by solar panels, reduce the amount accordingly.
For example, suppose we only need 50% of our house to be powered by solar panels and need 20 solar panels for 100% of the power. In that case, you'll only need 10 solar panels total instead.
Most solar panels produce around 250 watts per panel but will vary depending on the model you get. It mainly depends on how they're made and their efficiency levels. You also need to consider that the energy conversion decreases efficiency after output from the solar panel, typically by 25%.
You can look at What Affects Solar Power Efficiency to further narrow down the final total amount of power produced.
|Solar Panel Type||Pros||Cons|
|Monocrystalline (mono)||High efficiency / long lifespan||More expensive|
|Polycrystalline (poly)||Less expensive / most popular||Low efficiency|
|Thin-Film||Least expensive / flexible / lightweight||Very low efficiency / short lifespan|
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Solar panel efficiency can be severely reduced by any amount of shade covering the panels. Solar panels need direct sunlight to produce power, so any obstructions will block the sunlight from reaching it.
Cloud cover also reduces the efficiency of a solar panel by around 75%.
Average solar panels are around 15–20% efficient. This inefficiency is because only part of the sun's energy is capable of being absorbed by solar cells. Additionally, energy is lost due to the solar panel's glass reflecting part of the sunlight. Energy is also lost due to system inefficiencies while it is converted to usable power.
Solar panel efficiency of newer products is likely to increase further as technologies improve.
While solar panel efficiency is crucial, better efficiency doesn't matter if the panel is going to cost more overall per watt produced.
If you are tight on space available to place your solar panels, then efficiency is most important. If you have plenty of extra space, cost per watt is the most crucial factor when buying solar panels.
Solar Cell Efficiency: Amount of usable energy that's converted from the total amount of sunlight.
Cost Per Watt: Total cost for every watt of energy a solar panel is capable of producing. Solar panel cost ÷ solar panel wattage rating.
These are the main contributors to the power efficiency of a solar panel:
- Starting efficiency of a solar panel, based on design and manufacturing.
- Your location's average solar radiation, which depends on latitude, earth's orbit, and weather.
- Which direction your solar panel is facing and the amount of sunlight shining on it.
- Performance losses from the inverter, temperature, cables, shade, dust/snow, etc.
The International Electrotechnical Commission (IEC) sets the standards for solar panel hail resistance.
According to the IEC, almost all solar panels available today need to be able to withstand 1-inch sized hail that's traveling at 50 mph.
Most manufacturers guarantee that their solar panels can withstand 90 mph wind.
Beyond those conditions (90 mph wind and 1-inch size hail), solar panels can get damaged, resulting in reduced efficiency or failure.
You will need to contact your electric utility company to make sure you can sell solar power back to them. Every Utility company has their own policy regarding this question.
If you can, you might be able to receive monthly credits for your energy or cash refunds for excess solar energy.