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How Much Energy Do Solar Panels Produce | Electricity Output

July 3, 2020
By John Cole

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Solar Technology is a booming industry. As manufacturers strive to bring down the price and governments enact new environmental initiatives, this technology is sure to grow. But if you’re just starting out on your solar journey, you might be feeling overwhelmed. You might wonder if solar is as good as it sounds. Thankfully, we’re here to answer the question, how much energy do solar panels produce?

While there are many variations to solar panels (size, efficiency, type, mounting), one square foot of solar will generate 15 Watts of energy under full sun conditions. With an average of four hours of sun daily, it will produce 60 Watt hours every day. This means that 30 panels can power a house.

But this generalized answer hardly even scratches the surface. After all, what is a Watt? How is a Watt different from a Watt hour? How can I use solar energy at night? To understand how much energy solar panels produce, we need to begin with some key definitions. Even though solar terminology might feel like its own language at first, it’s simple enough to grasp.

Definitions

  • Volts (v) – The number of electrons moving in a circuit.
  • Amps or current (a) – The speed at which the electrons are moving.
  • Watts (W) – A measurement of electricity determined by multiplying the volts by amps.
  • Energy – A synonym for Watts.
  • Kilowatt (kW) – 1000 Watts. This is the typical measurement when discussing whole house or commercial electrical systems.
  • Kilowatts per hour (kWh) – the average amount of kW used over a period of time, specifically an hour. This is the measurement that utility companies use to determine your usage.
  • Direct Current (DC) – the form of power for solar and batteries. Depending on the time of day, solar panels might produce up to 35v of power.
  • Alternating Current (AC) – the form of power in American electric infrastructure. For example, American households function at 120v.

In the United States, residential electrical systems are usually in 120v AC. Most homes have a 200amp breaker. This means that if there is a surge that causes more than 200 amps to enter into the electrical wiring, the breaker will trip. Therefore, a residence can accept 24,000 watts, 24 kW, of energy at any given time.

All that being said, we use far less electricity than our actual wiring can handle. A typical household uses almost 40kWh per day. This translates to nearly 1200kWh in monthly usage. Both the utility company and the solar industry measure electrical usage in kWh. So let’s make sure we understand exactly what we’re talking about.

What Is a kWh?

Imagine that a typical light bulb is 60W. If you leave that light bulb on for one hour, you have used 60Wh. Remember that Wh is energy measured over time. If that same lightbulb is on for 17 hours straight, then the light bulb will use just over 1kWh.

You can determine your energy consumption by estimating the amount of time your run all of electric devices (electronics, refrigerators, HVAC) and find the amount of Watts they use. This method would be helpful in evaluating a custom solution on an RV. But for most people, your best bet is to review the past six to twelve months of your electric bills and average out your monthly usage. A small home might use 800kWh in a month while a large 3,000 sq. ft house might use 2,200kWh.

Solar Panel Energy Ratings

Now that we have a frame of reference, we can turn our attention toward the solar panels themselves. Solar panels come in different sizes. The energy they produce varies according to the number of solar cells they have. But regardless of their physical dimensions, each solar panel has a rating. This rating will vary between 200W and 450W. Generally speaking, the larger the physical dimensions, the greater number of solar cells and thus a higher energy output.

The actual rating of the panel (for example, a 300W panel) refers to the amount of energy, or Watts, a panel will produce under ideal circumstances, or under full sun. It is therefore vital to position the panels in the ideal location in order to keep them under full sun.

Follow this rule of thumb: 1 sq ft of solar = 15W of energy. This will help you estimate how much solar you can fit on your property.

Ideal placement

If you want the most out of your panels, then follow these helpful tips to avoid a drop in production.

  • Free from shade or other obstructions.
  • Plenty of air flow underneath to keep the panels and the wiring from losing energy due to excess heat. If mounted on the roof, place them several inches above the roof surface.
  • If in the Northern hemisphere, face the due south. If in the Southern hemisphere, face them due north.
    • If you can only mount them facing east or west, there will be an approximate 15% drop in production
  • Depending on your distance from the equator, tilt them appropriately. The closer to the equator, the flatter the tilt will be.
    • Northern US - 40-45 degrees
    • Central US - 40-30 degrees
    • Southern US - 30-25 degrees

Calculating Energy Production

While there are many solar panel calculators online, the basic math involved is actually quite simple. Let’s say you have a small rig of four panels at 200W. These four panels produce a total of 800W when the sun is shining. If the sun shines for a full hour, they generate 800Wh. And if it’s in the middle of summer and they receive 4 hours of full sun, this little rig will generate 3.2kWh daily! This number does not even include the bits of energy the trickle in in the morning and evening hours. This would be approximately 10% of a small household's daily energy consumption.

How Many Solar Panels Do I Need?

If you are looking to produce enough energy for all of your power needs, then you should expect to use 30 panels or so. In this example, we will estimate the yearly energy usage of a small home at 1000kWh per month. So 1000kWh is our target. Let’s break down the math and see if 30 panels will be sufficient.

We will start by assuming that a small home can get by with smaller solar panels. Therefore, the panels in this example are 300W. If they receive four hours of full sun, then each panel will produce 1.2kWh every day. On a good month, each panel will generate 36kWh.

Now if you factor the entire solar rig, on a good month, 30 panels will generate 1080kWh. If we consider that our example household needed only 1000kWh, then this solar rig generates more energy than the household consumed. Here’s what the math looks like:

300W panel * 4 hours = 1.2kWh per day.

1.2kWh * 30 days = 36kWh per year.

36kWh * 30 panels = 1080kWh annually

Limiting Factors

No system is perfect. Even in the best solar systems, there will be some inefficiencies. Here are a handful of limiting factors in solar technologies that will produce small amounts of energy loss.

  • Degredation – Solar panels have a lifespan. They don’t operate at 100% for years and fail immediately. Rather, there is a very gradual process of degredation that causes very small amounts of energy loss over time. The degredation rate varies from manufacturer to manufacturer.
  • DC/AC conversion – No, this is not some classic rock cover band. Solar energy is measure in DC, or direct current. While DC can charge batteries, which also function in DC, residential and commercial systems use alternating current, or AC. To use the energy in your home, you must process what energy through an inverter that converts the energy from DC to AC.
  • Mismatched Panels – If two solar panels are mismatched in their voltage output, then there will be energy loss. That’s one reason why it’s not recommended to add different manufacturers or different panels to the same system. Energy wants to be as consistent as possible when traveling through the wiring.
  • Wiring and Connectors – Anytime energy travels, there is some resistance. The longer your wiring, the more resistance and the more power loss will occur. This is especially important in DC systems. There will also be energy loss at every connection point along the way. These little losses add up over time.
  • Efficiency Ratings – You should also consider the efficiency rating of your panel. In the 1950’s, panels were known to have 6% efficiency. This meant that only 6% of the sunlight was processed into energy. Now, top of the line panels are known to have a 30% efficiency rating. As solar technology continues to increase, so will the efficiency rating.

Living Off the Grid

Whether you are in an RV or a cabin in the mountains, off-grid living is increasing in popularity. To use electricity off-grid, you must have a way to store the energy created by the panels. Most residential systems tie directly into the power grid. Any excess energy is then sold to the utility company and credited to your account. But if living off-grid, you’ll need to use that energy at night when the sun is not shining.

To do so, you’ll need solar batteries. Without going into the specifics of energy storage, which is its own topic altogether, just know that battery systems can be wired in different voltage configurations and are measured in ah, or amp hours, instead of kWh. These ah are measure in DC and must be converted to AC through the use of an inverter. This whole process the complexity of your system substantially but can be done.

If this is truly a desire, then you will do best if you use as little electricity as possible. Consider making some changes to your typical routines. For example, use a wood stove instead of electric heat during the winter. Consider a propane water heater instead of electric. For reference, here are the major electric hogs in a typical home:

  • Heating – 40%
  • Water heating – 20%
  • Air conditioning – 10%
  • Refrigeration – 5-10%
  • Everything else – 25-30%

If you find alternative solutions for the worst offenders, off-grid living will then be within reach.

Variations

There are two main types of solar panels. The above calculations are for the more common of the two, monocrystalline.

Monocrystalline – These panels have higher efficiency ratings, work better in high heat and shady environments, and are more expensive.

Polycrystalline – These panels are the opposite, lower efficiency, less effective in heat and shade, but less expensive.

Geographical impact - It is also important to factor in the amount of usable hours your geographic area receives. For example, Nevada gets approximately 4.8 peak sun hours daily while Minnesota receives only 2.8. This means that the kWh generated by solar systems in these two states will vary drastically.

map of sun exposure in US

Final Thoughts

Solar technology is both increasing in popularity and decreasing in cost. While the whole topic is overwhelming at first, there is an unending amount of resources. If you are considering doing a residential installation, do your homework. Get multiple quotes. There is enough evidence out there to suggest that homeowners who get 3 or more quotes save $5,000 - $10,000. Also, look for smaller solar companies. Don’t assume that bigger is better in this case.

If you are doing this for an RV, just take your time. It’s more than possible to put enough solar on the roof and run your devices. You should be able to handle everything except for heat and air conditioning off your solar on a regular basis. Of course, you always want a backup plan for those multi-day rainstorms.

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