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How to Calculate Solar Production

Solar for Homeowners | 7 min read
What goes into calculating the production of a solar system

One of the most important things you’ll get to see in your solar proposal is the estimated productivity of your system. This number will tell you how much free electricity your solar system will produce.

If this number is accurate, you’ll have a clear look at how much money you’ll save on electricity and how long it’ll take for your solar system to pay for itself.

But as you may have guessed, there are a lot of factors that go into the production estimate. Calculating your solar production is more than just multiplying the panels’ noted production on the spec sheet by the number of panels included with your system.

Everything from the location it’s installed to the way the components function can impact the production of your system.

What goes into calculating the production of a solar system? 

Starting with Your Area’s Sunshine

To find the production of a solar system, a solar installer will start with the solar panels and the amount of sunshine that hits a location (which will vary based on where you live and the weather patterns in your area). Then, they’ll start to calculate the losses for your specific site and system to come up with a good picture of what your system can do.

Calculating the Losses

In this blog, we’ll touch on the ways your solar system can lose production. This list may seem long - like there are too many drains on your system’s productivity to make solar worth it. But that isn’t the case. Each item may only cause a minuscule loss in productivity.

Despite these losses, solar can still be a very profitable investment with a front-loaded payback thanks to incentives. It will also serve as a means of producing your own, free electricity for decades.

Irradiance Losses

Irradiance losses are caused by factors that inhibit the sun’s energy from reaching your solar panels. They include things like the tilt of your solar panels, shading (whether from the surrounding terrain, trees, or buildings), soiling, snow, and something known as the incident angle modifier.

1. Tilt and Orientation

Solar panels produce electricity by drawing energy from the sun. The less energy that reaches the panels, the less electricity they’ll generate. So to optimize your production, you’ll want to install your solar panels at an angle and in the direction where they get the most direct sunlight.

Solar panels will generate less electricity when the sunlight enters at an angle, and more when the sunlight hits the panel perpendicularly. This is known as the incident angle modifier. The angle of the tilt will vary with the latitude in which your system is located. As a general rule of thumb, the closer the tilt of the solar array is to the location's latitude, the more the array will produce. .

At the equator, you’d want to install your solar panels completely flat. In our installation area, the ideal tilt ranges from about 37° (central Virginia) to 44° (Northern New York). Irradiance losses come into play, however, when your installer can’t install solar panels at this angle. For those installing roof-mounted systems, the tilt angle will be dependent on the tilt of the roof.

Best tilt angles for solar panel production

In addition to the tilt is the orientation of the panels. You’ll want the panels to face as close to true south as possible to extend their exposure to sunlight throughout the day.

While both the tilt and orientation play a role in how much energy your solar system produces, your property may not allow you to install a system in the optimal manner. If you opt for a roof-mount solar system, your panels will likely be installed at the tilt of your roof. And depending on the orientation of your building and available roof space, the panels may need to be oriented to the west or the east.

If you can’t meet these factors perfectly, installing solar can still be beneficial. The loss in production for both of these factors is relatively small in the grand scheme of your system and can be made up with the addition of a few extra solar panels.

2. Shade

Anything that casts a shadow on your solar panels stands between you and free electricity. Be that nearby trees or buildings, these can reduce the production of your system. If possible, shade should be avoided, either by finding a sunnier spot to install the panels or removing the source of shade (if possible).

Depending on the terrain surrounding your building, shade can also come from nearby hills, mountains, or plateaus. This is known as horizon shade. For example, if you live at the foot of a mountain, the sun will dip below the mountain/horizon far earlier in the day than if you live in a very flat area. This reduces the hours of the day your solar system is exposed to sunlight.

3. Soiling

Like shade, the more that comes between the face of your solar panels and the sun, the less electricity the system will produce. This includes dust and dirt that could accumulate on the solar panels.

In most areas of the country, it rains frequently enough to keep the dust and dirt from accumulating heavily enough to have a huge impact on the system. However, the dryer and dustier the climate is, the more likely it is that soiling will have a measurable impact on your system.

While the impact is small, it’s still a factor that should be taken into account to get the most accurate solar production estimate.

When is it worth cleaning your solar panels

4. Snow

When it snows, flakes can accumulate on your panels, block the sunlight, and inhibit your solar system’s production. Solar panels are designed to allow for snow to quickly melt and slide off the panels, shortening the time they stay covered.

Snow melting off of solar panels

Depending on what area of the country you live in, this may or may not have a measurable impact on the production of your system, but it’s something solar installers should account for.

Please note: the loss of production due to snow is minimal. Cleaning snow off solar panels can be a danger to you, your system, and your warranties. We don’t recommend clearing snow off your system.

DC Power Losses

DC (Direct Current) losses are electrical losses that happen after the solar panels harness sunlight but before the energy is transmitted to the inverter(s).

1. Environmental Losses

Various factors can impact the amount of irradiance of each panel, like soiling, shade, and snow. When one panel is producing more or less than the other panels in the system, this could lead to the loss of energy. The temperature of the panel can also be a factor here. The warmer a panel is, the less energy it will contribute.

2. Panel Rating

Solar panel manufacturers conduct rigorous testing to paint an accurate picture of their solar panels’ output. To control for variables and give a reliable baseline, these tests are conducted under what is known as standard testing conditions (STC), where temperature and irradiance are controlled.

The solar panel’s predicted output is a great way to estimate power production, but it isn’t always perfectly accurate. The module’s power rating may vary slightly. However, today’s solar modules are designed to produce at least their rated output, if not more.

3. Light-Induced Degradation

Degradation is a normal, unavoidable, and slightly frustrating part of solar panels. Just like so many appliances, it loses efficacy as it slowly ages over the 30+ year lifespan of the system.

Light-induced degradation is slightly different, in that it happens all at once during the first few hours that the panels are exposed to light. It’s when the panels degrade at the highest rates.

In just the first few hours, your panels can degrade 1 to 3%. However afterward, degradation slows down by a great amount to the manufacturer-specified degradation rate, which is protected under warranty.

4. Connections

When it comes to solar systems, where there’s resistance, there is often power loss. Such is the case when it comes to the wiring and soldering connected inside the solar panels. These connections reduce how much electricity solar panels can produce by adding electrical resistance to the circuit.

5. Panel Mismatch

If your solar system utilizes a string inverter as opposed to microinverters or optimizers, there will be a small reduction in energy produced due to the small variations between the solar panels themselves. Even the same solar panels manufactured by the same company won’t be exactly the same due to small variations in the electrical parameters.

6. DC Wiring

Solar panels are connected together in what is referred to as strings. In a string, each panel is connected to another through wiring. There is some loss as the energy travels through the wiring in the string due to the resistance to the circuit.

AC Power Losses

AC (Alternating Current) losses happen once the DC power goes to the inverter and before it is consumed in your building.

1. DC/AC Conversion

Your solar panels produce DC electricity, but your building most likely runs on AC electricity. It’s the inverters’ job to convert the DC power from your panels to the AC power your building needs.

Inverters do this with great efficiency, but not perfect efficiency, meaning some power is lost during the conversion process. Depending on the inverter brand, type, and model, you can expect your inverter(s) to run at about 96-99% efficiency.

2. Inverter Clipping

Sometimes, your solar panels produce a little more energy than your inverter(s) can handle. In order to keep things in check, the inverter will trim the output of electricity to the amount that it can safely convert. When this is happening, you’ll be losing some of the electricity generated by your panels.

3. AC Wiring

Just like the electricity lost on the DC side of your system, you may lose some production on the AC side of your system due to resistance in the circuit caused by wiring.

Other items that impact solar production

1. Age of the solar panels

We talked about light-induced degradation - when solar panels lose a bit of their production ability during the first few hours of exposure to sunlight. After the first few hours, the panels don’t stop degrading all together, but they do slow down a substantial amount.

On average, you can expect your solar panels to degrade less than 1% each year. The degradation rate is backed by a warranty from the solar panel manufacturer. For most solar panels, you can expect a 90% production guarantee for the first ten years, and then an 80% guarantee by year 25 or 30.

2. System Availability

Your solar system will experience productivity loss whenever the system needs to be shut off. If your solar system is connected to the utility grid, it will have to shut down when the power goes off. If your system needs maintenance, it will also have to be turned off, meaning it won’t be producing energy.

3. Other

There’s no concrete, defined scenario that is included in this category. Instead, it’s a catch-all for items not accounted for above. The company doing your solar estimate will go with a value they feel comfortably reflects the variability of your system’s production.

What Your Solar System Produces

Despite all these factors that eat away at your solar system’s production, it’s still a remarkably beneficial and efficient technology that can have a real positive financial impact on your home, farm, or business.

These factors will all be included in your solar proposal’s production estimate, meaning what you see is (just about) what you’ll get. And if you choose to go solar with Paradise Energy, you’ll get exactly what you see, thanks to our unique Triple Ten Guarantee. Included in this is our production guarantee, which states that if your solar system doesn’t produce at least what we estimate it will, we’ll pay you the difference.

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