How solar installers calculate solar production estimates

When you're evaluating your solar options, there are a lot of factors to take into account. Price is critical, whether you're paying out of pocket or financing monthly payments with a lease or loan. You should also think carefully about the installation company you choose and the type of equipment going on your roof. But arguably, the most critical data points are the amount of electricity your panels will produce and how much they'll save you in the long run.

When a solar installer starts to work with you, they will highlight a "production estimate" as part of their quote, which is the basis for your long-term savings. Here's how your installer estimates your solar panel system's electricity production.

Installers have a variety of design tools that they use for production estimates. These tools incorporate satellite imagery to model the specific characteristics of your roof that will affect electricity production, like shade, roof orientation, roof tilt, and geography. They also account for the equipment your installer will use.

Shade

Shade can dramatically affect the production of your system. Suppose you experience a good amount of shade on your roof throughout the day. In that case, installers may recommend trimming or removing trees to improve your panels' electricity production (and, therefore, experience a quicker payback for your solar investment!).

Azimuth/orientation

The azimuth or orientation of the solar panel system (whether on your roof or ground mounted) will impact your production. In the Northern Hemisphere, solar panels facing due south will produce the most electricity because they receive the most sunshine over the course of a day. While still suitable for solar, eastern and western-facing panels have less electricity overall because they see fewer "sun hours" throughout the day.

Tilt

The suggested tilt of your solar panel system can vary depending on geography. Still, most solar panel systems in the Northern Hemisphere will reach maximum electricity production at a 30 to 45-degree tilt. It's worth noting that while tilt impacts production, it has less of an effect on solar panel performance than the orientation of the panels.

Geography

Some areas of the country get more sun over the course of a year. Put the exact same roof with the same solar panel system in southern California and Massachusetts. The solar panel system in California will produce more electricity simply because there are more hours of sunshine there. Massachusetts experiences more seasonality differences throughout the year.

Size of the system

Production estimates also take into account the total size of the system proposed and its capacity to produce power.

Type of equipment

Not all solar panels are created equal. Some panels are more efficient than others and can produce more electricity. For example, a higher wattage and higher efficiency panel will produce more kilowatt-hours (kWh) than a standard panel in the same location. The type of inverter your installer uses can also impact production. Power optimizers and microinverters are known to be more efficient than a typical string inverter system, especially with more complicated installations.

If you're looking at a ground-mounted solar panel system, adding trackers will greatly impact production. While they will add to the cost of the entire system, trackers will enable the panels to follow the sun as it moves throughout the sky, producing as much electricity as possible.

Some of the more popular design tools installers utilize are Helioscope, Aurora, and Solargraf. Even without the use of design tools that draw up design images at the same time, there are also solar production estimate calculators that use the factors listed above to generate an estimate. One example is PVWatts, developed by National Renewable Energy Laboratory (NREL).

Your solar panel system's electricity production will determine how much you save on your electric bills in the long term, so it's important to understand the production estimates for each of the solar quotes you're comparing. Below are a few FAQs about production estimates from solar shoppers on the EnergySage Solar Marketplace.

Why isn't the production estimate the same as the system size? Wouldn't it make sense for a 6 kW system to produce 6,000 kWh?

With most solar panel systems in the United States (at least, those with adequate sunshine), your production estimate shouldn't be a 1:1 ratio. The size of the system (6 kilowatts, or 6,000 Watts) represents the capacity of a system to produce power, while kWh represents the energy output of a system over time.

The electricity production for a 6 kW system depends on many factors, including its location. An optimally placed 6 kW system in Massachusetts can produce over 7,000 kWh a year, while the same system somewhere in California would produce closer to 9,000 kWh in a year because California homes receive more sunshine.

That doesn't mean solar is only a good investment for the sunnier states. You'll benefit the most from solar energy if you have high electricity rates, spend a good amount on your monthly electric bill, and live in a state with good incentive programs.

Why are two installers quoting the same system size on my home and estimating a different amount of production? Which is right?

Are the panels proposed to be in the same place? Different system designs can impact production estimates. If one installer is proposing more panels on your southern-facing roof, that's likely to mean more production than the competing system that has more panels on your eastern-facing roof or in a location that experiences more shade throughout the day. It's always a good idea to ask installers for system design images so you can compare the options accordingly.

Even with the same system design and equipment, installers are likely to come up with different production estimates. Why? Well, it's important to understand that these are estimates. No installer can predict the weather, so they won't be able to tell you the exact amount of kWh your system can produce in a year.

Installers use different tools while generating these production estimates. Some installers will be more generous in their production estimates, while others are more conservative. If your estimates are dramatically different, your installer may be trying to push a system that's way too large for your energy needs or not generating enough electricity for your needs.

An effective way to gauge installers' production estimates next to each other is by calculating a production ratio:

This number will show you how much an installer is estimating their system will produce in relation to its size. These numbers will typically be above one unless it's an inefficient system or located in a very northern location of the country. But, production ratios can vary depending on your market. Below are rough ranges typically seen for production ratios in various markets (note: these are ranges for rooftop systems, not including ground mounts with trackers).

Solar panel system production ratio by region

What if production ratios for my quotes are within the normal range but still widely different?

If your competing quotes have similar production ratios, you can feel assured that both are reasonable estimates given the system size.

If you're seeing a conservative production estimate, it's worth asking your installer if the company offers a production guarantee. Some installation companies guarantee that your system will hit a certain amount of production for a set period (typically in the first 1-2 years of the system) to ensure that the system is operating as it said it would. If it doesn't meet the guaranteed production, there is usually a form of compensation promised to compensate for the difference in output.

When the installation company offers a production guarantee, it's common for their estimate to be more conservative, as they'd want to protect the company financially. Alternatively, if you're seeing a production estimate on the more generous end compared to competitors, it could also be worth asking them if they can guarantee that production.

With a big investment like solar, you want to look at multiple options to ensure you get a good deal. By signing up on the EnergySage Marketplace, you can receive multiple quotes from pre-screened installers to compare. These quotes will include production estimates as well as production ratios. Our quote comparison table will also flag production estimates that are outside the normal range for your area. If you'd prefer to start with an initial estimate of costs and savings associated with solar, our Solar Calculator can be a good place to start.