Why do solar installers quote different system sizes?

So you have a few quotes, sit down to evaluate them, and notice something immediately: installers A and B have proposed different sizes for your solar panel system. Why is this? And how can you truly compare apples to apples in this scenario?

In this article, we'll review why this might happen – and what you can do about it!

Before diving into why installers quote different system sizes, it's important to understand how installers come up with these numbers in the first place.

Three primary factors drive solar panel system designs:

• How much electricity do you use in a year

• Solar insolation (i.e., the amount of solar radiation that will hit your solar panels)

• The amount of space available for installing panels

#1. How much electricity do you use in a year

First, your annual electricity consumption: installers often look at past electricity bills to determine how many kilowatt-hours (kWh) you use over a year. This number is important because installers will generally design and recommend a system that will meet as close to 100 percent of your electricity needs as possible (the "as close to" bit is significant here, but more on that later).

#2. Solar insolation

Installers can't use electricity usage information alone to determine your ideal system size; they need to match your electricity consumption to the solar insolation at your property to determine how many watts of solar power you'll need to meet your annual needs.

To understand the importance of solar insolation, consider an average 8 kilowatt (kW) solar panel system: this size system will generate noticeably different amounts of electricity in Hawaii (~11,200 kWh per year) than in Vermont (~9,700 kWh per year) since Hawaii is closer to the equator and experiences more sun hours throughout the year. But even two 8 kW systems in Vermont located in the same neighborhood can–and likely will–produce varying amounts of electricity if site or system design conditions hinder access to sunlight (e.g., shade, the tilt of the panels, direction the panels face.)

Installers employ software design tools and satellite imagery to figure out this part of the puzzle – it's why most solar companies ask for your address before providing any numbers!

#3. Amount of available space for panels

Lastly, available space: unfortunately, not everyone has enough roof or land space to fit all the panels needed to cover their electricity needs. In scenarios like this, installers typically look to offset as much of your electric bill as possible and maximize production with high-efficiency equipment.

We've only brushed the surface of design considerations for solar panel systems here; however, we do have additional resources if you want to read more about determining how many solar panels you need, factors that make a home more or less suitable for solar panels, or how installers calculate production estimates.

Comparing two or multiple quotes with varying system sizes is never ideal – it can be hard to figure out what's causing the differences or which system size is right for you.

Below are a few questions that can help you sort this out:

Are the installers using the same assumptions for annual electricity usage?

We know that electricity usage plays a critical role in system size recommendations, so first ask yourself: are the installers providing quotes using the same numbers here?

When you receive quotes on EnergySage, all the installers will have access to the same bill details. However, if you're gathering quotes outside of our Marketplace, installers may use different assumptions for your annual electricity usage.

Keep in mind that it's very common for installers to update quotes throughout your shopping experience based on additional information or details they gather. Even if all the companies have access to the same electric bill info, one might suggest a larger system if you tell them that you expect your electricity usage to increase in the near future because of an EV purchase or other home electrification upgrades. It works the other way as well; if you've talked to an installer about energy efficiency upgrades and expect a decline in your usage, they may recommend a smaller system.

Does the placement of your panels vary between system designs?

Did the installers include system design images in their quotes? If so, compare one design to the next, particularly the placement of the solar panels. The tilt of solar panels and the direction they face both impact how much sunlight hits them and, consequently, how much electricity they will generate. Generally, most installers will recommend the same planes of your roof for panel placements, but slight variations in design could mean you need more or fewer panels to hit your electricity production target.

What panels did each company quote?

Calculating the size of a solar panel system is easier than you'd think: you have to multiply the number of panels by the wattage of each panel. For 20 320-watt panels, you'd be looking at a 6.4 kW system:

20 panels * 320 watts (W) = 6400 W / 1,000 = 6.4 kW

So, you may be dealing with two companies that recommend the same number of panels but quoted different system sizes because the proposed panels have different wattages:

20 panels * 320 W = 6.4 kW

20 panels * 340 W = 6.8 kW

Many installers will include details about the proposed equipment in their quotes, but if you aren't seeing it, ask! Solar equipment is one of the most important factors to consider when finding the right solar solution for you, and you want to make sure you're paying a fair price for the equipment being installed.

Are local policies influencing system size recommendations?

As mentioned above, most solar installers will recommend installing enough solar panels to meet your needs, assuming you have space. However, you might find that some installers will recommend a smaller system if policies from your utility company or government restrict system sizes. For example, in some states, systems must be under a specific size to take advantage of net metering or certain solar incentive programs.

Hopefully, all installers providing quotes will be familiar with your local utility company's interconnection policies and quote accordingly. However, installers new to your market or just breaking into the solar industry may be unaware that such restrictions exist – that's one reason why finding a reputable, experienced installation company matters!

Do any of your quotes include a battery?

Batteries are becoming an increasingly popular addition to solar panels, especially as costs continue to decline. If you install a solar-plus-storage system, the battery you choose and how you plan to use it (e.g., for backup power, self-consumption, rate arbitrage) plays an important role in sizing your solar panel system. Don't be surprised if an installer that offers a battery in their quote suggests a slightly larger system than one that doesn't – you'll want enough extra electricity to charge your battery, after all!

How do the production estimates compare?

The word "estimate" is critical: it's impossible to determine the exact number of kilowatt-hours a solar panel system will produce in real-life conditions since no one can predict the weather.

Because installers use different tools to generate system designs and production estimates, slight variations in both are bound to happen. Some installers are more inclined to provide generous production estimates, while others stick to more conservative numbers. Of course, more conservative estimates can lead to larger system size recommendations, while installers being more ambitious with their production estimates, will generally recommend smaller system sizes.

The best way to compare production estimates about system size is to calculate the system's production ratio:

Production ratio = year one production estimate/system size (Watts)

By comparing these ratios across your quotes, you can get a feel for what's typical for your property, whether your potential installer is being more or less aggressive with their production estimate, and how that could impact the system sizes in your quotes.

You can find more information about production ratios and how to evaluate them in this article.

Whether you've figured out why the installers quoted different system sizes or not, there are a few steps you can take to make sure your quotes are as accurate as possible:

• Provide the most up-to-date electric bill information, with any additional context: Above all, ensure all the installers providing quotes have access to accurate information. If you plan to increase or decrease your electricity consumption in the future, make sure the installers know this and why because it will influence their system size recommendation.

• Ask installers to update quotes according to your preferences: Your initial solar quote is not set in stone! Companies can adjust their proposals as you get a better idea of what you want. Whether you fall in love with a particular solar panel or decide you don't want solar panels on one portion of your roof, let the installers know so they can make sure their quotes reflect these preferences.

• Invite installers to a site visit: Site visits are an excellent opportunity to meet a prospective solar installer and ask them questions – they're also the perfect time for an installer to take measurements and fine-tune their system design. Fortunately, most solar companies offer free site consultations for prospective customers.

Try not to worry too much about slight variations in system sizes between quotes – we know... easier said than done! But at the end of the day, it's most important that you find a solar company with a proven track record of designing and installing high-quality systems for a fair price. Whether you land on a 6.6 kW or a 6.8 kW system, you'll still save money on electric bills!

On the EnergySage Marketplace, you can receive up to seven quotes from local solar companies. These quotes will include details about proposed system sizes and equipment and use the same standard assumptions, making it the most transparent and easy way to compare. If you'd like to start with some ballpark numbers on estimated solar costs and savings, try our Solar Calculator.