What Decides How Many Solar Panels You Need?

"How many solar panels do I need?" is usually the first question homeowners ask, and it's a good one. It also doesn't have a one-size answer. The right number depends on your home, your roof and the way your household uses electricity.

This article walks through each factor that decides the number, so you know what goes into a design and what to look for on a quote. For the bigger picture of how a system works, start with our guide to how home solar works.

Why is there no standard number of solar panels for a house?

You'll find charts online that promise a panel count for a "typical" home. They skip the things that make your home different. Two houses of the same size can use very different amounts of electricity, sit under different amounts of shade and face different directions.

The number of panels on your roof comes from four things working together:

• How much electricity you use in a year
• How much sunny, unshaded roof space you have, and which way it faces
• The wattage of the panels being installed
• Any changes coming, such as an electric car or a heat pump

Utility rules on how large a system can be also play a part. The sections below cover each one. The goal isn't to do the math yourself. It's to understand what a good design takes into account, so you can tell when a quote was built for your home and when it wasn't.

How does your electricity use decide the system size?

Your electric bills set the target. Each bill shows how many kilowatt-hours (kWh) your home used that month, and a solar system is sized to cover your use over a full year. The system's size, in kilowatts (kW), and the number of panels follow from that target.

A single month doesn't tell the whole story. Summer air conditioning, winter heating and a pool pump all change your use from season to season. That's why installers ask for 12 months of bills. Our article on why solar companies need your electric bill explains what they look for.

The size of your house matters much less than you'd think. A large home that heats with gas can use less electricity than a smaller one with electric heat and an electric car. What counts is the kilowatt-hours on your bills, not the square footage.

If you've recently moved, made big changes to your home or had a year that wasn't typical, mention it. A design built on an unusual year can end up too large or too small for the way you live now.

How much sunny roof space do you have?

Your roof sets the limits. Panels need sections that get good sun through most of the day, and not every part of a roof qualifies.

• Direction. South-facing sections collect the most sun, and east- and west-facing sections can work well too. Our article on the best direction for solar panels covers how each one compares.
• Shade. Trees, a neighbor's house or a chimney can shade panels for part of the day, and shade during the middle of the day costs the most. See how trees and shade affect solar panels.
• Obstacles. Vents, skylights, dormers and chimneys break up a roof and take away space where panels could go.

When usable roof space is tight, the design has to work around it. That can mean using a second roof section, choosing higher-wattage panels, or covering part of your use instead of all of it. Partial coverage still lowers your bill.

Does panel wattage change how many panels you need?

Yes. Every panel has a wattage rating, and a system's size is the total of the ratings of all its panels.

So the same size system can be built from fewer higher-wattage panels or more lower-wattage ones. On a roof with plenty of open space, the difference may not matter much. On a smaller roof, or one broken up by dormers and vents, higher-wattage panels can fit more system into the space you have.

That's also why comparing panel counts between quotes can mislead you. One company may show more panels than another simply because it uses a lower-wattage model. Compare the total system size and the production estimate instead. Evergreen Solar installs Tier 1 panels, and your quote names the exact model.

Should you plan for an electric car or heat pump?

If you expect your electricity use to grow, say so before your system is designed. Three changes make the biggest difference:

• An electric car charged at home
• A heat pump replacing oil, propane or gas heat
• An induction range or an electric water heater

Each one adds kilowatt-hours that your past bills don't show yet. Planning for them now is usually easier than adding panels later, when the layout and equipment are already set. Our article on how home solar fits into the electric future of living covers going all-electric with solar.

Is a bigger solar system always better?

No. More panels can sound like more savings, but the rules in each state reward a system that matches your use.

• Massachusetts. Extra power you send to the grid earns net metering credits that carry forward on your bill. With Eversource, they aren't paid out as cash, so producing far more than you use can leave credits you never spend.
• Rhode Island. Production up to 100% of your use earns full credits. For systems of 25 kW or less, production above your use is credited at the lower wholesale price.
• Connecticut. Under the RRES program, a system's production can't exceed the highest 12 months of use in the past five years by more than 5%. There are allowances for up to two electric cars and a heat pump.

An oversized system also costs more than it needs to. A system that undershoots leaves more of your bill behind. The right size sits close to your real use, adjusted for the changes you know are coming.

How does Evergreen Solar design your system?

Evergreen Solar's engineering team designs every system from your electric bills and your roof, and estimates its production in our own design software using historical weather data for your location. You don't have to work out any of the numbers yourself.

Here's how it comes together:

1. Consultation. A Solar Pro reviews your bills and goals, and our design team prepares a proposal showing panel placement on your roof and your expected savings. Our article on the solar consultation and site survey walks through it.
2. Site survey. Usually within 7 to 10 days of signing, our team checks your roof, attic and electrical system in person.
3. Blueprints. Over the next 2 to 3 weeks, the final design and permit plans are drawn up. If anything changes from the first design, we review it with you.

Every Evergreen Solar package includes a 25-year power production guarantee and a 25-year comprehensive solar warranty. To see how your roof looks for solar, start with a free Solar Fit roof scan, or call us at (508) 507-2908 to book a consultation.

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