Adding solar panels to an existing system: When is it worth it?

Already gone solar but need more energy for an EV, heat pump, or other newly electrified appliance? Most of the time, you can add more solar panels to your home. 

The economics will probably look a lot different then the first time you went solar. (And if you live in a state like California that upended its solar incentives, you’re probably better off leaving your old setup alone.) But with the right installer, enough space, and a willingness to compromise, new panels can help the environment and your bottom line. Here’s what to watch out for when you’re going solar again.

When you change an existing solar energy system, your utility company can force you to adopt newer incentives for your entire set of solar panels—not just the new ones. (Or at least any panels that are tied to your home’s electrical meter, and by extension the grid.) 

This is a non-issue in most parts of the U.S., where residential solar policies have barely changed in more than a decade. The incentives will be the same as they were the last time you went solar. Would-be solar expanders will still need to apply for new permits and a new interconnection agreement, but it won’t really affect your bottom line.

But in a handful of states, the incentives for residential solar power are much worse than they used to be, and that can make the economics of expanding your solar power system much dicier.

California is the most recent example. In 2023, the state utility commission instituted a new billing scheme (NEM 3.0) that completely changed the economics of new residential solar setups. The utility companies are entitled to buy your excess daytime solar energy at something like a 75% discount, but still charge you full price for any energy that you buy from their grid. Hawaii, Arizona, and Nevada made similar changes, where old solar customers are grandfathered in for a period but new customers have to accept less-favorable billing terms. 

If you’re on an older net metering plan that pays closer to 100% for your excess power, it’s usually best to keep your grandfathered interconnection agreement in place—and that means leaving your existing system alone, at least for a decade.

Does it ever make sense to add new panels to an old system that’s grandfathered into a better metering plan? “Adding extra solar might still be worth it, sometimes,” says Matt Schuler, a Senior Energy Advisor at EnergySage, who guides solar shoppers through the process of comparing quotes and choosing an installer. “But the payback period would be much longer, and you’d probably have to add a battery.”

Some states have incentives that became only a little weaker. In Massachusetts, for example, nearly all residential solar setups still qualify for full net metering, so adding additional solar should work out just fine, financially speaking. But you’d forfeit cash incentives from older, now-discontinued programs. These minor changes won’t usually be a dealbreaker, but it’s something to consider when you’re calculating a new payback period. Ditto for states like North Carolina or Michigan that have rolled back full net metering, but haven’t replaced it with a billing scheme as unfavorable as California’s.

The prime real estate for rooftop solar is on unshaded areas facing south, west, or east. If you have open space in any of those orientations, you’re in good shape to add new solar power. 

However, there’s a good chance you already used up your most productive rooftop space the first time you went solar. “At least in the past few years, most often if there’s an open space on a roof, the installers wanted to max that out,” says Jake Trubiano, co-founder at Gold Path Solar in Ohio (an EnergySage Approved installer). And as Bill Horbaly, president and CEO of Connected Technology in California (an EnergySage Elite installer) put it, “sometimes [finding] more roof space is rough.”

If a north-facing roof is all you have to work with, it can be hard to get a good return on investment. Panels that tilt towards the north get much less sunlight than panels facing any other direction, and the steeper the northerly tilt, the worse the production. Some people choose to get north-facing panels anyway, even when they’ve been warned about the long or non-existent payback periods. Horbaly says that tilt racks can sometimes be a cost-effective way to improve energy production on a north-facing roof.

Some other alternatives include:

• Ground-mounted systems. They always generate the most electricity because they can be installed at the ideal direction and angle. But most people either don’t have—or don’t want to sacrifice—enough land to make it worthwhile.

• A solar carport, pergola, or some other freestanding structure. But, “unless the customer is already planning to build a new structure, it is typically not the recommended approach,” Horbaly says.

• Replacing your old solar panels with new solar panels. Today’s solar panels generate about 25% more electricity from the same roof space as equipment from just 5 years ago, and even more compared to decade-old panels. Sometimes, replacing your old equipment can yield the biggest payoffs.

Try to go back to your original installer. As long as they haven’t gone out of business, or weren’t painfully incompetent the first time around, sticking with the first company is usually the smoothest path toward expanding an existing system.

Some pros won’t get anywhere near existing installations. “We don’t want to inherit, for lack of a better word, the [low-quality] installs,” says Trubiano (whose company actually will do system expansions). “We don’t have to want to add onto that system, and then [find out] there’s issues, and have to send out trucks and provide a lot of TLC for projects with issues that weren’t ours in the first place.”

Solar contractors who will take on expansions generally work around the old system rather than tweaking what’s already there. Here’s what they seem to have in common.

• They prefer to build a separate solar system. “A new system is not normally ‘integrated’ with an existing system, but is an ‘add on’ working independently,” says Horbaly, whose company will take add-on projects. Integrating new panels into an old string inverter setup, for example, can mean rewiring the entire array and replacing several other components. (It tends to be easier with microinverters.) All the undoing and redoing means extra labor. 

• They will not take on the warranty for old panels they didn’t install. “If anything happens to your old system, we can try to fix it” for a fee, Trubiano says, comparing it to hiring an HVAC technician to repair an old AC or furnace that they didn’t install themselves. “But we’re not going to assume [warranty] responsibility for the old system we did not install.” He adds that most homeowners understand and accept this.

Small projects cost more per watt than big ones: If you’re tacking on a relatively small addition (for example, a 1 or 2 kW add-on to avoid triggering NEM 3.0 in California), expect to pay more per watt of new solar than the current averages would suggest. 

We can see clearly in the EnergySage Marketplace data that the economies of scale get lost with small projects: A 10 kW project (roughly 25 panels, pretty typical for a new install) costs an average of $2.95 per watt, but it’s $3.83 per watt for a small 2 kW, 5-panel project. That’s 30% more expensive. Why the elevated prices? Regardless of the number of panels an installer puts on your roof, they need to cover the cost of their overhead and labor, and ideally a similar profit as what they’d be able to make if they were doing a larger project instead.

Replacements or integrations cost extra, too: If you’re replacing a roof full of old, inefficient, degraded panels, all the old equipment needs to be removed, and that means higher labor costs. You typically can’t re-use old parts, including the racks. (Solar panels aren’t all a uniform size, it turns out.) Ditto if you’re going to be mixing and matching old panels into a single setup, because it often means replacing old wiring and other parts.

The new panels won’t look exactly like your old panels: “The aesthetics are not going to match and that is something a customer has to be flexible about,” Horbaly says. Every panel is still a blue-black rectangle, but the precise size, color, pattern, and bezel can be subtly different from model to model.

With any solar project, big or small, it's good to shop around. By signing up on the EnergySage Marketplace, you can receive multiple quotes to compare costs for solar. If you want to add panels (or a battery) to your existing system, note that in your account, along with information about your current system so that installers can provide you with the most accurate quotes possible.