Why More Panels Doesn't Always Mean More Savings
Solar works by generating electricity you'd otherwise buy from the grid. If you generate more than you use, the excess goes back to the grid — and in Illinois, most utilities credit you for that excess at the net metering rate.
The problem: the net metering credit rate is often lower than what you pay for electricity you use directly. That means excess generation — electricity your oversized system produces that you can't use — returns you less value than the electricity your system offsets when it's generated at the right time for your usage.
An oversized solar system costs more to install. And the returns on the excess generation it produces are lower than the returns on the generation you actually use. That combination extends your payback period and reduces the long-term return on the investment.
Why Solar Companies Default to Bigger Systems
The incentives in solar sales point toward larger systems. A larger system is a larger sale. Commissions are often based on system size. Installers who price aggressively on small systems to win the sale still benefit from upsizing.
Many installers also size systems around your peak historical usage — the highest month of the year — rather than your average. That approach protects against the possibility of under-generating in a high-usage month, but it guarantees over-generation in the many months where your usage is lower.
None of this is necessarily intentional. Many solar companies genuinely believe that a bigger system is more future-proof, more valuable to the homeowner. But the math doesn't always support that belief.
The Payback Period Problem
Payback period is simple: how long until cumulative savings equal the cost of the system? A larger system costs more. If the additional generation from those extra panels produces less value per kilowatt-hour than the generation you use directly, the additional panels have a worse payback than the core system.
That doesn't mean the last panel on a larger system is a bad investment — it may still pay off over 25 years. But if your goal is the best economic outcome, right-sizing matters.
When a Larger System Makes Sense
The right-sizing principle isn't about minimizing system size for its own sake. There are legitimate reasons to install a system larger than your current usage:
- You're adding an EV — Electric vehicles are large loads. A typical EV adds 2,000–4,000 kWh per year depending on miles driven. If you're buying an EV now or planning to within the next year or two, account for that load in your solar sizing.
- You're switching from gas to electric heating — A heat pump replacing a gas furnace shifts a significant load from gas to electricity. The solar system needs to be sized for the total electrical load post-conversion.
- You plan to add a battery — Battery storage is most useful when paired with a system that generates excess, which the battery can store for evening use. A battery-paired system may warrant modest additional generation capacity.
- You have room on the roof and expect future load growth — If the incremental cost of additional panels is low and you anticipate meaningful load growth, there's an argument for building headroom now.
The Right-Sizing Framework
At SPM, right-sizing means building the solar proposal around your home's actual, optimized energy needs — not your current usage and not an assumed future maximum.
That process looks like this:
- Review your 12-month usage history to understand your real consumption baseline
- Identify efficiency opportunities — windows, insulation, HVAC — that will reduce consumption before solar goes in
- Estimate your post-upgrade usage, which becomes the target for solar sizing
- Account for any anticipated load additions (EV, heat pump) with a realistic timeline
- Size the system to cover that number — not your pre-efficiency, not your worst-case peak, not your roof's theoretical maximum
The result is a system that offsets what you actually need, pays back as efficiently as possible, and doesn't burden you with the cost of panels that spend most of their life over-generating.
Frequently Asked Questions
How do I know if I've been quoted an oversized system?
Ask the installer how they sized the system. If the answer is "based on your current usage," ask whether any efficiency improvements were factored in. If the system is sized to produce 110–130% of your current annual usage with no EV or major load additions planned, that's worth questioning. This guide on comparing solar quotes has more on what to ask.
What's the right percentage of usage to offset?
A common target is 100% of your net annual usage, after accounting for planned efficiency improvements and load additions. Some installers target 90–95% to avoid significant over-generation. The right number depends on your net metering policy and long-term plans — SPM will walk you through both.
Will the ITC or incentives change the calculation?
Tax incentives affect the after-incentive cost of the system but don't change the fundamental economics of oversizing. A larger system is cheaper after incentives — but it's also less necessary if the added generation produces lower returns. If you're in a legitimate high-usage situation, incentives make the larger system more attractive. But they shouldn't be the reason to overbuild.