The Definition
A right-sized solar system is one sized to offset your home's actual, optimized energy needs — not your current usage, not your theoretical maximum, and not whatever fits on your roof.
Right-sizing is the answer to a specific failure mode in solar sales: the tendency to overbuild. It doesn't mean minimizing system size for its own sake. It means building a system that's accurately matched to what your home will actually need, over the 25-year life of the equipment.
The Inputs to Right-Sizing
1. Your 12-Month Usage History
Your utility bills from the past year show how much electricity your home consumed each month. This is the starting point — but not the ending point.
2. Planned Efficiency Improvements
If you're planning to replace old windows, upgrade insulation, install a heat pump, or make other efficiency improvements, your future electricity consumption will be lower than your current consumption. A right-sized solar system accounts for that reduction. A solar proposal built on your current, pre-efficiency usage will be larger than you need after the improvements are made.
3. Anticipated Load Additions
If you're planning to buy an electric vehicle, add a pool, or make other changes that will increase your electricity consumption, those loads should be factored in. Right-sizing accounts for realistic future loads, not just today's snapshot.
4. Net Metering Economics
Under Illinois net metering, excess solar generation you send to the grid earns you a credit on your bill. The credit rate is typically the full retail electricity rate — but the rules vary by utility and can change over time. How the system's excess generation is valued should factor into the sizing decision, particularly if you're considering a system larger than your usage to create credits for future loads.
5. Available Roof Space and Shading
The right-sized system also needs to fit on your roof with acceptable shading. If roof constraints limit the system to a smaller size than the load analysis suggests, SPM will tell you — and the solar proposal will reflect what's actually achievable, not a theoretical optimum.
What Over-Sizing Looks Like
An oversized system consistently generates more electricity than the home uses. Under Illinois net metering, that excess is credited at the retail rate — but if credits accumulate faster than they're used, they may eventually be zeroed out at the end of an annual period (depending on your utility's policy) with no cash compensation.
More practically: an oversized system costs more than necessary. The extra panels add to the project cost. If their generation isn't fully offsetting billable consumption, the payback period is longer than it would be for a right-sized system.
What Under-Sizing Looks Like
An undersized system offsets only part of your consumption. You still have a meaningful utility bill. For homeowners who want to eliminate their bill entirely, this is disappointing. For homeowners who simply want to reduce their bill significantly at the lowest cost, it may be intentional.
The risk with undersizing: if future loads (EVs, heat pumps) increase consumption significantly and the roof can't accommodate an addition, you're locked into a system smaller than you need.
The SPM Approach
SPM's process starts with the question: "What will this home actually need over the next 25 years?" That means:
- Reviewing your current utility bills
- Identifying which efficiency improvements are realistic for your home, and estimating the consumption reduction each one would produce
- Understanding your plans for EVs, HVAC replacement, or other load changes
- Designing a system sized to cover the resulting need — not the current bill, not the roof's theoretical maximum
The result is a proposal you can trust to be accurate — not a number padded for margin or inflated to produce an impressive annual production figure.
The Right Question to Ask
When you receive a solar proposal, ask: "How did you size this system?" If the answer is "based on your current usage," the next question is: "Did you account for any planned efficiency improvements or load changes?" The quality of the answer will tell you a lot about the quality of the proposal.
Frequently Asked Questions
Is right-sizing just another word for a smaller system?
Not necessarily. Right-sizing means accurate sizing — which could result in a larger system than the installer first proposed if you're planning to add an EV or switch from gas to electric heating. The goal is accuracy, not minimization.
Does it matter which direction my roof faces?
Yes. A south-facing roof produces the most electricity annually. East- and west-facing roofs produce less. SPM models production estimates based on your specific roof orientation and pitch, so the system size is calibrated to what your roof will actually generate — not a generic average.
Can I add panels later if my needs change?
In many cases, yes — with some caveats. Adding panels to an existing system requires compatible inverter capacity, available roof space, and potentially additional permits. It's more expensive per watt to add panels in a second project than to include them in the original installation. SPM will factor in your plans and help you decide whether to build in headroom now or plan for a potential future addition.
Ready to see what a right-sized proposal looks like for your home? Start with a free assessment— we'll evaluate your home's full energy picture before recommending any system size.