The calculator above takes four numbers off your electricity bill and your address, and returns what solar would actually pay back at your building. It pulls real typical-year production for your location, applies the 30% federal investment tax credit, adds the MACRS depreciation you can actually use against your own tax liability, and reports a payback period with your electricity rate held flat.
That last part is the whole point. It is also why this number will usually be worse than the one an installer shows you.
Why our payback is longer than the one in your proposal
Nearly every commercial solar proposal assumes your utility rate rises three or four percent every year for twenty five years. Compound that and the savings in year twenty are more than double the savings in year one, which pulls the payback period in dramatically. The assumption is buried in a footnote, if it appears at all.
Your rate is held flat here. Not because rates never rise, but because we do not know that they will, and a number you might spend six figures against should not depend on a forecast we cannot defend. If your rate does climb, your real payback arrives sooner than the one shown. That is the direction an honest error should run.
What the calculator counts
The 30% federal investment tax credit. A credit against federal tax owed, based on the installed cost of the system. It is statutory, not a projection.
MACRS depreciation. Solar is five-year property, and the depreciable basis is reduced by half the credit you claimed, per the IRS basis-reduction rule. The resulting deduction shields income at your effective tax rate.
Your actual ability to use them. This is where most calculators quietly lie. A tax credit is worth nothing to a business with no tax liability, and depreciation is worth nothing to a business with no income to shield. The calculator asks how much federal tax you can offset in a year, draws the credit first and then the depreciation against that limit, and if any benefit is still unused after twenty five years, it drops it rather than counting it.
Real production for your location. Typical-year output for your coordinates, not a national rule of thumb multiplied by your roof size. Northern Illinois lands somewhere near 1,150 to 1,350 kWh per installed kW per year, and the exact figure depends on where you are.
What it refuses to count, and why that matters
The result names its own omissions. Each of these makes a real project better than what we showed:
USDA REAP grants. Rural small businesses and agricultural producers can win a grant covering a substantial share of project cost. It is competitive and application dependent, so we will not put a number on it. If you farm, or your business sits in a rural area, this is likely the single largest thing missing from your result.
C-PACE financing. Commercial Property Assessed Clean Energy financing can cover the whole project through an assessment on the property, repaid over a long term. It changes the shape of the deal rather than its size, and it can turn a project you cannot fund into one that cash flows from day one. Your result assumes you pay cash up front, which is the most conservative case.
Illinois Shines renewable energy credits. Illinois pays for the renewable energy credits your system produces. Block prices change by program year and eligibility has conditions, so we do not generate a figure for it. Most real Illinois projects carry meaningful value here.
What solar does not do to your bill
Solar does not reduce your demand charge. The calculator credits it with exactly zero, and that is not conservatism, it is the tariff.
Your demand charge is set by a single highest interval of power draw in the billing period. Solar can only reduce that interval if the peak happens to fall when the sun is out and the sky is clear. One overcast afternoon, or one peak that lands at seven in the evening, and the charge is untouched for the entire month. For many commercial sites the demand charge is a large share of the bill, and any proposal that credits solar with reducing it is describing something the tariff does not do.
The same logic applies to exported power. Illinois net metering has been supply-only since 2025, so the kilowatt-hours you push back to the grid earn roughly the supply half of retail rather than the full rate. Whether your production is consumed on site or exported therefore changes what it is worth, which brings us to the one assumption we cannot resolve from a bill.
The assumption only your meter can settle
The calculator assumes a share of your production is consumed on site rather than exported. That share is the largest unknown in a bill-only screen, and it varies enormously: a business running weekday daytime hours absorbs most of its midday production, while one that runs at night or only on weekends exports far more of it at the lower rate.
A bill cannot tell us which you are, because a bill reports monthly totals. Your interval meter data can, because it records your usage in short intervals across the whole month, which is exactly the resolution needed to line your load up against a solar production curve. If you want the assumption replaced with a measurement, that is the path.
