Section 179 solar depreciation is the phrase most business owners use, but it usually covers three separate tax rules that behave very differently: Section 179 expensing, bonus depreciation, and MACRS recovery. Conflating them is the single most common error we see in commercial solar proposals, including proposals written by other contractors.
Depreciation is not a rebate and not a credit. It is the recovery of an asset's cost against income over time, and its value to you is the deduction multiplied by your marginal rate. What Section 179 and bonus depreciation change is not how much you recover, but how fast. On a project of a few hundred thousand dollars, accelerating the write-off can be worth a meaningful share of the installed cost in year one.
This page separates the three mechanisms so you can hold a useful conversation with your accountant. It is not that conversation.
Verify before you rely on this
Federal depreciation rules change, and they have changed several times in recent years. Section 179 limits are indexed annually, bonus depreciation has been on a legislated phase-down that has itself been amended, and state tax codes do not all conform to the federal treatment. Every figure here is marked for verification for that reason. This page is a general explanation, not tax advice. You are responsible for verifying all tax information with a Certified Tax Accountant.
What is the difference between Section 179, bonus depreciation and MACRS?
MACRS is the default. It assigns an asset to a recovery class and spreads the deduction across that period on a fixed schedule. Solar energy property has historically been assigned a five-year class life, which is short for equipment expected to run for decades — that acceleration is deliberate policy, not an accounting quirk. Applied with the half-year convention, five-year property actually recovers across six tax years, and a mid-quarter convention can apply instead if too much of your year's asset purchases land in the final quarter.
Section 179 is an election to expense qualifying property immediately instead of recovering it over the class life. Two constraints define it:
- An annual dollar limit on the total you can expense — $1,220,000 — which phases down dollar for dollar once your total qualifying purchases for the year exceed a threshold of $3,050,000. Both are indexed and both change.
- A taxable income limitation. The deduction cannot exceed your aggregate taxable income from the active conduct of your trades or businesses. Section 179 cannot create a loss. Any disallowed amount generally carries forward.
Bonus depreciation applies a percentage of the asset's cost in the first year with no dollar cap and no income limitation. It can create or deepen a loss. The percentage has been on a legislated schedule — 100%, stepping down to 80%, 60%, 40% and 20% in successive years — and that schedule has been amended more than once, so the rate that applies to you depends on your placed-in-service year under the law as it then stands.
The short version: Section 179 is capped but selective, bonus depreciation is uncapped but blunt, and MACRS catches whatever is left.
How does the tax credit change the depreciation math?
You cannot treat the two independently. Claiming the 30% federal solar investment tax credit has historically required reducing the depreciable basis of the property by 50% of the credit claimed. The depreciation rules above then apply to the reduced basis, not to the full installed cost.
For rural and agricultural sites there is a third input. A USDA REAP grant for solar is federal money into the same asset, and it has its own effect on the basis you are entitled to depreciate and on the basis the credit is computed against. Get that modeled before you sign anything, not after.
An illustrative example
The figures below are illustrative only. They are not a quote, not a projection for your business, and not a representation that any of these amounts apply to you. Every number is marked for verification.
Take a commercial array with an installed cost of $500,000.
- Investment tax credit. At an assumed 30% rate, the credit is $150,000 against tax owed — assuming the entity has liability to offset.
- Basis reduction. Reducing basis by 50% of the credit removes $75,000, leaving a depreciable basis of $425,000.
- Accelerated deduction. Applying bonus depreciation at an assumed 60% gives a first-year deduction of $255,000, with the remaining $170,000 recovered over the MACRS schedule.
- Value of the deduction. At an assumed marginal rate of 21%, that first-year deduction is worth roughly $53,550 in reduced tax — not $255,000. A deduction is worth your rate; a credit is worth its face value.
Change any assumption — the rate, the entity type, whether Section 179 is elected first, whether your state conforms, whether a grant is involved — and the answer moves. That sensitivity is the point of the example.
Why does the order of these choices matter?
Because the elections interact, and each one changes the base the next one works from. The running order is Section 179 solar depreciation first, bonus depreciation next, regular MACRS recovery last, and all of it after the credit's basis reduction. Elect Section 179 on the wrong assets and you may waste a capped allowance on property that bonus depreciation would have covered anyway. Take a large deduction in a low-rate year and you convert a valuable deduction into a cheap one. Create a loss you did not intend and you may affect other planning entirely.
None of that is a solar contractor's call. It requires knowing your entity structure, your income across years, your state's conformity, your other capital spending and your owners' positions. We do not have that information and we do not want the liability of guessing at it. The mechanics of combining the credit with accelerated depreciation are laid out in stacking the solar tax credit with Section 179, and everything in it is still subject to your accountant's judgment.
Which businesses does this matter most to?
Section 179 solar depreciation matters most to profitable, tax-paying operations with substantial roof or ground area and a strong appetite for first-year deductions. In our own portfolio that describes two groups particularly well.
Car dealerships carry large service and showroom loads, large flat roofs, and an ownership structure that is typically tax-paying — we have built more than 1 MW across the Woody Folsom Chevrolet, Ford and Chrysler Dodge stores in Georgia. Manufacturing and warehousing facilities combine heavy demand charges with large roof areas; the 1.267 MW array and BatteryCube® storage at the Samsonite and TUMI distribution center in Vidalia is the clearest example we have.
To see how an accelerated write-off changes a payback range on your own numbers, the commercial solar savings calculator will produce a system size and a payback band from your monthly bill, and if you would rather talk it through with the engineering detail attached, ask us for a commercial solar proposal and send twelve months of utility bills. Whatever comes back, take it to a Certified Tax Accountant before you rely on the tax portion of it.