Mechanical Completion vs Substantial Completion vs COD
A project can hit every date in its EPC contract and still be catastrophically late under its PPA. The EPC contractor's clock and the offtaker's clock are set by two different documents, tested against two different standards, and stopped by two different events — and the space between them is uninsured, uncapped, and paid by the sponsor. On a 75-day COD slip where the contractor is only 36 days late, the sponsor can absorb seven figures without anyone breaching anything.
This is not a drafting curiosity. It is the most common way a project that "came in on schedule" still blows its first-year distributions. You will get precise definitions of the three completion milestones, the specific mechanisms that pull substantial completion and COD apart, a worked Excel model that sizes the uncovered gap in days and dollars, and the checks a lender runs before it funds.
What Are the Three Completion Dates in a Power Project?
A utility-scale project has three completion milestones that people use interchangeably in conversation and then discover, in a dispute, that the contracts never did. Mechanical completion and substantial completion live in the EPC agreement. Commercial operation date lives in the PPA. Two contracts, two counterparties, two sets of remedies.
The sequence runs mechanical completion → commissioning and testing → substantial completion → COD. Each one hands off a different thing: physical custody, performance risk, and revenue.
| Mechanical Completion | Substantial Completion | Commercial Operation Date | |
|---|---|---|---|
| Lives in | EPC agreement | EPC agreement | PPA |
| The test | All principal components erected and installed per spec | Performance and capacity tests passed; plant capable of operating | Commissioning complete; facility delivering at or near contract capacity |
| Punch list | Extensive; work continues | Minor items only, non-impairing | Not a PPA concept |
| Interconnection | Not required | Typically excluded from the test | Required — the grid operator must be dispatching the plant |
| What it triggers | Commissioning begins; milestone payment | Delay LDs stop; warranty and performance guarantee periods start; care, custody and control transfer; retainage released | PPA payments begin; PPA delay damages stop; construction-to-term loan conversion |
| Who is exposed if it is late | Contractor | Contractor | Sponsor |
That last row is the whole article. Two of these dates are the contractor's problem. The third is yours.
What Is Mechanical Completion?
Mechanical completion is the point at which all principal components of the plant have been erected or installed in accordance with the design, and the system is physically ready for commissioning. It is a construction milestone, not a performance one — nothing has been proven to work yet.
At mechanical completion the EPC contractor typically conducts a detailed walkdown, often with the owner or an independent engineer present, and the parties agree what remains. Per the Solar Best Practices commissioning guidance, this inspection is what opens the commissioning window rather than closing the construction one.
What it does not do: start any warranty, stop any delay clock, or transfer performance risk. A plant at mechanical completion is a very expensive object that has never generated a kilowatt-hour.
ℹ️ Note: Mechanical completion is where energization and backfeed usually sit in the schedule. That makes it the first milestone genuinely dependent on a third party — the interconnecting utility — even though the contract treats it as a contractor obligation.
What Is Substantial Completion?
Substantial completion is the date the plant is complete and functional — capable of generating electricity — with only punch list items and, critically, interconnection outstanding. It is the single most consequential date in the EPC agreement, because it is the one that stops the contractor's delay liquidated damages.
Stoel Rives' Law of Solar guide and the Akin Gump bankable-EPC checklist both frame substantial completion as the pivot point of the contract. It simultaneously:
- Stops delay LDs. The contractor's exposure to schedule damages ends here.
- Starts the warranty period. Equipment is placed in service and the clock on defect liability begins.
- Starts the performance guarantee period. The window in which the plant is monitored against its guaranteed output opens.
- Transfers care, custody and control. The asset becomes the owner's to insure and operate.
- Releases retainage and reduces or releases the performance security.
Note what is missing from that list: revenue. Substantial completion does not make the offtaker pay you.
⚠️ Warning: Substantial completion is conventionally defined to exclude interconnection. That is not sloppy drafting — the contractor genuinely cannot control when a utility energizes a line. But it means the contractor can achieve substantial completion, stop its LD clock, collect its milestone payment and hand you the keys to a plant that legally cannot reach COD.
What Is the Commercial Operation Date?
The commercial operation date is the PPA milestone at which the facility has completed all commissioning and is operating at or near its nameplate capacity and expected output, with the grid operator dispatching it or issuing it a generation schedule. COD is when the offtaker starts paying the contract price.
COD carries a different bundle of consequences, all of them financial rather than physical: the PPA payment obligation begins, the seller's delivery and availability obligations begin, any guaranteed-output regime starts running, the performance bond is typically released, and on the financing side the construction loan converts to term debt. In a project finance model built around DSCR, COD is the row where the revenue line switches on and the debt sizing assumptions start being tested against reality.
It is also the date with teeth on the other side. If the seller misses the guaranteed COD, a typical utility-scale PPA gives the buyer some combination of delay damages, the right to demand additional credit support, and — past a long-stop date — the right to terminate outright.
graph LR
A[Mechanical Completion<br/>EPC clock] --> B[Commissioning<br/>and Capacity Testing]
B --> C[Substantial Completion<br/>EPC delay LDs STOP]
C --> D{Grid operator<br/>dispatching?}
D -->|Yes| E[COD<br/>PPA delay damages STOP]
D -->|No| F[THE GAP<br/>no contractor liability<br/>sponsor fully exposed]
F --> E
Why Do Substantial Completion and COD Drift Apart?
Substantial completion and COD drift apart because they are tested against different standards by different counterparties. The EPC test is "can this plant run"; the PPA test is "is this plant running, at capacity, on the grid." Anything that satisfies the first without satisfying the second opens a gap the contractor is not liable for.
Four mechanisms do most of the damage.
1. Interconnection Sits Outside the EPC Test
This is the big one. Substantial completion typically carves out interconnection; COD absolutely requires it. When the utility is late energizing, the contractor's clock has already stopped and yours is still running. There is no breach anywhere in the contract stack — and you are paying PPA delay damages daily.
2. Capacity Is Measured Differently in Each Document
The EPC capacity test is run at reference conditions defined in the contract, often over a short test window with correction factors applied. The PPA's contract capacity is a commercial number. If the EPC test passes at corrected conditions but the plant cannot demonstrate contract capacity to the offtaker's satisfaction under actual conditions, you have satisfied one document and not the other.
3. Force Majeure Is Asymmetric
The EPC force majeure definition and the PPA force majeure definition are negotiated separately, usually by different lawyers, sometimes months apart. An event that extends the contractor's guaranteed substantial completion date does not automatically extend your guaranteed COD. Where the PPA's FM definition is narrower — and it usually is — the contractor gets relief and the sponsor does not.
4. The Dates Were Never Aligned in the First Place
The structural fix is well understood and frequently ignored. As Taft's analysis of aligning the EPC agreement and the PPA puts it: the EPC guaranteed substantial completion date must fall on or before the PPA guaranteed COD. If your PPA guarantees COD on 1 June, the EPC guaranteed substantial completion date must be 1 June or earlier — never later. An EPC milestone dated after the guaranteed COD builds the gap into the contracts on day one.
💡 Pro Tip: Model the two clocks as two separate date columns that never reference each other. The instant your model computes COD as "substantial completion plus 30 days," you have assumed away the entire risk this article is about.
How Do You Size the Delay Gap in Excel?
Size the gap by running two independent delay counts — one against the EPC guaranteed substantial completion date, one against the PPA guaranteed COD — and subtracting recovered LDs from PPA damages. The uncovered days, not the total days, are what the sponsor actually pays for.
All inputs below are illustrative assumptions, not market quotes. The structure is the point; substitute your own deal terms.
The Inputs
| Input | Cell | Illustrative value |
|---|---|---|
| EPC contract price | C3 |
$85,000,000 |
| EPC guaranteed substantial completion | C4 |
2027-05-15 |
| Actual substantial completion | C5 |
2027-06-20 |
| PPA guaranteed COD | C6 |
2027-06-01 |
| Actual COD | C7 |
2027-08-15 |
| EPC delay LD per day | C8 |
$42,500 (0.05% of contract price) |
| EPC delay LD cap | C9 |
10% of contract price |
| PPA delay damages per day | C10 |
$55,000 |
The Two Delay Counts
Count each clock against its own contract. Never derive one from the other:
=MAX(0, C5 - C4)
=MAX(0, C7 - C6)
That returns 36 days of EPC delay and 75 days of COD delay. The contractor is late. You are more than twice as late.
The Uncovered Days
=MAX(0, (C7-C6) - (C5-C4))
Thirty-nine days. On those days there is no contractor liability of any kind, because the contractor's obligation was discharged at substantial completion.
LD Recovery and the Cap
Recovery is the lesser of accrued LDs and the cap:
=MIN((C5-C4) * C8, C9 * C3)
That is MIN(36 × 42,500, 8,500,000) = $1,530,000. It is worth computing how long the cap would actually last:
=(C9 * C3) / C8
Two hundred days. The cap is not the binding constraint here — which is exactly the trap. Sponsors negotiate hard on the LD cap and then lose the money to date misalignment, a completely different failure mode that a bigger cap does nothing to fix.
The Net Exposure
=(C7-C6) * C10 - MIN((C5-C4) * C8, C9 * C3)
75 × 55,000 − 1,530,000 = $2,595,000 uncovered, on a project where the contractor was only 36 days late and never came close to its liability cap.
Sensitivity on the Alignment
The single highest-leverage variable is not the LD rate. It is the float between the EPC guaranteed substantial completion date and the PPA guaranteed COD. Build a one-way data table on C4 and watch the exposure move:
| EPC guaranteed date vs PPA COD | EPC delay days | Uncovered days | Net exposure |
|---|---|---|---|
| 45 days before | 64 | 11 | $1,405,000 |
| 30 days before | 49 | 26 | $2,042,500 |
| 17 days before (base case) | 36 | 39 | $2,595,000 |
| Same day | 19 | 56 | $3,317,500 |
| 15 days after | 4 | 71 | $3,955,000 |
Pulling the EPC guaranteed date 45 days ahead of guaranteed COD cuts the sponsor's exposure by roughly $1.2m in this example, without renegotiating a single LD rate. For the mechanics of building that table, see our guide to sensitivity analysis in Excel.
⚠️ Warning: PPA delay damages are frequently not the whole exposure. Where the offtaker has to buy replacement power, or where a missed COD costs you a tax credit vintage or a capacity award, the real number is larger and does not sit in the PPA's damages clause at all. Model those separately.
There Is a Fourth Date, and It Is the One With No Counterparty
This post has treated the problem as two contracts and two clocks, which is how it presents itself when the EPC and the PPA are on the desk. The full picture has more dates, and the two additions are both worse than the ones above because neither has anybody to negotiate with.
The placed-in-service date for tax purposes. A project's credit eligibility depends on being placed in service within a defined window, and under the continuity safe harbour that is the end of the fourth calendar year following the year construction began. There is a second, earlier constraint now: under the One Big Beautiful Bill Act, wind and solar must begin construction on or before 4 July 2026 to keep that four-year window, or else be placed in service by 31 December 2027.
Missing a contractual date costs liquidated damages. Missing the tax date costs the credit, and no amount of negotiating leverage recovers it — there is no counterparty on the other side of the deadline.
Financial completion. Distinct from COD, and it is the financing's own test: performance tests passed and certified, reserve accounts funded, the offtake and O&M agreements in force, a projected coverage ratio met on the lender's case, and no default subsisting. Satisfying it converts the construction facility to a term facility and releases the sponsor's completion support.
Financial completion can lag COD by months. Throughout that period the project is generating and selling power while the sponsor's completion guarantee remains live — a contingent liability with a real cost even when nothing goes wrong.
What Does the Gap Cost on the Financing Side?
More than the revenue the LD calculation covers, and the additional items are usually absent from the model.
Interest keeps accruing and is capitalised. Every day between substantial completion and COD adds to the interest capitalised into project cost, which permanently increases the balance repaid over the whole facility term. A delay is therefore not a one-off cost offset by LDs — it raises the debt.
The availability period can expire. A construction facility is drawable only within a defined window. A gap long enough to run past it requires lender consent to extend, sought at the moment the sponsor has least leverage.
Commitment fees continue on the undrawn portion, regardless of fault.
Contingency is consumed, and once it is, the certification that remaining funds suffice to complete fails — which stops drawings before any milestone is missed.
The practical instruction follows directly from the arithmetic this post has already run: the thirty-nine unrecoverable days are the visible cost, and the invisible one is what those days did to the capitalised interest, the fee accrual and the contingency balance. Model the gap as a package rather than as a revenue line, because the revenue line is the smallest part of it.
To build the two-clock model, the float analysis and the full cost of the gap, prompt Dezzmond with your EPC and PPA dates.
What Do Lenders Actually Check?
Lenders look for one thing above all: that the EPC delay LD regime is sized to cover debt service through the delay, and that the EPC guaranteed date sits ahead of the PPA guaranteed date with real float.
Delay LDs exist to replace lost revenue and cover debt service during a delay — that is their commercial purpose, not to punish the contractor. So the test a lender applies is arithmetic: does the EPC delay LD per day cover daily debt service plus fixed O&M through the expected delay window? If it does not, the coverage shortfall lands on the sponsor's equity and, in a bad case, on a debt covenant in the first test period after COD.
On caps, published ranges vary meaningfully by market and source. The Climate Solutions Legal Digest survey and practitioner guides such as SgurrEnergy's bankable-EPC guide describe delay LD caps commonly in the 10–15% band of contract price, performance LD caps in a similar range, an aggregate LD cap often around 20–25%, and overall contractor liability frequently capped near 100% of contract price — while other sources report delay caps as low as 5–10%. Treat any single figure as a negotiating anchor, not a standard.
The more useful question is not "is my cap market" but "how many days of delay does my cap actually buy" — the cap divided by the daily LD rate, as above. Two hundred days of cap against a 39-day uncovered gap tells you your money is being lost somewhere the cap cannot reach.
Frequently Asked Questions
What is the difference between mechanical completion and substantial completion?
Mechanical completion means the plant is physically built and installed per the design and is ready for commissioning; nothing has been tested. Substantial completion means the plant has passed its performance and capacity tests and is capable of operating, with only punch list items left. Mechanical completion opens the testing window; substantial completion closes it, stops delay LDs, starts the warranty, and transfers care, custody and control.
Can COD happen before substantial completion?
Yes, and it is a problem when it does. If the PPA defines COD against a lower threshold than the EPC capacity test, the project can start delivering under the PPA while the contractor still holds care, custody and control. You then have offtake obligations on an asset you do not yet control, with unresolved questions about who bears an outage during that window. Align the definitions or make COD expressly conditional on substantial completion.
What happens if the utility causes the interconnection delay?
Usually nothing recoverable from the contractor, because substantial completion is typically defined to exclude interconnection. The contractor's clock stops on time; yours does not. Interconnection agreements rarely provide meaningful damages for utility delay, so this exposure generally has to be managed through float in the guaranteed COD, an explicit COD extension for interconnection delay in the PPA, or delay-in-start-up insurance.
How big should the EPC delay LD cap be?
Sized to your debt service, not to a market percentage. Work out daily debt service plus fixed O&M, multiply by a realistic worst-case delay window, and test whether the cap covers it. Published caps commonly cluster in the 10–15% range of contract price with aggregate LD caps around 20–25%, but those are anchors. A 15% cap on a project with thin coverage can still leave equity exposed.
Do delay liquidated damages cover debt service?
That is their intended function — replacing lost revenue and covering debt service and related losses during the delay period. Whether they actually do is a calculation you must run rather than assume. Divide the LD cap by the daily LD rate to get the days of cover, compare the daily LD rate to daily debt service, and check both against the delay scenario your independent engineer considers plausible.
Closing: Two Contracts, Two Clocks, One Balance Sheet
Mechanical completion, substantial completion and COD are not three names for finishing a project. They are three different allocations of risk, and only one of them protects the sponsor. The contractor's exposure ends at substantial completion. Yours ends at COD. Every day between those two events is paid for out of equity.
The fix is unglamorous and almost entirely front-loaded: align the guaranteed dates with deliberate float, define capacity the same way in both documents, reconcile the two force majeure clauses, and size the LD regime against debt service rather than against a percentage someone quoted. Then build the two clocks into the model as independent inputs, so the gap is visible in the base case instead of arriving as a surprise in the first quarter after COD.
Model it before you sign it. The 39 days you cannot recover are much cheaper to find in a spreadsheet than in a term sheet.
This post is general information about common contract structures, not legal advice. Contract definitions vary materially between deals — read your own documents and take advice on them.
Sources: Taft — Power Project Development: Aligning the EPC Agreement and Power Purchase Agreement · Akin Gump — A Legal and Commercial Checklist for Bankable Solar EPC · Stoel Rives — The Law of Solar: Design, Engineering and Construction · SgurrEnergy — Preparing a Bankable EPC Contract