Define the comparison boundary
Use the same fence quantity, substrate, geometry, installation location and required performance for both options. Identify which costs are common and which genuinely differ. If one proposal changes the panel or post as well as the coating, separate those changes rather than attributing every saving to the finish.
Choose a comparison period that suits the project. State whether the calculation includes installation, inspections, cleaning, repairs, access equipment, closures, disposal and any residual value. Avoid including a cost in one option and silently omitting it from the other.
The calculation is a decision model, not proof of coating life. Maintenance intervals and repair quantities need an evidence basis or a clearly labelled assumption. Keep warranties separate from those assumptions because their conditions and covered costs may differ.
Build an undiscounted example first
The following invented currency-unit example covers the same fence over 20 years. It is not a price list, life prediction or financial recommendation. Assume routine inspections are identical and excluded from both options. All listed costs are on the same constant-price basis, with no tax, inflation or discounting.
| Cost item | Option A | Option B |
|---|---|---|
| Initial coating-related supply cost | 8,000 | 10,000 |
| Cleaning: 300 every 2 years through year 20 | 3,000 | 3,000 |
| Repairs at year 5, 10 and 15: 1,200 each | 3,600 | 0 |
| Repair at year 10 | 0 | 1,500 |
| Access per repair visit: 400 | 1,200 | 400 |
| Total over the stated period | 15,800 | 14,900 |
Option B starts 2,000 higher but totals 900 lower under these assumptions. There are ten cleaning events, three A repair visits and one B repair visit. Repairs at year 20 are excluded from both because the model assumes no further service within its boundary; document a different end condition if the project requires one.
Test the assumption that decides the result
Suppose Option A needs only two repair visits rather than three. Removing one 1,200 repair and its 400 access cost reduces A to 14,200. It is then 700 lower than B. The decision has reversed without changing the initial quotations.
That sensitivity tells the buyer where more evidence is valuable: the likely repair requirement and access burden. It does not justify choosing whichever interval produces the preferred answer. Seek relevant service records, condition surveys or a project-specific maintenance assessment.
Access can also dominate. If a repair requires traffic management or closure of an operating facility, use the actual project estimate and define which party bears it. Do not insert an arbitrary percentage for disruption and call the result precise.
Add timing only with an agreed method
An undiscounted constant-price total is easy to audit but treats a cost next year and the same cost in year nineteen equally. If the organisation uses discounted cash flow, apply its approved rate and consistent treatment of inflation to every option.
The basic present-value expression is cost at year t divided by (1 + r) raised to t, where r is the agreed annual discount rate. Record the selected rate, price basis and timing convention. A rate chosen only to favour one proposal makes the comparison less useful.
There is no need to add complexity when uncertain maintenance quantities dominate the decision. First fix the scope and test plausible scenarios; then use the organisation's financial method where it improves the assessment.
Keep performance evidence beside cost
A low modelled total cannot compensate for a finish that fails a mandatory project requirement. Confirm the substrate, preparation, coating route and acceptance evidence before comparing cost. Laboratory corrosion results should not be converted into years to populate the model without a justified relationship.
Include repair compatibility and replacement availability. A small local repair that preserves the system is different from repainting an entire elevation for colour consistency. State what event triggers maintenance and what condition marks the end of the comparison.
Record the decision and revisit it
Save the quantities, quotations, assumed events and sensitivity results with the selected option. During service, record actual cleaning, repairs and access costs against the model. Those records improve the next purchase far more than a generic claim of long life.
Use the coating specification reference to preserve the technical scope and the local repair guide to define what a repair includes. The final decision should explain which assumptions matter and what evidence supports them.