What goes wrong with Excel feasibility models

The structural problem

Excel is a blank canvas. There is no structure to enforce that your construction rate appears once, that your scenarios stay in sync, or that your formulas reference the right cells. Every error is silent until it isn't.

The eight failure modes

1. Hard-coded numbers inside formulas.The classic. Search for any return value that doesn't trace back to a referenced input.
2. Hidden tabs. Common in inherited models. Always unhide before circulating.
3. Broken named ranges. The Name Manager is the second most useful feature in Excel and the least used.
4. Circular references in finance.When interest depends on a balance that depends on interest. Excel lets you turn iterations on and pretend it's fine.
5. Out-of-date pricing. Comparable sales from six months ago, used without an adjustment.
6. Out-of-date contributions.Statutory schedules update annually. Most templates don't.
7. Annual cashflow only. Peak debt and IRR both fail under annual aggregation.
8. “Final” version that isn't. File naming is not version control.

The structural fix

Each of these errors is fixable in Excel, once you've spotted it. They're harder to spot the longer the model gets, the more analysts have edited it, and the more time has passed since it was built. A structured tool eliminates the class of error rather than catching individual instances.