- \n
- Unit cost often relies on standard times, not on the time actually consumed.<\/li>\n
- Every minute of stop, micro-stop or changeover has a cost you can calculate.<\/li>\n
- Spreading those minutes over good parts explains the cost variance, cause by cause.<\/li>\n
- Measured machine time makes cost accounting more reliable.<\/li>\n<\/ul>\n<\/div>\n
What cost accounting expects from the shop floor<\/h2>\n
Tree Partners, a French chartered accounting firm, published Expert-comptable industrie : pourquoi la comptabilit\u00e9 analytique est cl\u00e9<\/a> on its website, written by Ali Tazi (24 June 2026). The article covers the accounting specifics of manufacturing: the weight of inventory on the balance sheet, the choice of valuation method, the rational allocation of indirect production costs and, above all, calculating unit cost by product, customer, line or site, which, according to the author, requires an information system that tracks resource consumption. It separates margin on direct cost, margin on production cost and net margin, and concludes plainly: “Une comptabilit\u00e9 qui ne reconstitue pas ces marges ne sert pas \u00e0 piloter, elle sert seulement \u00e0 cl\u00f4turer.” (in other words: an accounting system that does not rebuild these margins is not a steering tool, it only closes the books).<\/p><\/blockquote>\n
The accounting method is in place. One question remains that an accountant cannot settle alone: how much machine time did each product really consume? If that time comes from a theoretical routing or a hand-entered average, the calculated margin is right on paper and wrong on the floor. That is where OEE measurement becomes financial data.<\/p>\n
Standard time or measured time: where the gap comes from<\/h2>\n
In many plants, unit cost relies on a nominal rate and a loading assumption set once a year. Yet the time actually spent producing a part number depends on what happens on the line: breakdowns, material waits, adjustments, micro-stops of a few seconds, slow running, changeovers that overrun, parts scrapped on restart. None of these events appear in the standard time.<\/p>\n
Controllers know the result: a production cost variance that shows up at month end, booked as a lump, with nobody able to say which line, which part number or which cause produced it. Micro-stops are the most misleading: too short to be logged, they can add up to a lot over a shift. Changeovers are often spread evenly across part numbers, even though a short run carries far more setup time per part than a long one.<\/p>\n
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<\/figure>\nCalculating the cost of a minute of downtime, step by step<\/h2>\n
Translating OEE into money starts with one simple value: what one minute of the line costs. Here is a five-step method to build with your controller.<\/p>\n
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- Set the hourly cost of the line.<\/strong> Add up the costs that run whether the machine produces or not: depreciation, assigned labour, base energy, maintenance, share of overheads. Divide by planned production time.<\/li>\n
- Derive the cost of one minute.<\/strong> Hourly cost divided by 60. This is the cost of a lost minute, whether it is lost to a stop, a micro-stop, a changeover or making a part that ends up as scrap.<\/li>\n
- Add lost margin if the line is a bottleneck.<\/strong> On a saturated line, a lost minute is also a lost sale. On a line with spare capacity, the cost is limited to the resources consumed. That distinction completely changes which projects come first.<\/li>\n
- Measure lost minutes by cause and by part number.<\/strong> Stops, micro-stops, changeovers, slow running, scrap: each OEE loss category is a number of minutes you can put a value on.<\/li>\n
- Spread those minutes over good parts.<\/strong> The real cost of a good part is the cost of planned time divided by the number of good parts. The gap to standard cost then breaks down cause by cause.<\/li>\n<\/ol>\n
Example: breaking down the cost of a part<\/h2>\n
For example, take a line with a full hourly cost set at 300 euros, or 5 euros per minute, a nominal rate of one part per minute, on a 480-minute shift. The figures below are illustrative: they show the method, not a real situation.<\/p>\n
- Derive the cost of one minute.<\/strong> Hourly cost divided by 60. This is the cost of a lost minute, whether it is lost to a stop, a micro-stop, a changeover or making a part that ends up as scrap.<\/li>\n
- Set the hourly cost of the line.<\/strong> Add up the costs that run whether the machine produces or not: depreciation, assigned labour, base energy, maintenance, share of overheads. Divide by planned production time.<\/li>\n