Registering a formula allows ExcelWriter to calculate a function a certain way. However, once the workbook opens inside of Microsoft Excel, Excel will calculate the formula again with its own calculation engine. This means that if you registered a function name that does not exist in Excel, your formula will evaluate to #NAME? once Excel recalculates it. If you overwrote a formula with you own implementation, the formula will be recalculated using the implementation that is built into Excel. To keep Excel from re-calculating the formulas upon opening the workbook or upon changing values in the workbook, go to File -> Options -> Formulas. Under the "Calculation Options" header, there is an option called "Workbook Calculation". Please select the "Manual" option and uncheck the "Recalculate workbook before saving" checkbox. Now Excel will not re-calculate formulas. These settings will allow you to preserve the values that ExcelWriter calculated.