How do I compare two products when their specifications do not line up?
One quotes capacity, the other quotes dimensions; one quotes annual cost, the other quotes cost per cycle. How to normalise both onto the same scale before you compare anything.
- Difficulty
- intermediate
- Time
- 25 min
- Read
- 3 min
Short answer
Convert everything to one unit and one time period before comparing. Cost per year, capacity in litres, energy in kilowatt hours per year, warranty in months. Where a figure is quoted under different test conditions, either find the standard both are tested to or discard the number entirely — a comparison of two incomparable numbers is worse than no comparison.
Specification sheets are not written to be compared; they are written to present each product at its best. That means different units, different time bases and different test conditions, and if you compare the numbers as printed you will reliably reach the wrong conclusion. Normalising takes ten minutes and is the single highest-value step in any product comparison.
Step by step
- Build one table with one row per attribute.Write the attribute in the unit you want, not the unit each brand used. Every product then has to be translated into your table rather than your table accommodating each product.
- Put every cost on the same time base.Annual is usually clearest. Multiply monthly by twelve, convert per-cycle figures using how many cycles you actually run in a year, and add one-off costs spread over the years you expect to keep it.
- Convert capacity and size into what you care about.Litres, kilograms, number of place settings, square metres of coverage. If one brand quotes gross and the other usable, find the usable figure for both or note that the comparison is unsafe.
- Check the test conditions behind any performance figure.Energy, range, runtime, noise and coverage figures are all measured under a defined programme or condition. Two figures from different standards are not comparable, and a figure with no stated method is not a figure.
- Mark the gaps honestly rather than guessing.Leave a cell blank if a manufacturer does not publish it, and treat a blank as a mild negative. Filling gaps with estimates produces a table that looks complete and is not.
- Compare only the rows you can trust.A table with four verified rows beats one with twelve rows of mixed quality. Decide on the verified ones, and use the unverifiable ones only to break a tie.
Tips
- Warranty length is a comparable number and a useful proxy — manufacturers price warranties against expected failure rates.
- Where one product quotes a range and the other a single figure, compare like with like by taking each one's typical or rated value, not its maximum.
- Keep the normalised table. It is the thing you will want again in five years when the same product needs replacing.
Common mistakes
- Comparing a headline monthly price with a total price — It is the most common trap in contract comparisons. Convert both to total cost over the same period, including any up-front payment and any end-of-term charge.
- Assuming the same word means the same thing across brands — Capacity, coverage, runtime and even warranty are defined differently by different manufacturers. Read the footnote that defines the term before using the number.
Questions people ask
What if a manufacturer will not publish a figure I need?
Ask them directly, in writing, and treat a refusal as information. On regulated categories the figure is often in the energy label or the standards documentation rather than the marketing page.
How do I convert a per-cycle cost to annual?
Multiply by how many cycles you genuinely run, not by the manufacturer's assumed number. Household usage varies enormously and the assumed figure is usually optimistic.