We set up our dehumidifiers in a temperature- and humidity-controlled test chamber. For each model, we measure the amount of water removed during three test "runs" at different temperature/humidity combinations that reflect humid winter climates (between 8°C and 16°C and 65% to 90% relative humidity).
Cold, humid conditions present a worst-case scenario for a refrigerative dehumidifier; it's easier to extract moisture from warm air than cold air (which is why manufacturers usually quote removal rates at conditions which make their machines look the best, typically 30°C and 80% relative humidity). Refrigerative models that perform well in this test will perform even better in warm climates. Desiccant rotor models can be much more effective in cold climates because they don’t use cooling to extract the moisture; in fact, their performance should be largely unaffected by the air temperature.