And that's directly related to the voltage difference. In both countries, electric kettles have to be designed on the assumption that they can pull only 13 to 15 amps of load from the mains. This puts a hard limit on the wattage rating — but wattage is volts times amps, so the higher UK supply voltage makes higher power kettles possible. Microwave ovens, on the other hand, are typically powered by 20-amp dedicated circuits in the USA, so they can be higher power than kettles at the same supply voltage. I don't know how they're wired in the UK.

@zwol FWIW, I've never seen a microwave with a 20A plug.

@bcs I'm not sure about this but I have the impression that it's OK per US electrical code to use a NEMA 15 socket on a 20A circuit *as long as it's a dedicated circuit*, and this is one of the reasons why 20A plugs are so rare on US kitchen appliances. That said, something else is also going on, because I just checked and my microwave is rated at 17kW, which is 14.2 amps at 120V, but I can't find any electric kettle for sale that goes higher than 1.5kW (12.5A at 120V). Possibly the real concern here is that a kettle *can't* assume a dedicated circuit, so the designers have to leave some headroom in case there are lamps or something plugged into the same circuit.

@zwol you can 100% put a lower amp outlet on a higher amp circuit, and you don't need it to be dedicated. (It's the same as plugging an 8A lamp cord into a 15A socket; the load is responsible for protecting it's own cord.) In fact, 20A wires and 15A sockcts are very common. What you can't do is sell an appliance that draw more than 15A but plugs into a 15A socket.

Microwaves in the UK: all the ones I've seen (Brit living here 50+ years) are simply plugged into a 13 amp socket, like the kettle is. They're normally rated 1 kW, but some make it to 1.2kW.. Cookers (oven, hob) are usually wired into a separate 45 amp circuit.