When I've looked at one for my home to replace a 24kW gas boiler, all I can find is 14kW kits. Am I missing something? Can I not match the output of a big gas boiler?
You're right, it can't match the _peak_ output, but the whole system is designed so this isn't necessary.
With a "traditional" combi-boiler, it will turn on occasionally to heat the water in the central heating system, and continue to pump it around. When you need hot water, it switches over the heat-exchanger to directly heat the incoming water and this is when it will ramp up and burn the most gas (24kW+).
For an air-to-water heat pump system, the system will be on almost continually, but will heat the water to a lower temperature, the peak power output is less, but it averages out the same. For hot water, the system will heat a storage cylinder.
One of the catches here is that's it not always an easy retrofit as the lower water temperature can necessitate larger radiators.
https://www.heatgeek.com/do-i-need-to-upgrade-my-radiators-f...
edit: In-depth geeky video on this:
https://protonsforbreakfast.wordpress.com/2022/04/05/what-si...
Not sure why the market has moved this way but it is what it is. That unit probably is underspeced
https://carbonswitch.com/heat-pump-water-heater-buyers-guide...