How long does a solar inverter last? What’s the replacement cost?

Inverter warranties are 10-15 years, depending on the make and model. Inverter replacement costs are hard to forecast 15 years out. The inverter manufacturers are just now starting a price war similar to what the solar panel manufacturers went through over the past three years. So costs for equivalent models will likely fall considerably over the next 15 years. However, more and more value-added features are getting built into inverters, so those additional features will likely offset the price reductions going forward.