Doing most of this now (swithing from gas to electric heat pumps, installing solar).

Energy Monster and other solar installers can look at both your historical electricity use, plus do estimates based on the heat pump system you are considering and size the panels accordingly. We're doing solar first, then heat pumps. Another thing you'll want the estimate to factor in, that the panels will degrade over time and the system should be sized accordingly over their likely life (20 years).