A heat pump replacing a gas furnace typically adds somewhere between 3,000 and 5,000 kWh a year to a home’s electricity use, depending on climate zone and home size. If you’re planning both, size the solar system for the combined load, not just your current usage.
The cheapest way to add capacity is during the original install — adding panels later means a second permit, a second inspection, and often a less efficient roof layout than if it had been planned as one system from the start.