Upgrades / adding to existing systems

Add the existing panels and inverter to the system design, just as you would for quoting new ones.
Then on the Solar and Battery Designer page, click the 'pre-existing' box next to each inverter and MPPT. That way their energy production is factored into the site and battery charge, but they are not added to the quote price.

The consumption profile on the Energy page needs to be “before solar”, i.e., the customer’s actual consumption - not the net energy imported from the grid (ie reduced consumption due to existing solar).
The solar export earnings from the existing system do not contribute towards the payback/ROI of the battery
The payback periods for battery-only upgrades on existing solar are usually quite long.

Hybrid inverters with a battery, but no panels

If you are upgrading an existing system by adding a hybrid inverter and battery but no solar panels, and just using it as if it were an AC-coupled battery, there is a solution:

Add the existing panels and inverter to the system design, just as you would for quoting new ones.
Then on the Solar and Battery Designer page, click the 'pre-existing' box next to each inverter and MPPT. That way their energy production is factored into the site and battery charge, but they are not added to the quote price.
Click add inverter
Select the hybrid inverter you wish to use
In the PV module section, select ‘Blank panel’
Check the ‘pre-existing’ box next to the panel name
Add the battery
Do not place the ‘Blank panel’ on the map