fix stacking of multiple power sources

[eringerli]

Proposed change

The power sources should be stacked and thus added, not averaged. This allows for multiple power sources per category (solar, battery, grid).

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for [www.home-assistant.io][docs-repository]

[MindFreeze]

Thanks @eringerli