Thanks, @anon11644107, I'd not found that link in my searching.
No, the source entity is not a template. It's provided by a custom integration. It's a single entity (sensor.wattpilot_totally_charged) and has State class : Total, Unit of measurement : kWh, Device class : energy. The utility meter is not intended for use in the energy dashboard - I just want the previous month total & the mtd total.
There is not an option to include an availibility condition with a UI-created utility meter. If this is a necessary component, shouldn't it be an option in the UI?
Found this link to some detail about yaml-created utility meters Utility Meter. I'll try this.