Add a filter to timeseries pairs
You have two subqueries that both contain a column named datetime. When you say:
where datetime >= DATEADD(Hour, -25, GETDATE())
You need to tell SQL Server which one you mean. So either:
where tOn.datetime >= DATEADD(Hour, -25, GETDATE())
... or ...
where tOff.datetime >= DATEADD(Hour, -25, GETDATE())
I'm going to guess it should be tOn.