SELECT SUM(Boiler1)
FROM Energy_Centre_Availability eca
INNER JOIN Energy_Centre_Boiler_Maintenance_Schedule s
ON eca.t_stamp > s.startDate
AND eca.t_stamp < s.endDate
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden