declare @t table(N int,T datetime,D int, primary key clustered (N,T));

insert @t
SELECT * FROM 
(VALUES
(25623,'20130620 14:30',5)
,(25621,'20130620 14:31',5)
,(25622,'20130620 14:32',5)
,(25624,'20130620 14:33',5)
,(25625,'20130620 14:34',5) 
,(25625,'20130620 17:34',10) 
,(25624,'20130620 17:53',7)
) as T(N,T,D);

declare @be table(N int, T datetime, E int, primary key clustered (T, N) );

insert @be
select N, T, 1 from @t
union all
select N, dateadd(minute, D, T), -1 from @t;

select t1.T, sum(t2.E)
  from @t t1 inner join @be t2 on t2.T <= t1.T
  group by t1.T
  order by t1.T
;