SQL Server 2012 :: Group By On Different Timelines?

I would like to do a group by when the timestamp is different from an usual pattern,

create table #tbl ( V1 int, V2 datetime, V3 float)
insert into #tbl values (1,'2012-12-12 10:15', 12.5)
insert into #tbl values (1,'2012-12-12 10:35', 2.5)
insert into #tbl values (1,'2012-12-12 10:45', 1.5)
insert into #tbl values (1,'2012-12-12 11:15', 10.5)
insert into #tbl values (1,'2012-12-12 11:30', 11.5)
select sum(v3) [value], v1 from #tbl group by v1
drop table #tbl

i would like to have result set as, aggregating results of every 15 minutes and when it exceeds 15 minutes then add the same in next group by. This 15 minute interval can be specified in the query itself.

value v1
16.5 1
221