Run this query and study the results. If you are still unsure then ask:

SELECT

DateAndTime,

AbsoluteMinutes, -- minutes since 19000101.

AbsoluteMinutes/60.00, -- INT divided by DECIMAL: decimal fraction retained.

AbsoluteMinutes/60, -- INT divided by INT: decimal fraction truncated.

(AbsoluteMinutes/60)*60, -- round down to nearest hour boundary. Brackets for clarity.

RoundedDateAndTime = dateadd(minute,AbsoluteMinutes/60*60,0)

FROM ( -- some sample data

SELECT DateAndTime = GETDATE() UNION ALL

SELECT DateAndTime = DATEADD(minute,10,GETDATE()) UNION ALL

SELECT DateAndTime = DATEADD(minute,20,GETDATE()) UNION ALL

SELECT DateAndTime = DATEADD(minute,30,GETDATE()) UNION ALL

SELECT DateAndTime = DATEADD(minute,40,GETDATE()) UNION ALL

SELECT DateAndTime = DATEADD(minute,50,GETDATE()) UNION ALL

SELECT DateAndTime = DATEADD(minute,60,GETDATE()) UNION ALL

SELECT DateAndTime = DATEADD(minute,70,GETDATE()) UNION ALL

SELECT DateAndTime = DATEADD(minute,80,GETDATE()) UNION ALL

SELECT DateAndTime = DATEADD(minute,90,GETDATE()) UNION ALL

SELECT DateAndTime = DATEADD(minute,100,GETDATE()) UNION ALL

SELECT DateAndTime = DATEADD(minute,110,GETDATE()) UNION ALL

SELECT DateAndTime = DATEADD(minute,120,GETDATE())

) t

CROSS APPLY (SELECT AbsoluteMinutes = datediff(minute,0,DateAndTime)) x

Your query has much scope for improvement. Try this - if the figures are out, it shouldn't take much effort to tweak:

;WITH temp56 AS (

SELECT

DateAndTime,

kwh= SUM(CASE WHEN tagindex = 0 THEN Val ELSE 0 END),

PF= SUM(CASE WHEN tagindex = 1 THEN Val ELSE 0 END),

LLAVG= SUM(CASE WHEN tagindex = 2 THEN Val ELSE 0 END),

LNAVG= SUM(CASE WHEN tagindex = 3 THEN Val ELSE 0 END),

[avg CURRENT]= SUM(CASE WHEN tagindex = 4 THEN Val ELSE 0 END),

frequency= SUM(CASE WHEN tagindex = 5 THEN Val ELSE 0 END)

FROM dbo.FloatTable

WHERE tagindex BETWEEN 0 AND 5

AND dateandtime >= @st

AND dateandtime <= DATEADD(dd,1,@st)

GROUP BY DateAndTime

)

SELECT

DateAndTime = x.DateGroup,

KWH= AVG(kwh),

PF= AVG(PF),

LLAVG= AVG(LLAVG),

LNAVG= AVG(LNAVG),

AVGCURRENT= AVG([AVG CURRENT]),

FREQ= AVG(frequency)

FROM temp56

CROSS APPLY (

SELECT DateGroup = CASE

WHEN @typ IN (15,30,60) THEN DATEADD(MINUTE,DATEDIFF(MINUTE,0,DateAndTime)/@typ*@typ,0)

ELSE DateAndTime END

) x

GROUP BY x.DateGroup

ORDER BY x.DateGroup