Why not simplify the whole thing by doing something like this:

SET @reportDate = '2009-01-10 00:00:00.000' --hard coded date for now

SELECT

sr.SiteName,

b.BldgName AS BuildingName,

mm.CommodityType,

@reportDate AS DaySelected,

DATEPART(hour, dls.TimeStampUTC) AS [HOUR],

MAX(dl.value) AS MaxValue,

MIN(dl.value) AS MinValue,

(MAX(dl.value) - MIN(dl.value))AS Consumption

FROM

Ion_Data_Test..DataLogStamp dls

INNER JOIN Ion_Data_Test..DataLog dl

ON dls.ID = dl.DataLogStampID

INNER JOIN ION_Data_Test..Source s

ON s.ID = dls.SourceID

INNER JOIN TEE..MeterMapper mm

ON mm.Meter_Name = s.Name

INNER JOIN TEE..TEGInstance ti

ON mm.TEGInstanceID = ti.ID

INNER JOIN TEE..SiteRef sr

ON sr.ID = ti.SiteRefID

INNER JOIN TEE..Building b

ON b.ID = mm.Bldg_ID

WHERE

dls.SourceID = 8

AND dl.QuantityID = '10005' --KWH

AND mm.CommodityType = 'ELECTRICITY'

AND dls.TimeStampUTC >= @reportDate

AND dls.TimeStampUTC < @reportDate + 1

GROUP BY

sr.SiteName,

b.BldgName,

mm.commodityType,

DATEPART(hour, dls.TimeStampUTC);

Also, there's a more efficient way to round to the nearest hour:

declare @TimeVariable datetime;

select @TimeVariable = '6/9/09 4:11 PM';

select dateadd(hour, datediff(hour, 0, @TimeVariable), 0);