Geography Inside a CTE
Using the examples from SQL Geography data type column in GROUP BY clause, I was able to group by a geography location.  See if this works for you.

--demo set up
Declare @Locations table (ID int, StaticLocation GEOGRAPHY)
Declare @Capacity table (LocationId int, SpareCapacity int)
insert into @Locations(id, StaticLocation) values
(1,geography::Point(41.878899,-87.636297, 4326)),
(2,geography::Point(40.749014,-73.985317, 4326))

insert into @Capacity(LocationId,SpareCapacity) values
(1,100),
(1,1000),
(2,200),
(2,2000)

--Solution
SELECT GEOGRAPHY::STGeomFromText(L.StaticLocation.STAsText(), 4326) as StaticLocation
    ,sum(SpareCapacity) as SumSpareCapacity
FROM @Capacity UC
JOIN @Locations L
    ON L.Id = UC.LocationId
--WHERE u.StaticLocation.STDistance(geography::Point(53, -2, 4326)) / 1609.344 &lt; 10   
GROUP BY L.StaticLocation.STAsText()




| StaticLocation                                 | SumSpareCapacity |
|------------------------------------------------|------------------|
| 0xE6100000010C821FD5B0DF5F444057CC086F0F7F52C0 | 2200             |
| 0xE6100000010C59BE2EC37FF04440BF620D17B9E855C0 | 1100             |