SUM Mixed With Case In Select Statement

Question

I am attempting to run this SELECT statement, but I keep getting error -

Msg 130, Level 15, State 1, Line 9
Cannot perform an aggregate function on an expression containing an aggregate or a subquery.

This is my syntax, how should this be re-written in order to succesfully execute? I also add here make-up DDL to better illustrate question.

Declare @info TABLE (region varchar(50), p1 float, p2 float, p3 float, p4   float, p5 float, necombs float, d1 datetime)
INSERT INTO @info (region, p1, p2, p3, p4, p5, necombs, d1) VALUES
('west', 1.11, 2.22, 3.33, 4.44, 5.11, 22.00, '01/01/2015 08:00:00.000')
,('east', 2.11, 3.22, 3.33, 1.44, 2.11, 12.00, '01/03/2016 08:00:00.000')
,('north', 1.11, 2.22, 3.33, 4.44, 5.11, 22.00, '01/05/2015 08:00:00.000')

Select
region
,SUM(CASE WHEN YEAR(d1) = '2015' THEN ISNULL(p1, 0)+ISNULL(p2, 0)+ISNULL(p3, 0)+ISNULL(p4, 0)+ISNULL(p5, 0)/NULLIF(SUM(ISNULL(necombs, 0)), 0) ELSE 0 END) [Answer]
FROM @info
Group By Region

Answer 1

The accepted answer doesn't show the 2016 row, which I would expect to be 0 based on the initial SQL in the question.

Here's an alternative that would give the additional 2016 row:

 SELECT i.region,
        DATEPART(YEAR, i.d1) AS YearNumber,
        SUM(rd.Calc) / ISNULL(SUM(ISNULL(necombs, 0)), 0) Answer
 FROM @info i
 CROSS APPLY (VALUES(CASE WHEN DATEPART(YEAR, d1) = '2015' THEN ISNULL(p1, 0) + ISNULL(p2, 0) + ISNULL(p3, 0) + ISNULL(p4, 0) + ISNULL(p5, 0) ELSE 0 END)) AS rd(Calc)
 GROUP BY Region, DATEPART(YEAR, i.d1);

Answer 2

DECLARE @info TABLE
(
                    region  VARCHAR(50),
                    p1      FLOAT,
                    p2      FLOAT,
                    p3      FLOAT,
                    p4      FLOAT,
                    p5      FLOAT,
                    necombs FLOAT,
                    d1      DATETIME
)
INSERT INTO @info
(region,
 p1,
 p2,
 p3,
 p4,
 p5,
 necombs,
 d1
)
VALUES
('west',
 1.11,
 2.22,
 3.33,
 4.44,
 5.11,
 22.00,
 '01/01/2015 08:00:00.000'
),
('east',
 2.11,
 3.22,
 3.33,
 1.44,
 2.11,
 12.00,
 '01/03/2016 08:00:00.000'
),
('north',
 1.11,
 2.22,
 3.33,
 4.44,
 5.11,
 22.00,
 '01/05/2015 08:00:00.000'
);
WITH RawData
     AS (SELECT i.region,
                i.d1,
                DATEPART(year, d1) AS YearNumber,
                ISNULL(p1, 0) + ISNULL(p2, 0) + ISNULL(p3, 0) + ISNULL(p4, 0) + ISNULL(p5, 0) AS Calc,
                necombs
         FROM @info i)
     SELECT rd.region,
            rd.YearNumber,
            SUM(rd.Calc) / ISNULL(SUM(ISNULL(necombs, 0)), 0) Answer
     FROM RawData rd
     WHERE YearNumber = '2015'
     GROUP BY Region,
              YearNumber

Hope this helps.