;WITH Keys AS (
  SELECT TradeDate, SellerId FROM inserted
  UNION
  SELECT TradeDate, SellerId FROM deleted
),
Agg AS (
  SELECT t.TradeDate, t.SellerId,
         SUM(t.kWh) AS TotalKWh,
         SUM(t.kWh * t.Price) AS TotalAmount
  FROM dbo.Trade t
  JOIN Keys k ON k.TradeDate = t.TradeDate AND k.SellerId = t.SellerId
  GROUP BY t.TradeDate, t.SellerId
)
MERGE dbo.DailySellerSummary AS s
USING Agg AS a
ON s.TradeDate = a.TradeDate AND s.SellerId = a.SellerId
WHEN MATCHED THEN
  UPDATE SET s.TotalKWh = a.TotalKWh, s.TotalAmount = a.TotalAmount
WHEN NOT MATCHED THEN
  INSERT(TradeDate, SellerId, TotalKWh, TotalAmount) VALUES(a.TradeDate, a.SellerId, a.TotalKWh, a.TotalAmount);
DELETE s
FROM dbo.DailySellerSummary s
LEFT JOIN dbo.Trade t ON t.TradeDate = s.TradeDate AND t.SellerId = s.SellerId
WHERE t.TradeId IS NULL
  AND EXISTS (SELECT 1 FROM Keys k WHERE k.TradeDate = s.TradeDate AND k.SellerId = s.SellerId);

DECLARE @d date, @s int, @kwh decimal(12,3);
SELECT TOP (1) @d = TradeDate, @s = SellerId, @kwh = kWh FROM inserted;
UPDATE dbo.DailySellerSummary
  SET TotalKWh = TotalKWh + @kwh
WHERE TradeDate = @d AND SellerId = @s;

INSERT INTO dbo.DailySellerSummary(TradeDate, SellerId, TotalKWh, TotalAmount)
SELECT TradeDate, SellerId, SUM(kWh), SUM(kWh*Price)
FROM inserted
GROUP BY TradeDate, SellerId;

UPDATE s
  SET s.TotalKWh = s.TotalKWh + i.SumKWh,
      s.TotalAmount = s.TotalAmount + i.SumAmount
FROM dbo.DailySellerSummary s
CROSS APPLY (
  SELECT SUM(kWh) AS SumKWh, SUM(kWh*Price) AS SumAmount
  FROM inserted
) i;