Calculate total size of transaction log backups in the last 24 hours

I wanted to get the total backup sizes in KB, MB and GB for each database on an instance. I have pieced together the below but haven't been able to do the sum part.

SELECT  
   A.[Server],
    A.DATABASE_NAME,   
   A.last_db_backup_date,  
   B.backup_start_date,  
   B.expiration_date, 
   CAST(CAST(b.backup_size / 1000 AS INT) AS VARCHAR(14)) + ' ' + 'kB' AS bkSize,
   CAST(CAST(b.backup_size / 1000000 AS INT) AS VARCHAR(14)) + ' ' + 'mB' AS bkSize,
   CAST(CAST(b.backup_size / 1000000000 AS INT) AS VARCHAR(14)) + ' ' + 'gB' AS bkSize,    
   B.logical_device_name,  
   B.physical_device_name,   
   B.backupset_name, 
   B.description 
FROM 
   ( 
   SELECT   
       CONVERT(CHAR(100), SERVERPROPERTY('Servername')) AS Server, 
       msdb.dbo.backupset.database_name,  
       msdb.dbo.backupset.backup_finish_date AS last_db_backup_date 
   FROM    msdb.dbo.backupmediafamily  
       INNER JOIN msdb.dbo.backupset 
       ON msdb.dbo.backupmediafamily.media_set_id = msdb.dbo.backupset.media_set_id 
   WHERE   msdb..backupset.type = 'l' 
   --GROUP BY 
   --    msdb.dbo.backupset.database_name  
   ) AS A 

   LEFT JOIN  

   ( 
   SELECT   
   CONVERT(CHAR(100), SERVERPROPERTY('Servername')) AS Server, 
   msdb.dbo.backupset.database_name,  
   msdb.dbo.backupset.backup_start_date,  
   msdb.dbo.backupset.backup_finish_date, 
   msdb.dbo.backupset.expiration_date, 
   msdb.dbo.backupset.backup_size,  
   msdb.dbo.backupmediafamily.logical_device_name,  
   msdb.dbo.backupmediafamily.physical_device_name,   
   msdb.dbo.backupset.name AS backupset_name, 
   msdb.dbo.backupset.description 
FROM   msdb.dbo.backupmediafamily  
   INNER JOIN msdb.dbo.backupset ON msdb.dbo.backupmediafamily.media_set_id = msdb.dbo.backupset.media_set_id  
WHERE  msdb..backupset.type = 'l' 
   ) AS B 
   ON A.[server] = B.[server] AND A.[database_name] = B.[database_name] AND A.[last_db_backup_date] = B.[backup_finish_date] 
WHERE backup_start_date BETWEEN DATEADD(hh, -24, GETDATE()) AND GETDATE()   
ORDER BY  
   A.database_name