Calculate Daily/Monthly Energy consumption in SQL Server

Question

I have a SQL Server table Energy_CC with two columns: time [int] (Epoch time) and E_CC14 [float]. Every 30 minutes, the total amount of my energy (kWh) is appended to the data table - something like this:

time	E_CC14
1670990400	5469.00223
1670992200	5469.02791
1670994000	5469.056295
1670995800	5469.082706
1670997600	5469.10558
1670999400	5469.128534

I tried this SQL statement:

SELECT 
    MONTH(DATEADD(SS, i.time, '1970-01-01')),
    i.E_CC14 AS mwh, 
    iprev.E_CC14 AS prevmwh,
    (i.E_CC14 - iprev.E_CC14) AS diff
FROM   
    Energy_CC i 
LEFT OUTER JOIN
    Energy_CC iprev ON MONTH(iprev.time) = MONTH(i.time) - MONTH(DATEADD(month, -1, i.time)) 
                    AND DAY(iprev.time) = 1
WHERE
    DAY(i.time) = 1
GROUP BY
    MONTH(i.time), i.E_CC14, iprev.E_CC14;

I expect the result for monthly like this :

time	E_CC14
DECEMBER-22	10223

Any help would be greatly appreciated.