What is the biggest monthly consumption of the customers who use euro as their currency?
Monthly consumption = SUM(consumption) / 12
SELECT SUM(T2.Consumption) / 12 AS MonthlyConsumption FROM customers AS T1 INNER JOIN yearmonth AS T2 ON T1.CustomerID = T2.CustomerID WHERE T1.Currency = 'EUR' GROUP BY T1.CustomerID ORDER BY MonthlyConsumption DESC LIMIT 1