83 AGL Energy Limited Annual Report 2011 The ageing of trade receivables at the reporting date is detailed below: The consolidated entity’s policy requires customers to pay in accordance within agreed payment terms. Depending on the customer segment,  settlement terms are generally less than 30 days from date of invoice. An allowance for doubtful debts is recognised when there is objective  evidence that a trade receivable is impaired. Financial difficulties of the debtor, default payments or debts overdue are considered objective  evidence of impairment. An impairment loss for doubtful debts of $62.6 million (2010: $50.0 million) has been recognised in the year and has  been included in other expenses in Note 6. At the end of the reporting period, trade receivables with a carrying amount of $133.3 million (2010: $147.1 million) were past due but not  considered impaired. These trade receivables relate to customers for whom there has not been a significant change in credit quality and the  amounts are considered recoverable. Other balances within trade and other receivables are neither impaired nor past due. It is expected that these other balances will be  received when due. Unbilled gas and electricity revenue is not collectible until such time as customers’ meters are read and bills rendered. For terms and conditions relating to amounts owing by associates and jointly controlled entities, refer to Note 49. Due to the short-term nature of these receivables, their carrying amount is assumed to approximate their fair value. The maximum  exposure to credit risk at the reporting date is the carrying amount of each class of receivables. Collateral is not held as security. Derivative financial instruments – at fair value (Note 52) Note 16 – Trade and other receivables (non-current) Note 15 – Other assets (current) Note 14 – Other financial assets (current) Unbilled revenue Amounts owing by associates and jointly controlled entities Fair value and credit risk 2011 2010 $m $m Note 13 – Inventories Raw materials and stores – at cost  28.9 28.0 Finished goods – at cost  98.5 66.2 127.4 94.2 – Forward foreign exchange contracts – cash flow hedges  – 0.1 – Energy derivatives – cash flow hedges  22.7 2.3 – Energy derivatives – economic hedges  222.2 136.6 244.9 139.0 Futures deposits and margin calls  56.3 86.3 301.2 225.3 Green commodities scheme certificates and instruments  250.0 147.1 Prepayments  26.6 27.0 276.6 174.1 Other receivables  0.5 0.6 2011 2010 Total Allowance Total Allowance $m $m $m $m Not past due  360.7 (6.7) 360.6 (5.0) Past due 31 – 60 days  49.8 (3.3) 49.2 (5.6) Past due 61 – 90 days  20.8 (3.5) 24.4 (3.3) Past 90 days  120.6 (51.1) 132.0 (49.6) 551.9 (64.6) 566.2 (63.5)