pre_text: note 4 : property , plant and equipment the following table summarizes the major classes of property , plant and equipment by category as of december 31 : 2015 2014 range of remaining useful weighted average useful life utility plant : land and other non-depreciable assets . . . . . . . . . . $ 141 $ 137 sources of supply . . . . . . . . . . . . . . . . . . . . . . . . . . 705 681 12 to 127 years 51 years treatment and pumping facilities . . . . . . . . . . . . . . 3070 2969 3 to 101 years 39 years transmission and distribution facilities . . . . . . . . . 8516 7963 9 to 156 years 83 years services , meters and fire hydrants . . . . . . . . . . . . . 3250 3062 8 to 93 years 35 years general structures and equipment . . . . . . . . . . . . . 1227 1096 1 to 154 years 39 years waste treatment , pumping and disposal . . . . . . . . . 313 281 2 to 115 years 46 years waste collection . . . . . . . . . . . . . . . . . . . . . . . . . . . 473 399 5 to 109 years 56 years construction work in progress . . . . . . . . . . . . . . . . 404 303 total utility plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18099 16891 nonutility property . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 405 378 3 to 50 years 6 years total property , plant and equipment . . . . . . . . . . . . . . . $ 18504 $ 17269 property , plant and equipment depreciation expense amounted to $ 405 , $ 392 , and $ 374 for the years ended december 31 , 2015 , 2014 and 2013 , respectively and was included in depreciation and amortization expense in the accompanying consolidated statements of operations . the provision for depreciation expressed as a percentage of the aggregate average depreciable asset balances was 3.13% ( 3.13 % ) for the year ended december 31 , 2015 and 3.20% ( 3.20 % ) for years december 31 , 2014 and 2013 . note 5 : allowance for uncollectible accounts the following table summarizes the changes in the company 2019s allowances for uncollectible accounts for the years ended december 31: .
post_text: .
table: 2015 2014 2013
table: Balance as of January 1 $(35) $(34) $(27)
table: Amounts charged to expense (32) (37) (27)
table: Amounts written off 38 43 24
table: Recoveries of amounts written off (10) (7) (4)
table: Balance as of December 31 $(39) $(35) $(34)
question: Analyse this data from a financial earnings document. by how much did property , plant and equipment depreciation expense increase from 2013 to 2015?