"The order is not in accordance with Section 62 (4) of the Electricity Act, 2003 which allows amendment of tariff only once in a financial year except in respect of any changes expressly permitted under the terms of any surcharge formula," the letter said.
The letter, written by Delhi Power Secretary Sukesh Kumar Jain, also referred to the ongoing CAG audit of discoms.
"Therefore any tariff hike in the name of PPAC (power purchase adjustment cost) adjustment may not have been made at this stage," it said.
A senior government official had said the DERC's move is "completely illogical" as the CAG audit of the power discoms is still under way.
The letter further noted that DERC had neither given any notice to the public or the Delhi government before taking the decision nor any public consultation was made with Department of Power in this matter, it said.
"The detailed working on the basis of which DERC has arrived at the PPAC percentage spread over next 3 quarters, ranging from 4-6 percent is also not provided in the said order," it added.
DERC on Friday had approved a 4-6 percent increase in power purchase adjustment cost (PPAC), or the fuel surcharge, as it restored a levy charged to compensate the discoms for rise in cost of buying electricity on account of increase in coal and gas prices.