It serves over 8.4 million customers throughout Peninsular Malaysia and the eastern state of Sabah through Sabah Electricity Sdn Bhd. TNB's core activities are in the generation, transmission and distribution of electricity.

Other activities include repairing, testing and maintaining power plants, providing engineering, procurement and construction services for power plants related products, assembling and manufacturing high voltage switchgears, coal mining and trading. Operations are carried out in Malaysia, Mauritius, Pakistan, India and Indonesia.

The Lembaga Letrik Pusat (Central Electricity Board, CEB) was established and came into operation on 1 September 1949. The Board was to become heir to three major projects considered by the Electricity Department following its re-establishment in April 1946 which were the Connaught Bridge Power Station, Cameron Highlands Hydroelectric Project & the development of a National Grid. CEB eventually became the owner of 34 power stations with a generation capacity of 39.88 MW, including a steam power station in Bangsar with a capacity of 26.5 MW, a hydroelectric power station at Ulu Langat with a capacity of 2.28 MW, and various diesel powered generators with a total capacity of 11.1 MW.

The TNB Group has a complete power supply system, including the National Grid which is energised at 132, 275 and 500 kilovolt (kV), with its tallest electricity pylon in Malaysia and Southeast Asia being the Kerinchi Pylon located near Menara Telekom, Kerinchi, Kuala Lumpur. The National Grid is linked via 132 kV HVAC and 300 kV HVDC interconnection to Thailand and 230 kV cables to Singapore.

The Distribution division conducts the distribution network operations and electricity retail operations of TNB. The division plans, constructs, operates, performs repairs and maintenance and manages the assets of the 33 kV, 22 kV, 11 kV, 6.6 kV and 415/240 volt in the Peninsular Malaysia distribution network. Sabah Electricity provides the same function in the state of Sabah.

To conduct its electricity retailing business, it operates a network of state and area offices to purchase electricity from embedded generators, market and sell electricity, connect new supply, provide counter services, collect revenues, operate call management centres, provide supply restoration services, and implements customer and government relationships.

According to wikipedia