Billing and account problems made up the majority of customer complaints to the Commission for Regulation of Utilities (CRU) last year, according to the CRU customer care annual report.
The body provides an independent complaints resolution service for customers who have an unresolved dispute with their energy supplier, network operator or Irish Water.
Following the publication, the CRU is encouraging customers to carefully read and review their bills.
If customers complain to their supplier or network operator and are not happy with their response after completing their complaints process, they can log a formal complaint with the CRU.
The CRU can investigate the matter and issue a decision that is legally binding on the energy company.
According to today's report, the CRU investigated 179 complex energy complaints last year, down 37% on the previous year.
It said 33% of these complaints were upheld in favour of the customer.
According to the CRU, complaints were on an upward trend from 2016 to 2019.
However, the volume decreased in 2020, which it said may have been due to the Covid-19 pandemic.
Last year, the customer care team opened its lowest level of complaints since its formation in 2008.
A large portion of complaints to the CRU were as a result of 'catch-up bills', where an actual meter read is provided following estimated reads.
Billing issues accounted for 46% of energy complaints to the CRU during 2020.
It said this may be due to the suspension of meter reading during certain periods of the year, leading to estimated bills.
The CRU said once meter reading resumed, customers would have received actual bills reflecting customers' usage which may have been higher than anticipated due to the previous estimated bills.
The CRU said that contact to the CRU regarding Just Energy, Go Power, Panda Power and SSE Airtricity 'greatly reduced' last year, along with a reduction in contacts for BGE, Electric Ireland, Energia, and Pinergy, while contacts for two suppliers increased.
Pinergy and Iberdrola had the lowest volume of complaints per customer, with zero complex complaints lodged against it in 2020.
Prepay Power had the second lowest level of complaints.
Flogas and Panda Power had the highest level of complex complaints per 10,000 customers referred to the CRU - however, both suppliers have a small market share when compared against the other suppliers.
The CRU received 240 customer contacts in relation to Irish Water in 2020.
It said most contacts were in relation to billing and account problems.
Customers also noted that Irish Water was failing to respond to their queries 'in a timely manner and/or not escalating their complaints', the report stated.
There were 22 complex complaints received, a decrease of 35% compared to 2019.
"Customers can feel assured that the CRU will provide an independent and free complaints resolution service if they have an ongoing issue with a supplier, to ensure their rights are fully protected," said Aoife MacEvilly, CRU Chairperson.