The firm said the programme would "significantly" reduce costs and allow it to bring teams together, enabling "quicker and more effective decision-making."
The plans were communicated to staff after the German-owned company reported a 3% fall in half-yearly results, saying: "This is mainly due to the cost of delivering Government energy-efficiency schemes and increased network charges, as well as the cost of implementing new simplified tariffs for domestic customers.
"RWE npower currently has 26 sites across the UK and it is envisaged these will be consolidated to around 10 over the next five years. This consolidation programme would significantly reduce costs and allow RWE npower to bring teams together, enabling quicker and more effective decision-making.
"RWE npower has also started reducing management layers across the business, bringing all employees into closer contact with customers, and has been undertaking end-to-end reviews of customer processes to improve performance and satisfaction.
Colin Smith of the GMB union said: "We are looking to assess the implications for GMB members of this announcement. We are seeking further clarity from npower as to what this will mean for jobs and redeployments.
"We do not know how many jobs are likely to be affected.
"GMB will be looking for assurances that any job losses will be mitigated and that there will be no compulsory redundancies and that npower will redeploy any employees who will be impacted by closures of these sites."