v3.24.1.1.u2
Subsidiary undertakings, joint ventures and associates - Disclosure of interests in joint ventures (Details)
12 Months Ended
Mar. 31, 2024
£ / shares
shares
Mar. 31, 2024
€ / shares
shares
Disclosure of joint ventures [line items]    
Par value per share (EUR/GBP per share) | £ / shares £ 0.1243129  
BritNed Development Limited    
Disclosure of joint ventures [line items]    
Proportion of ownership interest in joint venture 50.00%  
BritNed Development Limited | National Grid Interconnector Holdings Limited | C Ordinary shares    
Disclosure of joint ventures [line items]    
Number of shares held in joint venture (in shares) | shares 284,500,000 284,500,000
Par value per share (EUR/GBP per share) | € / shares   € 0.20
BritNed Development Limited | National Grid Interconnector Holdings Limited | A Ordinary shares    
Disclosure of joint ventures [line items]    
Number of shares held in joint venture (in shares) | shares 1 1
Par value per share (EUR/GBP per share) | £ / shares £ 1.00  
National Places LLP    
Disclosure of joint ventures [line items]    
Proportion of ownership interest in joint venture 50.00%  
Nemo Link Limited    
Disclosure of joint ventures [line items]    
Proportion of ownership interest in joint venture 50.00%  
Community Offshore Wind LLC    
Disclosure of joint ventures [line items]    
Proportion of ownership interest in joint venture 27.27%  
Clean Energy Storage Systems, LLC (previously Clean Energy Generation, LLC)    
Disclosure of joint ventures [line items]    
Proportion of ownership interest in joint venture 50.00%  
Emerald Energy Venture LLC    
Disclosure of joint ventures [line items]    
Proportion of ownership interest in joint venture 51.00%  
Island Park Energy Center, LLC    
Disclosure of joint ventures [line items]    
Proportion of ownership interest in joint venture 50.00%  
LI Energy Storage System, LLC    
Disclosure of joint ventures [line items]    
Proportion of ownership interest in joint venture 50.00%  
LI Solar Generation, LLC    
Disclosure of joint ventures [line items]    
Proportion of ownership interest in joint venture 50.00%  
IFA2 SAS    
Disclosure of joint ventures [line items]    
Proportion of ownership interest in joint venture 50.00%