Jillian Ambrose (Report, 11 June) claims that a 50MW battery system is a “significant step”. But how significant a step does not depend upon the instantaneous energy output in megawatts. Most significantly, it depends on how long it can keep this energy flow going. So the relevant measurement, to compare with rival systems, would be in MW hours (which are what we pay for in our electricity bills).
(Emeritus professor of physics and astronomy at UCL), London
• Join the debate – email email@example.com
• Read more Guardian letters – click here to visit gu.com/letters
• Do you have a photo you’d like to share with Guardian readers? Click here to upload it and we’ll publish the best submissions in the letters spread of our print edition