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).
David Miller
(Emeritus professor of physics and astronomy at UCL), London

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