(1) On a bad winter day, at present, we would need 40 GW*; this equates to about 1 TWh/day*. This estimate is based on a 30 GW average daily demand during the last year [1].

(2) Thus, if 50% is from (variable) renewables, then on a challenging *Dunkelflaute *(dark doldrums) day, we would need to store 0.5 TWh in this one day – assuming that the remaining 50% is composed of non-renewable sources of gas, nuclear and biomass (arbitrarily, we denote these as non- renewable) and interconnector supply.

(3) So, for a period of 10 successive *Dunkelflaute* days – together with this (accepted) electricity need – we would need a total of 5 TWh of battery storage.

(4) We know that 1 GWh of battery storage costs about £300M (London Gateway, [2]), so 5 TWh would cost £1.5T* (Trillion).

(5) However, if we depended entirely (100%) on renewable electricity, the corresponding battery storage cost would be £3T.

**It is assumed that there is adequate excess renewable generation capacity pre-Dunkelflaute to ensure that stored electricity is available**.

(6) This approach can be scaled up to estimate battery storage costs for any other demand value, present or future.

(7) For context: UK public sector spending 2020/21 is reported to be £928B [3]: ca £1T.

* The gigawatt (GW) is equal to one billion (10^{9}) watts or 1 gigawatt = 1000 megawatts (MW). This unit is often used for large power plants or power grids.

The terawatt (TW) is equal to one trillion (10^{12}) watts. The total power used by humans worldwide is commonly measured in terawatts.

Gigawatt-hour (GWh) and terawatt-hour (TWh) are units of energy equal to one gigawatt and one terawatt of power sustained for one hour. This can refer to both the consumption or the storage of electricity.

£1T (Trillion) = £1000B (Billion) = £10^{12}.

[1] National Grid: Live Status https://grid.iamkate.com/

[2] InterGen gains consent to build one of the world’s largest battery projects in Essex, InterGen press announcement 30 November 2020 https://www.intergen.com/news-insights/categories/news/intergen-gains-consent-to-build-one-of-the-world-s-largest-battery-projects-in-essex/