This post is a follow-up to the previous post, where I looked into the scenario of an unlimited storage device topping off the excess electricity at peaks and filling in the gaps when there is a shortage. I found that a storage capacity of about 2,500 GWh was needed to fill in all the gaps. That number seemed very high to me, so I wanted to check whether other people also found such large numbers.

I quickly found a back-of-the-envelope-calculation by the late David MacKay. He proposes that 33 GW of wind power, delivering on average 10 GW, needs roughly 1,200 GWh backup. This is his calculation:

10 GW × (5 × 24 h) = 1,200 GWh.

He starts from the assumption that it is necessary to bridge five consecutive days of no wind. The difference is that he only considers wind, while I also include another intermittent energy source (solar).

The average delivered power is rather similar in both cases. In my scenario I have ((3,369.05 MW x 0.12) + (3,157.185 MW x 0.24)) x 8.57 = 9,957.95 MW.

Which is a tad below the 10 GW of MacKay is working with. Yet, my result is almost twice as high. Is this the influence of another intermittent power source in the mix? Or just a coincidence? Or did I do something wrong?

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