24/7 renewables power?

Suppose wind power is available every day for 30% of the time. For 24/7 power supply, effective generation capacity must be about 3.3 times as much as the equivalent effective fossil fuel generation capacity. Daily power must all be generated when the wind blows.

These 'every day is the same' assumptions are unrealistic. We can face several days, or more, without sun or wind. All the power generation needed for reliable 24/7 power must occur in the day or days before the sunless or windless days. Why? Electricity must be used as soon as generated, or immediately stored in batteries. Otherwise it's lost. Assume power must be generated the day before.

Suppose we have a three day 'drought' of sun or wind. Using our numerical example, solar generation capacity now needs to be 26.7 times the equivalent fossil fuel capacity. All the power for four days must be generated in 15% of the first day. Wind generation capacity needs to be 13.3 times the equivalent fossil fuel capacity. All the power for four days must be generated in 30% of the first day.

Most of the power generated is not used immediately. It's stored in batteries. Storage capacity therefore is a really big deal, too.

If solar power is available every day for 15% of the time, for 24/7 power supply, battery storage capacity must be the equivalent of 5.7 times as much as the equivalent effective fossil fuel generation capacity. For a three day solar 'drought', this generation-equivalent storage increases to 25.7 times the equivalent effective fossil fuel generation capacity.

If wind power is available every day for 30% of the time, for 24/7 power supply, battery storage capacity must be the equivalent of 2.3 times as much as the equivalent effective fossil fuel generation capacity. For a three day wind 'drought', this generation-equivalent storage increases to 12.3 times the equivalent effective fossil fuel generation capacity.

These required generation and storage capacities are additive.

For reliable renewables, therefore, the equipment costs comprise at least three elements that must be added together.

i. Generation equipment needed to harvest enough 'free' solar, wind, or hydro energy.
ii. Plusmanufactured battery equipment sufficient to store most of that for later discharge when needed.
iii. Plusnew and upgraded transmission (FCAS requirements, etc) to transport most power from where generated to where used.

Batteries don't generate power. They store it, more or less inefficiently. Separate energy generation to charge and re-charge them is needed to support them. Electric cars aren't a new source of electricity supply. They are a new source of electricity demand.

Governments usually announce 'big battery' investments (such as Hornsdale in SA) with assertions about how many homes they can power. Typically, these announcements are misleading. For 24/7 power, the relevant questions are: (a) how many homes can be powered 24/7, and (b) for how long? The answers are 'not many' and 'not for long, without re-charging'.

Given the physics – multiplied generation-equivalent capacity requirements across generation, storage, and transmission – what would a 24/7 system, as reliable as we now enjoy, cost? Less than our current system, including costs of now-expensive fossil fuel inputs?