Wind most certainly isn't the back-up.
Anyone who's ever worked around grid management will tell you that the suitability of a generating technology as a back-up source is a function of it's ability to be "dispatched" (i.e. called on) on demand.
Not least because if a back up can't be called on reliably when needed, you need a back-up for the back-up.
The measure of dispatchability is "capacity credit" or "firm rating" (the latter is the UK term). It basically says that if I've got "n" megawatts of that technology, what proportion of it am I 90% likely to be able to rely on if needed.
For conventional technologies (CCGT, hydro, nuclear, coal) it's basically the lifetime availability of the plant, so around the 90-95% mark.
For variable renewables, it's never higher than the capacity factor. Onshore wind's capacity factor tends to be about 27% in the UK. Firm rating (depending on the site) is typically 8-14%. Solar's capacity factor in Germany is about 9%, suggesting a firm rating of 5-6%
If I've got 10,000MW of solar, it means I can rely on it to provide 50-60MW operating as a back-up.
Here's a thought. If I came to you and sold you car insurance that had a 6% of paying off whem you needed it, would you buy it?
WilliamAshbless: This doesn't make sense. The UK has ~ 10.5GWe of wind 'capacity'. Wind has been delivering an average of about 5% of rated capacity for an entire week now. Sometimes ,last week, it's dropped well below even 5% [e.g. it was 4% earlier this morning]
PatLogan: It's a probability based number, William - the basic definition (although it tends to get more complicated in applications) is what proportion of rated (i.e. nameplate) power am I 90% likely to get if the plant is called on in any given half-hourly period.
So yes, there will be periods when available capacity is below that, just as there'll be others when it's higher. 90% of the time, I'd be able to get that amount or higher.