only 2%.

Let’s quantify the fluctuations in country-wide wind power. The two
issues are short-term changes, and long-term lulls. Let’s find the fastest
short-term change in a month of Irish wind data. On 11th February 2007,
the Irish wind power fell steadily from 415 MW at midnight to 79 MW at
4am. That’s a slew rate of 84 MW per hour for a country-wide fleet of
capacity 745 MW. (By slew rate I mean the rate at which the delivered
power fell or rose – the slope of the graph on 11th February.) OK: if we
scale British wind power up to a capacity of 33 GW (so that it delivers
10 GW on average), we can expect to have occasional slew rates of

 84 MW/h × 33 000 MW = 3700 MW/h, 745 MW

assuming Britain is like Ireland. So we need to be able to either power
up replacements for wind at a rate of 3.7 GW per hour – that’s 4 nuclear
power stations going from no power to full power every hour, say – or we
need to be able to suddenly turn down our demand at a rate of 3.7 GW per
hour.

Could these windy demands be met? In answering this question we’ll
need to talk more about “gigawatts.” Gigawatts are big country-sized units
of power. They are to a country what a kilowatt-hour-per-day is to a person:
a nice convenient unit. The UK’s average electricity consumption is
about 40 GW. We can relate this national number to personal consumption:
1 kWh per day per person is equivalent to 2.5 GW nationally. So if
every person uses 16 kWh per day of electricity, then national consumption
is 40 GW.

Is a national slew-rate of 4 GW per hour completely outside human
experience? No. Every morning, as figure 26.3 shows, British demand
climbs by about 13 GW between 6.30am and 8.30am. That’s a slew rate of
6.5 GW per hour. So our power engineers already cope, every day, with slew
rates bigger than 4GW per hour on the national grid. An extra occasional
slew of 4 GW per hour induced by sudden wind variations is no reasonable
cause for ditching the idea of country-sized wind farms. It’s a problem

Figure 26.3. Electricity demand in Great Britain during two winter weeks of 2006. The left and right scales show the demand in national units (GW) and personal units (kWh/d per person) respectively. These are the same data as in figure 26.1.