Diversification vs. Volatility

Let's start with what "diversification" means here, because the idea is borrowed straight from investing. If you put all your savings into one stock and that company blows up, you lose everything. If you spread your money across many stocks whose ups and downs aren't tightly linked, a disaster in one is cushioned by the others, so your total wealth bounces around far less. The same logic applies to a country's electricity supply. If you generate power from a single fuel and that fuel's price spikes or its supply gets cut off, your whole grid feels the full blow. Mix several sources whose risks aren't correlated, and a shock to any one of them only moves a fraction of your total cost.

Here, "volatility" just means how much prices and supply swing around. High volatility is bad for almost everyone: factories can't budget, households get hit by surprise bills, and traders face wild risk. So the natural question is whether throwing more energy sources into the mix reliably calms things down. The honest answer is mostly yes, but not always, and you should be able to argue both directions in an interview.

Why diversification usually reduces volatility

The first reason is that you stop being a hostage to one supplier. When a country leans heavily on a single fuel imported from a single region, any disruption there passes straight through to prices. The clearest recent example is Europe after Russia cut pipeline gas flows in 2022. Countries that had built out nuclear, renewables, and access to seaborne liquefied natural gas (LNG is natural gas chilled to a liquid so it can be loaded onto ships and bought from anywhere in the world, rather than only from whoever you are connected to by pipeline) rode out the shock far better than countries that had bet almost everything on that one pipeline. Wholesale power prices in the most gas-dependent markets multiplied several times over; more diversified neighbours saw smaller moves.

The second reason is that different sources break for different reasons, and those reasons rarely line up. A drought hurts hydro power but does nothing to a gas plant. A pipeline outage spikes gas but leaves your solar farms untouched. A heatwave that cuts nuclear output (reactors need river water for cooling, and warm rivers force them to throttle back) is often the same heatwave delivering blazing sunshine to your solar panels. Because these risks are roughly uncorrelated, when one source has a bad day another is usually fine, and your blended cost barely moves.

Let's make that concrete with a tiny numerical example, because this is where the portfolio analogy really earns its keep. Suppose two sources each have a price that swings with a standard deviation (a standard measure of how much a number typically deviates from its average) of 20 units, and you split your generation 50/50 between them.

If the two sources were perfectly linked, so they always spike together, the blended swing would just be the average, still 20 units, no benefit at all:

• Perfectly correlated
  the combined standard deviation is $0.5 \times 20 + 0.5 \times 20 = 20$ units.
• Completely independent
  the variances (the square of the standard deviation) add rather than the standard deviations, so the combined standard deviation is $\sqrt{0.5^2 \times 20^2 + 0.5^2 \times 20^2} = \sqrt{200} \approx 14.1$ units.

So with genuinely independent sources your price volatility drops from 20 to about 14, a roughly 30 percent reduction, without changing your average cost at all. That free lunch, lower risk for the same expected price, is exactly why diversification is attractive.

Sanity check

Does the 14.1 figure make sense? Splitting evenly across $n$ independent sources of equal volatility $\sigma$ gives a blended volatility of $\sigma / \sqrt{n}$. With $n = 2$ and $\sigma = 20$, that is $20 / \sqrt{2} \approx 14.1$, matching the line above. Push it further: with four independent sources the blend would be $20 / \sqrt{4} = 10$, half the single-source swing. The more (uncorrelated) sources you add, the smoother the ride, which is precisely the intuition we wanted.

Why diversification can sometimes add volatility

Now the catch, because "more sources is always calmer" is too glib. The new sources you add bring their own behaviour, and some of it is jumpy.

The big one is intermittency, meaning wind and solar only produce when the wind blows and the sun shines, and you can't dispatch them on command. On a windy sunny afternoon they can flood the grid and crush prices, sometimes to zero or even negative (yes, power prices can go negative, meaning generators pay to offload electricity because it's cheaper than shutting a plant down). Then at dusk the sun sets, wind drops, and prices can leap as expensive backup gas plants scramble to fill the gap. So adding a lot of renewables can raise short-term, intraday price swings even as it lowers your long-run exposure to fuel-price shocks. The two effects pull in opposite directions.

The second catch is transition cost and uncertainty. Rewiring a grid for many sources means new transmission lines, storage, and rule changes, and the policy around all of that, subsidies, carbon prices, connection rules, keeps shifting. That regulatory uncertainty can itself make investors and prices nervous in the near term.

How to land the answer in an interview

Diversification is portfolio theory applied to electricity. By spreading generation across sources whose risks aren't correlated, you cushion any single shock and, as the numbers showed, you can cut blended price volatility meaningfully without giving up anything on average cost. That dominates over the long run, which is why diversified systems weathered the 2022 gas crisis better. But it isn't a pure free lunch: the variable renewables that make a portfolio look diversified can increase short-term, within-day price swings, and the cost and policy churn of building all that out adds its own noise. So the sharp answer is: diversification lowers long-horizon, fuel-shock volatility, while potentially raising short-horizon, intermittency-driven volatility. Knowing which volatility you're talking about is the whole game!