In a previous post I talked about a lot of websites or apps that will show you the average carbon emissions for the UK grid and use this as a guide to when you could use your energy-hungry electrical appliances. The problem I have is that most of the traffic light systems based on averages are actually misleading. To make a decision to run an energy-hungry appliance or not, you don't use an average, you need to look at the marginal cost of producing that extra electricity to run your appliance.
Let's look at this in more common business terms. The marginal cost is used to describe the cost of producing one extra unit. Economically, things tend to get cheaper to produce per unit as you produce more and more of them but that after some number of units produced, the units actually get more expensive due to dis-economies of scale (you need a bigger warehouse, pay staff overtime, a complicated computer system, communication errors etc). In our case, the marginal cost can be described as the cost (in carbon) of producing 1 extra gigawatt (GW) of electricity for the grid. For that extra GW of electricity, the carbon emissions cost will depend from where that electricity is sourced. Intermittent electricity sources such as solar and wind cant contribute as they can't produce any extra to meet demand, that's not how they work, and nuclear is nearly always maxed-out due to the economics of investing in such high-cost power. This leaves natural gas (or fossil fuels in general) as the marginal generators in the UK electricity grid most of the time. This means that the marginal cost of electricity production is nearly always the cost of making a gas-fired power plant work harder (roughly 400 metric tonnes of CO2 per extra GW or more).
You can go all-out on this kind of analysis and use machine learning and other mass data analysis techniques to churn out a number on the marginal cost, and this is what the people at tmrow.co have done. But whether you just look at the grid in a simple way or look at all the data, the answer is the same, for the UK it's usually gas-power stations as the marginal generators of electricity. All of this is no surprise, there are lots of sources available on the web that talk about the marginal cost (described as the marginal or displacement emissions factor) of electricity production, so it seems strange that websites with traffic lights don't take account of this or at least mention it (kudos to Earth Notes for doing so).
So, sadly, there is currently (2018) no obvious best time to run your dishwasher to reduce your carbon emissions. The best way forward is to still use your dishwasher or washing machine as effectively as possible (full-loads and at a low temperature). On a positive note, the high cost of marginal electricity production is a good thing when you are trying to reduce carbon by energy saving. Any energy we use comes at a high marginal carbon emissions cost and so energy saving has a large carbon reducing effect. In fact energy saving is by far and away the best method of achieving all the goals of our energy revolution, making energy cheaper, greener and more reliable. Every bit of electricity saved (electricity nega-watts), means a customer doesn't have to pay for it, there are fewer carbon emissions in making it, and the electricity grid doesn't have to be upgraded to carry it. This is the reason for my whole website, there is no downside to energy efficiency for anybody.
Finally, I would be bad if I suggested to consumers that you can ignore grid 'signals' and to use your energy hungry appliances whenever. In fact, I believe the opposite, and that we should start opening up to the idea of relying on signals on when to use our biggest appliances. This is because time-of-use electricity pricing is almost definitely coming, so using your energy-hungry appliances at peak times is going to get more expensive.
About this blog
I have been involved in the field of energy for nearly 20 years, so I have a deep relationship with the subject.