Running generators: is it unsustainable?

March 23, 2021

By Thomas Jennings, Head of Optimisation 

At Kiwi, our customers are often worried that running their generator might have a negative impact on the environment.  Our response is always that the reality is a little bit more nuanced than that, and that perhaps we need to reframe the question to: what service does a generator provide right now, and is it better than the alternatives? And how can it help us transition to a more sustainable energy system?

Filling the gaps

When National Grid is asking a generator to provide flexibility it is more often than not because they are lacking generation. This might be because demand is higher than expected, or the level of generation is not as high as anticipated. These fluctuations in demand and supply are increasingly driven by the intermittent and unpredictable nature of renewable energy, from both large in Front of Meter Offshore Wind farms, and smaller Behind the Meter solar roof tops.

To fill the gaps caused by these fluctuations, and facilitate our move to Net Zero, predictable flexible energy assets are needed. Historically, this flexibility has been provided by large centralised generators, such as Coal and Gas power plants. However, there is an increasing role to play for smaller decentralised assets to provide this flexibility.

Decentralised vs centralised: which is more efficient?

Now, at this point another question is thrown at me… 

Surely, a large centralised asset is more carbon efficient than a small generator? You can’t be suggesting that decentralised generation will get us to Net Zero faster? 

That is exactly what I am suggesting.

The reason behind this is the energy acronym MNZT or Minimum Non Zero Time.  Whilst a centralised asset is more efficient on a CO2e per MW basis, this is only true on a MW for MW basis.  When you start looking at the total run time for the asset the picture becomes clearer. But before we go into that, let’s muddy the water a bit first with some more acronyms:





Minimum Non Zero Time

The amount of time a generator must be generating once turned on.


Stable Export Limit

This is the minimum level of generation the asset must achieve to provide a stable load, efficiently operating and start providing flexibility.


Maximum Export Limit

This is the maximum output available, the difference between this and the SEL indicates the plant’s flexibility.


Minimum Zero Time

This is the time a generator must be turned off for, before it can be turned on again.


When we compare the parameters of a centralised generator to a decentralised one the picture becomes a lot clearer.

To achieve 40MWs of flexibility with a centralised generator that is not currently generating, it needs to be warmed up over three hours in advance to first get to its SEL (in this case 175MWs) and then increase to the additional 40MWs for exporting. 

To achieve the same flexibility with a decentralised generator requires considerably less CO2e, despite not being as efficient when running.

Putting it all together 

Now consider a scenario where we need to run additional flexibility in the next few hours after this 40MW has been generated. The centralised plant must be off for at least 3 hours (its Minimum Zero Time) before being started again, so it cannot provide flexibility during this time. Often, this means large centralised generators are left on and generating in case additional flexibility is required at short notice.

This in turn means that sources of renewable energy don’t have a chance to be deployed because generation requirements are being met by large centralised assets that are left running – which builds a strong case for the need for small decentralised generators to be providing flexibility, as they can be switched on and off more quickly, allowing more room for renewable generation and thus supporting the move to Net Zero.

The ace up your sleeve 

At Kiwi Power we are seeing an increasing interest in Kiwi Core as Energy Managers, and their consultants, look to use our software to provide the necessary flexibility to enable the grid to transition to renewables, whilst generating revenue. Many are seeing Kiwi Core as an ‘ace up their sleeve’ to pull out during difficult conversations.

We’d love to help you transition into making the most of energy flexibility. We invite you to talk to us about what that might look like for you as we quickly shift into this very different, much more sustainable future with energy flexibility at its core.