Calculating kWh at NBP

2025-10-25

Before we get into NBP kWh, there's an update on the previous post on finding DNO tariffs. I've heard from the industry body DCUSA that they're working on an API for DNO tariffs, which is great news. They caution that it's still in the early stages of development though.

So, on to NBP. Many electricity tariffs are charged at NBP, for example the RAB charge I've talked about previously. With the RAB interim charge, the cost is calculated as:

(RAB Interim Charge £) = (Rab Rate) x (NBP kWh)

MSP (Meter Supply Point) kWh is the amount of electricity measured at the meter. The kWh you see in the half-hourly data collected from the meter is at MSP. Before getting to the meter, the electricity has to go through the transmission and distribution networks. Transmission refers to the national grid:

and distribution refers to the local DNO network:

NBP kWh is the amount of electricity before transmission and distribution losses occur. The amount of electricity before distribution losses is known as Grid Supply Point (GSP) kWh:

GSP kWh = MSP kWh * LAF

where the LAF is the Loss Adjustment Factor. NBP kWh is:

NBP kWh = GSP kWh x TLM

where TLM is the Transmission Loss Multiple. We'll concentrate on the TLM, which takes us from GSP to NBP. There's a different TLM for each half-hour, and for each region of the country. As a programmer working on Chellow, I'm pleased that Elexon makes the TLM data available for automatic download via their portal. The regions of the country are known as GSP Groups, and every electricity supply is within a GSP Group. Here's a screenshot from Chellow showing the GSP Groups:

Then if we run a virtual bill in Chellow, it would do the calculations for each half-hour and give us the following example result:

I've used RAB Interim as an example here, but the same principle applies to any element that's billed at NBP kWh.

Hope you enjoyed this post, and see you next time!