Summary

• Electricity grids operate at a precise frequency; in the UK and EU, it’s 50 Hz
• When supply doesn’t equal demand, frequency deviates. If the imbalance isn’t corrected quickly, it may lead to blackouts
• Grid operators procure frequency services to respond quickly to deviations
• The fastest frequency services require response times as fast as 0.1 seconds, and are exclusively supplied by grid-scale batteries; slower services (around 30 seconds) can be supplied by residential batteries and other smaller assets

Why we need frequency services

All modern grids operate an alternating current, at a precise ‘heartbeat’; 50 Hz (changes in direction per second) in Europe, 60 Hz in the US. When there’s an imbalance on the grid (supply doesn’t precisely meet demand), the frequency of the grid gets pulled away from its setpoint. Small imbalances result in small deviations; large, sudden imbalances (like a large powerplant going offline) can result in big spikes or falls.

Grid frequency is like a tottering statue; react quickly and steady the wobble with ease; react slowly, and your best efforts may not prevent a crash. Frequency services are a fast-acting response, automatically snapping into action at the earliest sign of a deviation. Frequency services are typically differentiated by their response time and duration; the fastest response times (0.5 seconds) typically deliver power for a short period, while slightly slower responses (30 seconds) may deliver power for 30-60 minutes. Together, these services are able to deliver both a fast response and an enduring response, keeping the grid in balance in times of stress.

GB frequency services

The GB grid operator (NESO) is required to keep the grid within 1% of 50 Hz. They do so by 1) managing the supply and demand of the grid in real-time to minimise mismatches, 2) procuring frequency services to kick in whenever there is an imbalance.

A live frequency feed from Axle’s central frequency monitor. If the frequency drops below 49.7 Hz, we trigger our sFFR fleet, discharging hundreds of batteries to restore the grid to 50 Hz.

Frequency services can be differentiated by a few key criteria:

• Static vs dynamic: Static services (like static FFR) deliver a formulaic response; e.g., when frequency drops to 49.7 Hz, increase generation or decrease demand for 30 minutes. Dynamic services (like Dynamic Containment) deliver a variable response depending on the real-time frequency of the grid.
• Pre- vs post-fault: Pre-fault services (like Dynamic Regulation) anticipate needs before faults occur, stabilising frequency before any large deviation. Post-fault services (like static FFR) kick in to respond to issues on the grid.
• Direction: Services can either be single-directional (like static FFR) where response is required only when frequency increases or decreases, or it can be bidirectional, where response is required for both increases and decreases. Bidirectional products can be procured together (one site needs to deliver both directions) or separately.
• Response time: response times (the time of initial response, and the time of full delivery) range from 0.5 seconds (Dynamic Containment) to 30 seconds (static FFR)
• Delivery duration: faster response-time products (like Dynamic Containment) deliver for shorter durations, as short as 15 minutes. Slower response-time (like Dynamic Regulation) deliver for longer, as long as 60 minutes.

Due to the technical requirements, the Dynamic suite of products is best suited to grid-scale batteries, while static FFR is best suited to smaller-scale batteries, diesel generators, and industrial load. (Note that there’s an additional product, Mandatory Frequency Response, which is mandatory for some power plants and not tendered for other technologies).

static Firm Frequency Response (FFR)

static FFR is a straightforward product; when frequency drops below 49.7 Hz, participants are required to respond within 30 seconds, and then increase generation (or reduce demand) for 30 minutes.

sFFR is dispatched when frequency falls below 49.7 Hz. The response must hold constant for the next 30 minutes

Unlike other frequency products, participants are able to aggregate individual assets into a single participating unit, and may monitor frequency using a central frequency meter (as opposed to frequency metering embedded in each asset).

sFFR is procured day-ahead, over 4 hour bidding windows. The product is rarely dispatched (~7-12 times per year). Payment is based on availability, not utilisation.

sFFR summary

Which assets are best suited to participate?

Today, static FFR is predominantly delivered by diesel generators and industrial load. Diesel generators, if fast-acting, are typically installed as backup, and thus are unused (and available to deliver frequency response) much of the year. Industrial load, particularly that which is always-on with a relatively low opportunity cost, can switch off when required for sFFR.

Residential & distributed batteries are also well-suited to static FFR. Assuming batteries are able to provide a quick (30 second) and consistent response, they can tap into this revenue stream without constricting their ability to maximise solar self-consumption or arbitrage power prices. (Other residential products like EV chargers and heat pumps are limited by their utilisation rates).

Dispatching 1MW worth of batteries in 30s: each line is the response of a single inverter.

To participate, static FFR Units must complete a pre-qualification test, demonstrating the ability to deliver the required response within the required standard deviation. This test must be certified by an Independent Technical Expert (ITE). Registration is then completed via NESO’s Single Markets Platform (SMP).

How much does it pay?

So far this year, static FFR has cleared at an average price of £3.85/MW/hr. (Note that the average price varies per 4-hour EFA block, with the evening block typically the most lucrative).

An always-available battery would make £34/kW/year (based on 2024 prices). Assuming 50% availability (due to low state of charge or inconsistent connectivity), a 3.5kW battery would make £59/yr, and a 5kW battery would make £84/yr. Paired with infrequent utilisation, this provides an attractive, and mostly passive, additional revenue stream for small-scale batteries.

Battery revenue (at an average price of £3.85/MW/hr):

How to learn more and get started

You can find all the nitty gritty details from the NESO here.

Axle is a software platform that connects assets like residential & distributed batteries to flexibility markets, including static FFR. We can help you with the soup-to-nuts of participating, from prequalification through registration, aggregation, bidding, and delivery.

If you’re keen to learn more, feel free to drop us a note.