Before you quote for solar panels or a battery, you need to know how much power the client’s local network operator, or DNO, will let the site send to the grid. If the system you’re planning could export more than the maximum, the DNO may still approve it if your design includes G100-compliant export limitation controls that automatically keep the site’s exports below that figure.
In this article, we explain the G100 export limitation requirements, how to get your design approved by the DNO, and how to prove the controls work when you’ve finished the job.
What is the G100 export limitation scheme?
A G100 export limitation scheme automatically caps how much power a site can send to the grid at any one time, with the client’s DNO setting the actual maximum.
The following is needed to achieve this:
- Physical equipment: Sensors at the site’s grid connection measure how much power is flowing in or out. A controller connects those sensors to the solar inverter, battery inverter or other devices it can control.
- Software: Software running in the controller compares the sensor readings with the DNO’s limit. If the site gets close to exporting too much, it tells the connected devices to change what they’re doing so less power reaches the grid.
The Energy Networks Association is the body which outlines the technical requirements that a G100-compliant installation needs to follow.
The DNO limits how much power the whole site can send to the grid. The business may use some of the power supplied by the solar panels or battery, so only what is left is exported.
This means you may be able to design a solar and battery system that can supply more power than the site is allowed to export. However, the DNO will only approve your design if it believes the controls you put in place will keep the amount of power being sent to the grid within the agreed limit.
When is a G100 export limitation scheme required?
On some sites in Great Britain, the DNO will require you to include a G100 export limiting scheme before it approves a new solar or battery connection.
At the start of a project, look at the client’s existing connection documents to find out how much power, if any, the site is already allowed to export. Then work out the most power the proposed solar and battery system could send to the grid at any one time.
If the client doesn’t have any documents showing the export allowance, or you believe the proposed system could go above it, ask the DNO to assess your proposal. You can ask for permission to export all the power the system could send to the grid or propose a lower limit enforced by G100 controls.
If you propose a lower export limit, you’ll normally send the G100 details to the DNO with the relevant G98 or G99 application.
Here’s what each set of requirements covers:
- G100: The controls that keep the amount of power the whole site sends to the grid within the limit agreed with the DNO.
- G99: The connection of solar, battery and other generating equipment that is above G98’s limit of 16A per phase or doesn’t meet its other rules.
- G98: The connection of fully type-tested solar, battery and other generating equipment with a combined rating of no more than 16A per phase.
Once it receives the connection application and any G100 details, the DNO works out whether the local network can take the amount of power you want the site to export. It may agree to that amount, give you a lower limit, ask you to change the design or tell you that it needs to work on the network before it can give the connection the green light.
If the DNO gives you a lower limit, it will put the figure in the connection offer. It may call this the Maximum Export Limit, or MEL, and give it in kW or amps. Use the number and unit in the connection offer when you design and set the controls.
How does a G100 export limiting scheme work?
A G100 export limiting scheme keeps a site below its export limit by:
- Reducing solar output
- Reducing battery discharge
- Charging the battery or switching on equipment that the DNO has agreed can use the extra power
The system you install may take one or more of these actions. However, the DNO must approve the actions included in your design and how they work together.
The site may also use energy management or trading software to control the battery. If that software calls for a discharge that would take the site over its agreed export limit, the G100 controls must override the instruction and reduce or stop the discharge. The battery can only discharge for bill savings or trading when the site remains within its export limit.
A solar-and-battery example
Take a site where the solar panels are producing 500 kW, the building is using 300 kW and the export limit is 100 kW. Without the controls, the remaining 200 kW would go to the grid and put the site 100 kW over its limit.
If the battery has room to store more electricity and can charge at 100 kW, it can take the extra 100 kW while the allowed 100 kW goes to the grid. If the battery can charge at only 40 kW, the controls must reduce solar production by at least 60 kW.
If the battery is full, the controls must reduce solar production by at least 100 kW unless other equipment approved by the DNO can use that power.
What happens if part of the scheme fails?
Before the DNO approves the connection, your design must show what actions the controls you’ve put in place will do if a sensor, controller, power supply, communication link or connected device fails. Options include reducing solar or battery output, shutting down equipment or switching to backup protection. Whichever response you use, the site must always stay within its export limit.
You must also stop anyone who doesn’t have the right permissions from changing the export limit. For this, a password, PIN or physical seal is generally enough.
What does the export limit mean for your solar and battery proposal?
When you first work out the figures for the project, use the export limit you plan to ask the DNO for. This will help you estimate how much electricity the solar panels will produce, how much the battery could cut the client’s bills and how much it might earn from trading. Once the DNO tells you the limit it will allow, work out the figures again using that amount.
Start with the export limit you plan to ask for. When the DNO gives you its decision, update your figures using the amount it approves.
Solar output and curtailment
Sometimes the solar panels will be able to produce more power than the building is using. If the battery can’t take all the extra power and exporting it would take the site over its limit, the controls reduce the output from the panels. This is called curtailment.
When you estimate how much electricity the solar system will produce over a year, take off any amount you expect the controls to curtail. The panels never produce that electricity, so the client can’t use, store or export it later.
Battery charging and use at the site
A battery can prevent some curtailment by storing surplus solar electricity. It can supply that electricity to the building later, when the panels produce less or electricity from the grid costs more.
How much surplus the battery can store at any one time depends on two things. Its charge power in kW tells you how much power it can take in at once. The amount of usable storage space it has left, measured in kWh, tells you how much more electricity it can hold.
For each period in your model, compare the solar output and the site’s electricity use with how quickly the battery can charge and how much room it has left. Allow for the electricity lost when the battery charges and discharges.
If the battery is full by midday or the solar panels produce surplus power faster than the battery can take it in, the controls will still have to reduce the solar output.
Battery trading
The battery and solar panels share the same export allowance. If the panels are already exporting 80 kW and the site has a 100 kW export limit, the battery can add no more than 20 kW, even if its inverter could discharge faster.
Before you add trading income to the proposal, work out how much battery power and stored electricity the client wants to keep available for the building. Base the trading figure only on what is left.
The battery must also stay within the site’s import limit when it charges from the grid and its export limit when it sends electricity back.
Show bill savings, normal export payments, wholesale trading income and flexibility payments as separate figures. Don’t count the same electricity twice. If one kWh supplies the building, it can’t also be counted as electricity exported during the same period.
What to show the client
The client should be able to see which export limit you’ve used, what your figures are based on and what could change the result. Include:
- The export limit: Give the figure and its unit. Say whether you’ve requested it from the DNO or the DNO has approved it.
- The system you’ve modelled: Show the existing and proposed solar, battery and other generating equipment. Keep their ratings separate from the amount the whole site is expected to export.
- The information you’ve used: State which meter readings, solar data and other assumptions the figures are based on.
- The battery figures: Give its charging and discharging power in kW, usable storage capacity in kWh, charge limits and expected losses.
- What happens to the solar electricity: Show separately how much the site uses immediately, how much the battery stores, how much goes to the grid and how much is lost through curtailment.
- The savings and income: Separate bill savings, normal export payments, wholesale trading income and flexibility payments. Include a base case that doesn’t rely on trading.
- A downside case: Show what happens when you use less favourable assumptions.
- Who is responsible: State who will commission the controls, protect the settings and provide help after installation.
What must the installer submit and prove?
You send information to the DNO at two stages. Before you start work on your client’s export limitation scheme setup, you submit the proposed design for approval. After installation, you submit the commissioning results to prove that the finished controls work as the DNO approved them.
Before installation: the G100 application
You need to show the following in your application pack:
| What the DNO needs to see | What you need to provide |
|---|---|
| What is already at the site and what you plan to add | List the existing and proposed solar, battery storage and other equipment that can export power. Include how much power the site can take from the grid and the export limit you’re requesting. |
| How everything will be connected | Provide a single-line diagram showing the grid connection, meters, CTs, controller, inverters, controlled equipment and protection. |
| Which equipment will limit the export | Give the make and model of the controller, power measurement unit and connected devices. If the scheme has been fully type tested against the current G100 requirements, include its Energy Networks Association Type Test Register reference. |
| What the controls will do | Explain how the controls will respond as the site approaches its export limit. Also explain what will happen if they lose measurement, power, communications or control of a connected device. |
The DNO may come back with questions or ask you to change part of the design. It may also want someone present to witness when you run the commissioning tests. Don’t assume acceptance, instead waiting to get approval before getting started with the installation.
National Grid’s current export limitation application form shows the sort of information one DNO asks for. Use the form and guidance provided by the DNO responsible for your client’s connection.
After installation: commissioning evidence
At the end of the installation, test the solar, battery and other controlled equipment separately and then together. Check that the G100-compliant export limitation controls keep the site’s total export within the limit agreed with the DNO.
For every test, record what fault or issue caused the controls to act, how quickly they responded and how much power the site exported. Do the same for each failure covered by the approved design, including a loss of power or communications. Use the response times and settings the DNO approved for this project.
The commissioning record normally includes:
- The make and model of each installed device, its firmware version and any type-test reference.
- An updated single-line diagram showing what was actually installed.
- The final export limit and how the setting is protected from unauthorised changes.
- The tests you carried out and the results you recorded.
- Any settings used by the backup protection.
- Your signed installer declaration and details of any tests witnessed by the DNO.
Keep an up-to-date diagram and operating instructions at the site. Send the commissioning record to the DNO by the deadline it gives you. National Grid’s commissioning form shows what it asks installers to record. Other DNOs may request the information in a different format or by a different date.
Get more from the battery without exceeding the export limit
GridVolt connects its Energy Manager software and an on-site controller to compatible batteries and inverters made by other manufacturers. The software uses the site’s electricity use, solar generation, tariff, forecasts and battery charge level to work out when the battery should charge, discharge or hold. Whatever instructions it sends, the site must remain within its approved G100 export limit.
At suitable sites in Great Britain, GridTrade can use only the battery power and stored electricity that the client doesn’t need to keep available for the building. Before you add trading income to a proposal, GridVolt checks the battery, inverter, firmware, meter and connection limits.
Send us the proposed equipment, the site’s electricity data and its connection documents. We’ll check compatibility and model how much the battery could save on the client’s bills or earn through trading while staying within the site’s import and export limits.
Frequently asked questions
Can the G100 export limit be increased later?
Only after the DNO agrees the higher export capacity and accepts any design or setting changes it requires. Don't change the protected setting because the site appears to have spare network capacity at a particular time.
Who can change the G100 setting?
Only someone authorised under the accepted scheme should change it. The project records should name who can do this and explain how to record an approved change.
Can G100 limit imports as well as exports?
Yes. The current G100 framework covers customer import limitation as well as export limitation, but the DNO agrees the import and export values and control design separately. An export-limited project shouldn't assume it also has permission to increase or change the site's import capacity.