When in the planning and design stages of a solar PV project, you may come across the term export limitation. Essentially the process involves fitting a device to cap exported power going from the solar system to the grid. But why would you want or need one? Read on to find out...
What is export limitation?
Solar PV generates electricity when the panels are exposed to sunlight. This electricity must be used immediately, either on site, stored in a battery or sent to the national grid. But often your supply and the local grid managed by the District Network Operator (DNO), may not be able to cope with large amounts of power flowing from multiple properties. To protect this infrastructure, or avoid costly upgrades, it’s necessary to restrict the power that might be exported. To do this, the DNO can either reject proposals for a large solar PV system, or insist that an export limiter is installed alongside it.
There is always a chance they will want an export limitation device fitted when applying to install a system with an AC output above 16A per phase (3.68kW single phase / 11.04kW three phase), which is the permitted limit beyond which an application is required for connecting a system.
Most inverters on the market nowadays have in-built export limitation functionality, and so the typical approach to configuring an export limitation system is to install a metering device at the grid connection point to feed back power flow data to the inverter so that it can adjust its output power accordingly to keep below a preset export limit.
Another benefit of installing such a metering device is that you then get full 360-degree monitoring of the power flows in your home: generation, self-consumption and export. Consumption and export data is normally visible on the inverter monitoring platform where an export meter is installed, in addition to the generation data. Of course, the generation data will only reflect what is actually produced - it’s not possible to record any ‘lost’ generation curtailed due to the export limit. The amount of generation lost over a year could be considerable if the export limit is low and the home’s electricity usage is not well-matched to the solar generation.
You can also maximise the power you send to the grid for export tariff payments.
How does this work in the real world?
Let’s say you apply for a 6kW PV system (inverter rating) without battery storage, and the grid says you can have a 6kW system installed but with export limited to 4kW.
This means on a perfect sunny day at peak production if the system is generating 6kW, and you are using 4kW in your home, the surplus 2kW will be exported.
If you were to suddenly switch all of your loads off while the system was still generating 6kW, there would be a brief moment where the export power would be the full 6kW. At this point the inverter would receive this power flow data from the metering device at the grid connection and quickly (within 5 seconds) throttle its power output down to 4kW to satisfy the export limit. The 2kW of potential generation would be ‘lost’.
If you were then to switch on your 2kW kettle in this ‘loads off scenario’, the new power flow at the grid connection point would be 4kW generation - 2kW load = 2 kW export, and so the inverter would receive this power flow data from the metering device and increase its output back up to the 6kW peak generation, which would bring the exported power up to the 4 kW limit. Once the kettle switches off, the solar generation will again be clipped to 4kW to maintain the 4 kW export limit.
Note that this example assumes perfect conditions and realistically the kW output of the system will vary all the time according to the local weather conditions.
A battery can help with a system expected to have frequent export limitation, as Powerwall for example would allow you to have an extra 5kW of generation capacity for a couple of hours when the 13.5kWh battery is empty. This means the battery can soak up excess power that would otherwise be ‘clipped’ by the inverter to satisfy the export limit.
When do you need export limitation?
Large commercial systems generate a lot of electricity, but this may not be used on weekends or other days when the business is shut. The existing local grid infrastructure wouldn’t be able to cope with such high generation, and the upgrade of transformers and cabling would be prohibitively expensive. In this case export limitation can control energy sent back to the grid to ensure it’s always a manageable level.
Similarly, some households have a high energy demand (larger properties, those with heat pumps or swimming pools etc.) and would benefit from a large PV system. But the DNO may want a guarantee that not too much is exported back to their network if demand falls on the property when the panels are at peak generation, in a ‘worst case scenario’. With export limitation, you can install the maximum solar capacity possible to meet your demands, without exceeding the limit set by the DNO.
All export limitation schemes must meet G100 standards in order to be approved by the DNO, including that the system must:
- be fail safe,
- not rely on wireless communication between components,
- meet all relevant power quality requirements,
- react within one second,
- reduce output within 5 seconds.
We can only install export limitation when it has grid approval based on the DNO's network study.
Sometimes the DNO insists on a witness test when the equipment is installed.
Are there alternatives to export limiting?
If the DNO rejects the desired size of your solar system, the only alternatives will be those that they suggest - usually a smaller system, export limitation or an expensive 3-phase electrical upgrade of your supply.
How much does export limiting cost?
For a domestic system, export limitation can add around 10% to the installation cost. For commercial arrays, it can add about 5%.
If you would like to discuss in further detail or get a quote for your project, please give us a call on 0118 951 4490.