Quick answer
Measure net import and export where the whole site connects to the grid, not just at the battery inverter. If the inverter already receives accurate whole-site meter readings, the EMS may be able to use them. If not, it may need a direct meter connection or its own CT clamps.
A third-party energy management system, or EMS, should measure net site import and export at the point where the client’s premises connect to the grid.
Check the available options in this order:
- Check whether the battery inverter already receives readings from a grid meter and current transformer, or CT, clamps at the site connection point. In many cases, the EMS can read the same figures through the inverter.
- If the inverter cannot provide reliable net import and export readings for the whole site, the EMS may need a direct connection to the site meter or its own CT clamps at the grid connection point.
- If the EMS also needs to know the building’s actual gross load, it may need separate readings for the solar system and battery. Another option is to fit CT clamps around a common load bus that does not include the solar and battery connections.
Gross load means all the electricity that the building uses before allowing for any electricity supplied by the solar panels or battery.
Where should we measure net site import and export?
The EMS should measure net site import and export at the point where the premises connect to the grid, not at the inverter output.
On a typical three-phase site, the meter normally needs one CT clamp around each incoming phase conductor. A CT clamp measures the electrical current flowing through the cable.
Ask the installer to confirm that the CT clamps sit after the electricity supplier’s meter and incoming protection, but before the supply divides between the site’s distribution boards. This position shows the net power entering or leaving the premises.
The meter must also take its voltage reading from the phase that matches each CT clamp. For example, the CT clamp on phase L1 must match the meter’s L1 voltage input. The same rule applies to L2 and L3.
Ask the installer to confirm that they have:
- Followed the CT manufacturer’s direction markings
- Entered the correct CT ratio in the meter
- Included every route through which electricity can enter or leave the part of the premises that the EMS will control
- Checked whether the existing export-limitation equipment already uses the same measurement point
- Confirmed that the meter can send readings to the EMS
Do not assume that the arrow on every CT clamp should point towards the electrical load. Manufacturers use different markings to show direction.
When does the EMS also need actual site consumption?
A meter at the grid connection point measures the net flow of electricity between the premises and the grid. It does not always show the building’s actual gross load.
For example, a factory may use 500 kW while its solar panels provide 200 kW. The grid connection meter will show that the factory imports 300 kW.
If the battery also supplies 100 kW, the meter will show an import of 200 kW. However, the factory still uses 500 kW in total.
If the EMS needs to know the building’s actual gross load, the installer can use either:
- CT clamps around a common load bus that does not include the solar and battery connections
- A meter at the grid connection point, plus separate readings for the solar system and battery
A client may need more than one meter or set of CT clamps when the premises have several incoming supplies, private transformers, bus couplers, or generators. The EMS calculation must include every route through which electricity can enter or leave the part of the premises that the EMS controls.
Before commissioning, verify the reading that the EMS will use. Compare it with the electricity supplier’s meter where possible. Then check that the figure moves in the correct direction when the building’s electricity use, solar output, or battery output changes.
Choose the metering point based on what the EMS needs to control. Net grid flow and the building’s actual gross load will differ whenever the solar panels or battery supply part of the building’s electricity use.
Related questions
What equipment does a third-party EMS need on a commercial battery site?
How do we check whether a third-party EMS can control an existing commercial battery?
How does a G100 export limit affect commercial battery savings?