At the switchboard
This is how I use a control-volume approach to practically monitor circuits. It’s half maths and half “it’s easy that way”. The area below is a switchboard, monitored with multiple CTs. See phisaver.com.

Referring to the above:
- Net
- Import is positive, export is negative
- Production is positive
- Loads are positive and monitored
- Unmonitored loads are same as Loads, just without a CT
- Battery is a special case of Load. Positive when changing, negative when discharging. It is not considered in the following.
We can then work out some useful equations. I use them in iotawatts.
- N + P – L – U = 0
- U = N + P – L
Derived Measurements
We can further derive some measurements with cannot be directly sensed. There can be found on the Iotawtt:
- Import : Net max 0
- Export: (Net max 0) x -1
- Unmonitored: Net + Production – (monitored) Loads
- Consumption: Net + Production (since Consumption = U+L and U+L = N+P)
Some further notes on these measures.
- Production and consumption are not at the meter, as production can be self-consumed.
- Production is of any kind (e.g. solar, battery, whatever). Battery storage within the building is consider production, it’s just shifted in time.
- Net is “at the meter”.
Given that:
Net = Import - Export
Net = Consumption - Production
.. then putting this together:
Production - Export + Import - Consumption = 0
Percent Grid v Solar
We want to state (e.g.) “your solar system covers x % of your energy”. This is a bit tricky. Imagine over two days:
- 10 kWh consumption, no production (broken)
- 10 kWh consumption, 20kWh production (exactly balanced)
So, do we mean:
- Fraction of consumption directly (i.e. behind the meter) provided by solar. If so, then that’s 10kWh / 20 kWh or 50%. This is, perhaps, a more accurate measure of solar contribution.
- Fraction of consumption provided by solar (either directly, or exported). If so, then that’s 20kWh / 20kWh or 100%. The general definition of “Net Zero” is 100%.
Let’s break consumption up:
Consumption = SelfConsumption (i.e. behind meter ) + GridConsumption (i.e. import)
Option 1: Direct:
GridFraction = import / consumption SolarFraction = 1 - GridFraction = SelfConsumed / consumption = (production - export) / consumption
Option 2: “Net”
SolarFraction = production / consumption