## 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:

**N**et- Import is positive, export is negative

**P**roduction is positive**L**oads are positive and monitored**U**nmonitored 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/consumptionSolarFraction = 1 - GridFraction = SelfConsumed /consumption= (production-export)/ consumption

#### Option 2: “Net”

SolarFraction =production/consumption