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:

1. 10 kWh consumption, no production (broken)
2. 10 kWh consumption, 20kWh production (exactly balanced)

So, do we mean:

1. 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.
2. 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