Solar PV, Batteries and … Maths

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Some, it all seems pretty simple, right? It’s just import, export, consumption. This post clarifies the sign (+ or -) conventions I use which seem to make things easiest.

Import+veEnergy from grid
Export+veEnergy to grid.
Consumption+veConsumed in the building
Net+ve Import and -ve ExportPower reading at meter
Production+veProduction. Can be solar, battery, wind. Note battery is just time-shifted solar or wind.

Each of the three images has a ‘control volume’ where we measure power flow across.

CV1 is the meter: the ‘Net’ reading is positive (import) or negative (export) at any time.

CV2 simply breaks Net into positive (importing) and negative (exporting).

Import - Export = Net

C3 shows production and consumption. Note production is of any kind (e.g. solar, battery, whatever). Battery storage within the building is consider production, it’s just shifted in time.

Production - Consumption = Net

So putting this together:

Production - Export + Import - Consumption = 0

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