EffectOfBatteries - The Psalter

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The effect on emissions of adding batteries to a solar system

The red circle represents the total energy consumed over some time interval.
The green circle represents the emissions free energy generated by PV panels over the same time.
It is shown slightly larger that the red circle, as is the case for GWUC over a year.
The intersection of the circles represents energy both produced and consumed, that is self consumption. It has no effect on the grid.
The remaining part of the red circle is energy bought from the grid. It adds to our emissions at a rate depending on the emissions intensity of the grid.
The remainder of the green circle represents energy generated but not consumed, that is the surplus that is exported to the grid. This reduces emissions from the grid by the same intensity.
The size of both circles and the degree of overlap will all vary with time of day and year, weather etc. However we can neglect that by taking a long time interval, say 1 year.

Installing a battery does not change the energy consumed, i.e.the size of the red circle, nor the energy generated, the size of the green circle.
What it does change is the level of overlap. It increases self consumption by allowing some of what would otherwise have been exported, to be stored and used when instantaneous generation is less than consumption, e.g. at night.
This means that both export and import are reduced by the same amount, equal to the increase in self consumption.
Import and export have different prices but the same emissions intensity, so that a battery saves money but not emissions.

Notes

Adding the battery will actually increase consumption slightly as charging and discharging cause some losses as heat. See here.
The tradable carbon credits for the saved emissions due to exported energy will be counted by the power company if they were signed over at installation, as is very common.
Emissions are logically proportional to the difference between Consumption and Generation, neither of which change. This is the same as Bought minus Sold, as self consumption neither causes nor displaces grid generation, and Bought and Sold change by equal and opposite amounts.
Increasing self consumption in other ways, such as time shifting loads, have the same effect.
Widespread batteries do facilitate decarbonisation of the grid by reducing the variations in demand.
Batteries do decrease the likelihood of the power company reducing the output of inverters. to stabilise the grid. Such reductions of solar generation would increase emissions as long as the grid retains fossil fuelled generation.

DavidMorgan