After installing solar panels on my roof I got my electric meter replaced with a bidirectional one. This meter can measure both energy that I draw from the network and the energy that I send, depending whether my consumption is higher or lower than my production.
It got me thinking though. If the network uses AC which has no constant direction, then how can a meter by measuring the current and voltage know if the power is consumed or given back?
In: 7
The meter looks at instantaneous current and voltage.
In an AC system, the voltage is positive for half the time and negative the other half. If you are importing energy, then the current will be into your house when voltage is positive and out when voltage is negative.
The meter takes the voltage and current readings, multiplies them and sums the result. This way if you are importing, the calculation will give a positive power, or negative if exporting.
In fact, all electricity meters do this.
Regular electricity meters have a block (either mechanical or software) which stops the counter going down when export power is measured.
A bidirectional meter has two separate meter counters. One only goes up during import and one only goes up during export. They are both blocked from going down.
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