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?
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It’s easier to describe in terms of DC but it wouldn’t really answer your question.
With AC, as you say, the voltage is changing ‘direction’ all the time, but so is the current.
So power = volts x amps. When a device is USING power the voltage is positive the current is also positive, and when the voltage goes negative, so does the current.
Plus x plus = plus.
Minus x minus = plus.
So power is still flowing the same ‘positive’ direction towards the device that’s using it even when the voltage and current are negative.
So when a device PRODUCES power, from its perspective, the current and voltage are opposite.
Think about a water pump and a hose nozzle. The water flows towards the hose nozzle and away from the pump, but the water pressure is positive in both cases.
Plus x minus = minus.
Minus x plus = minus.
So back to your original question. The way your meter can tell which way the power is flowing is to measure which direction the current is flowing when the voltage is positive.
I hope that helps but if youre interested and want to go further then try to read about ‘passive sign convention’ and ‘power factor’
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