# GT source energy question

A couple of question from a new forum member. I've recently had a 5k system installed and am waiting final inspections before turning on.

While sitting back and waiting for finals I started to wonder about how AC source to the load is determined.

Why does a GT system use energy from the PV system before that from the utility?

What process/mechanism determines which source (utility,PV) is used to satisify the load?

My GT system uses a line side tap in the meter box to connect the AC from the inverter. At this point I would say that there are '2' ac sources capable of suppling power to the load (house), utility & inverter, on the load side of the meter.

I'm curious about: What determines which source is used - is there a circut (or other mechanism) within the meter that detects the AC souce from the inverter as sufficient for the load and blocks forward utility flow while allowing excess inverter power to flow into the grid for net metering.

As I've scanned through the threads I've not seen any comments on this subject so any information on how this works will be appreciated

Re: GT source energy question

Welcome to the forum SRJAN,

While the GT inverter is certainly a complex piece of equipment, how it interacts with the utility is pretty simple from an electrical point of view...

Basically, the Utility Power looks like a giant AC battery (120/240 VAC 60 Hz). I.e., in electrical terms it is a Voltage Source... No matter how much load you place on the "Battery" the voltage (and frequency remains the same). And if you try to pump energy (current) back into the utility, the voltage/frequency will remain the same.

This is all just like the battery in your car--The battery there holds ~12-14 volts no matter if you are discharging (using power) or charging (supplying power) to the battery.

The GT Inverter, on the other hand, is a high impedance "current source"... The GT inverter takes whatever energy it can collect from the solar array, and pumps out sine wave current (Power=Voltage*Current) to your home's wiring.

Now, that current has to go somewhere, either to the Air Conditioner, electric heater, TV, etc. in your home or out to the utility.

And that division is very simple... If your GT inverter is outputting 10 amps at 240 VAC (2,400 watts). And you have 10 amps worth of load, your GT power goes directly to the load...

If you have only 5 amps of load, then the other 5 amps @ 240 volts AC (1,200 watts) goes out through the electric meter spinning it backwards.

Or, if you have 20 amps of load, the GT inverter will supply 10 amps, and the electric utility will supply the other 10 amps (and spin your meter forwards).

This is really pretty much the same way your car works. You have a battery (utility power). An alternator (GT inverter). And your loads (starter, headlights, ignition, etc.).

The battery is a big "buffer" of energy and the Alternator adjusts its output current to keep the battery charged... Some energy going to/from the battery, and other energy to support the loads.

Of course there are various safety items... The inverter checks for 212-264 Volts AC, 60 Hz +/- 0.1%, clean voltage sine wave, open neutral, etc... And it has a five minute timeout--if there are any line problems, it waits 5 minutes before retrying to generate power again.

A GT inverter cannot operate unless your electric power is turned on and within specifications--That is why a GT inverter cannot (by itself) run during a power failure.

Does my explanation make sense?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset