# Which Source of Energy Comes First?

When there are multiple sources of power, how is it determined which one is used? or are they all split?

Easy example is a grid tied solar system. If you have grid power in a breaker box and also solar power coming into the breaker box, how is the decision explained? Does the load use the incoming solar energy first? If so, why?

From observation, it seems the inverter monitors the voltage from the grid and outputs very slightly higher voltage. If i'm correct then the determining factor is voltage. If voltage is like pressure, is a higher voltage source pushing back the lower voltage source?

• If they're just wired in parallel, then Kirchhoff's laws apply. If there's a box of electronics connecting them all, then it depends on what's in the box of electronics, because the people who designed the electronics get to decide. Commented Jun 10, 2020 at 21:09
• In a real world system the decision is made by design rules. Technology allows any source or sources to be used regardless of voltage levels or other technical parameters. The usual aim is the minimisation of long term amortised cost. This includes the cost of implementing more technically complex systems, feed in tarriffs, unit cost of power, storage costs and more. In simple systems it may just be "the laws of physics" that determine what flows where but even that is a design decision - ie allowing cheap rather than complex solutions. Commented Jun 12, 2020 at 2:37
• || FWIW - your question is not as clear to many as you seem to think it is. Commented Jun 12, 2020 at 2:38

To expand on a comment by user253751, it has to work because of Kirchhoff's current law.

There will be a point in the system where the grid supply, the solar input and the house load all meet. That could be in a breaker box, or some other sort of junction box.

Kirchhoff's current law says that the total current going into a point must equal the total current going out. If it were not so, electrons would be piling up at that point.

So imagine it's a sunny day. The solar inverter is pushing out 10A. The house is drawing 6A. That means 4A must be going out to the grid.

Suppose you turn on a high power appliance, and the load increases to 16A. The solar is still generating 10A, so 6A must be coming in from the grid.

So nothing "decides" where the power comes from. It's all done by the laws of physics.

I answer to the comment below, solar inverters are clever computerised things. The constantly monitor the output of the solar panels, and suck out as much power as they can, given how sunny it is. They then adjust the voltage and phase of the AC that they generate so that they are always pushing out as much power as they can.

So as the grid voltage fluctuates slightly throughout the day, the inverter is adjusting its own output to match.

• Either I completely don't understand any of this or I am asking the completely wrong question. Everyone seems to be giving tons of great information, but no one is actually answering the question. If I have a 20A load and 2 lines that can provide 20A of power, there must be a determining factor to which line is providing that power. I thought it might be based on the voltage, like.. the line with the higher voltage is the one that will provide the power, but someone already said no to that theory. So i'm still stuck with no answer. Commented Jun 11, 2020 at 22:01
• @Grimshad Does the extra bit on my answer help? Commented Jun 12, 2020 at 13:35
• It helps but still doesn't answer the question > Why does the load use the power from the inverter instead of the power from the grid? They are both pushing power to the load, but only the power from the inverter is used... until it is not longer generating power (at night) and then automatically the loads is now using grid power instead. What makes the inverter the authority? Why does the inverter come first? Why does the grid provide the difference in power instead of the inverter? Commented Jun 15, 2020 at 14:39
• @Grimshad The inverter always pushes out as much power as it can. It will increase the voltage and adjust the phase to make sure that happens - essentially, it is pushing harder than the grid. Given that it always pushes out as much power as the panels are generating, the laws of physics decide where the power goes. I don't think I can say any more than that. Commented Jun 15, 2020 at 20:53
• what number or numbers or formulas are used to determine which source is "Pushing harder" ? Commented Jun 16, 2020 at 15:32

A grid-tied solar system would probably store solar energy first. If the storage is full, then it would use the solar energy for local loads. After that any excess energy available would be sold to the grid. The decision is explained by determining the strategy that minimizes cost to the solar system owner. Another strategy would be to minimize the risk of not having energy available. If the risk of losing grid power is high. Batteries would not be used except for powering critical loads when the grid power is not available and be recharged from the grid whenever it is available.

Your observation about inverter vs. grid voltage is essentially correct. To return power to the grid, the inverter must match the grid frequency and phase and exceed the grid voltage slightly. The voltage increase in controlled by monitoring the effect of current.

From comment:

I wanted to know what the determining factor is for which source of energy the load draws from... scientifically. IE: If there is a grid source and a solar source, how does the load determine which source energy to use? and why?

The load does not determine which source to use.

The solar panel provides power to a grid-ted inverter. In order to get the most power from the solar panel that it can produce for a given level of received sunlight, the inverter draws the level of current from the panel that tracks the maximum power point on the panel's voltage vs. current curves.

The output of the inverter is connected to the grid to form a microgrid. The local loads and the battery charge controller are loads connected to the microgrid. The loads draw power from the microgrid without regard to which source is supplying the microgrid. If the loads don't use all of the power that the inverter supplies, the inverter increases its voltage sufficiently to force the utility grid to take power from the microgrid. If the loads draw more power than the inverter can supply, the inverter voltage drops slightly to allow the utility grid to supply the power shortage.

For example, if the solar panel can supply 20 amps, the inverter will supply the minimum voltage required to supply 20 amps to the microgrid. If the load requires 20 amps it will draw 20 amps from the microgrid. In order to supply 20 amps, the inverter voltage must be equal to or enough above the above the utility voltage to prevent it from supplying any current. If the inverter voltage does not prevent current from coming from the utility, it can not supply 20 amps.

In the diagram below, the 100 V ideal voltage source represents the utility grid. The 20 A ideal current source represents the inverter. The 5 ohm resistor represents the load. The grid supplies 100 volts with the polarity indicated regardless to the magnitude or direction of the current it supplies or receives. The inverter supplies 20 amps in the direction indicated regardless of the voltage. The resistor current and direction is determined by by the sources. So the current source determines the current flossing towards the node where the resistor is connected. The voltage source determines the voltage across itself, the resistor and the current source. Since 20 amps through 5 ohms results in 100 volts, all of the current supplied by the current source and no current flowing in or out of the voltage source is the only situation that satisfies the basic circuit principles. There is no one determining factor. All of the above principles must be satisfied.

As mentioned above, a battery charge controller and the loads may be controlled also to suit the needs of the overall energy use strategy.

• I think you may have misunderstood the first half of my question. Probably due to my poor wording. I wanted to know what the determining factor is for which source of energy the load draws from... scientifically. IE: If there is a grid source and a solar source, how does the load determine which source energy to use? and why? Commented Jun 10, 2020 at 20:05
• What I wrote is the determining factor. The load would draw energy from the solar source if the solar source is available and not fully loaded by charging batteries. I believe a grid-tied solar system needs a controller that monitors and directs everything. If there are no batteries, to may be a little simpler. Someone else may have a better answer.
– user80875
Commented Jun 10, 2020 at 20:22
• @CharlesCowie I think the asker means what physical condition determines which power source gets used--not how it should be controlled, but what quantity needs to be controlled. Commented Jun 10, 2020 at 21:26
• I find it funny that so many people seem to know what i'm asking, but no one has actually answered the question yet. Commented Jun 11, 2020 at 0:09
• @CharlesCowie Thatnks for the update. So is the answer to my question "Voltage"? Will the load always take the higher voltage line first? Commented Jun 11, 2020 at 16:03

In a real world system the decision is made by design rules.

Technology allows any source or sources to be used regardless of voltage levels or other technical parameters. Energy flow magnitude and direction and power factor of two or more nominally equal voltage AC sources can be controlled by phase-angle variation.

The usual aim is the minimisation of long term amortised cost. This includes the cost of implementing more technically complex systems, feed in tariffs, unit cost of power, storage costs and more.

In simple systems it may just be "the laws of physics" that determine what flows where but even that is a design decision - ie allowing cheap rather than complex solutions.