Real Power

[jt_retro]

Hey Folks

My understanding is that some appliances, namely inductive loads such as motors, will cause real and reactive power to flow in AC circuits.

However, I have come to learn that apperently, domestic properties are only billed for the real portion of the power (in kwh), even though the power company has to supply both real and reactive power.

Where my confusion lies is when comparing how many kwh my solar system has generated, vs how much my home has consumed from the grid. I've been taking the kwh readings from my epever SCC and Merross smart switches (which are on the solar inverter) to be equal to the kwh that my power company grid meter says.

Eg, I would usually say to myself: "my epever generated 5kwh of power today. This means I saved 5kwh of grid power, less whatever solar system losses there might be".

But is this wrong? For appliances such as a tumble dryer, does this mean that 5kwh of solar would only cost eg 2-3kwh on grid? (Assuming a tumble dryer is 50% reactive power)

While getting monetary payback was never my priority for building this system, my calculations always landed on ~15 year payback. But if this reactive power is the case, perhaps I'll never see payback for the life of the system.

Thanks

 

[svetz]

Hopefully one of the EEs will correct anything I say that's hopelessly wrong... but... it's my novice understanding that the device consumes more power than it actually needs... that is a device consumes the "real power" and "reactive power" - so you're always paying for it. The grid meter readings (or an energy meter) should tell you the actual consumption. A better-designed device will reduce the reactive power and hence lower the overall consumption.

The reason why commercial energy rates vary with reactive power is because their usage is typically a lot more significant and it's to cause them to do things to correct the imbalance rather than the utility needing to expand more capacity.

 

[Woody]

Boy that’s getting into the technical weeds.
Let’s start you out with some reading on the topic. Search for: real power vs reactive power explained

Here’s a couple links to start you out with.

https://www.arrow.com/en/research-and-events/articles/real-vs-reactive-power

Understanding the basics of reactive power

energyinnovationproject.com

A crude simple explanation would be real power is the power consumed vs reactive power is power you borrow. Sorry rather crude but to fully understand this one needs to look at the voltage/amp waveforms with an understanding of capacitance/reactant loads….

 

[jt_retro]

So in a nutshell, domestic users do indeed pay for reactive power and 1kwh of solar generated == 1kwh of grid power saved? (Hypothetically, assuming no inverter or cable losses, which of course wouldn't happen in real life)

Thanks

 

[nonoceb]

So in a nutshell, domestic users do indeed pay for reactive power and 1kwh of solar generated == 1kwh of grid power saved? (Hypothetically, assuming no inverter or cable losses, which of course wouldn't happen in real life)

Thanks

Yes but at the cost of IR loss in cable, not a big deal in house and short ac cable.
it's like you want drink 1liter of water with one plastic bottle and you don't finish the water inside to refill the source water tank then refill your water bottle to finish your 1liter drink. a bit inefficient but it's work fine

 

[jt_retro]

Aahhh interesting! I think I get it....

Basically, you "give the reactive power back" to the inverter (and thus don't loose it), however in the process of transferring it to and from the load, you loose some of it due to the IR loss in the cable. But as you say, not a big deal in a 240V circuit.

Is this correct?

Cheers

 

[wattmatters]

Where my confusion lies is when comparing how many kwh my solar system has generated, vs how much my home has consumed from the grid.

Just to add to your confusion, grid tied solar PV inverters can also have their power setting adjusted so they also have different real and apparent power output. In one distribution zone in Australia (SE QLD) it's a requirement for grid tied solar inverters to have a lagging PF setting of 0.9.

So in a nutshell, domestic users do indeed pay for reactive power and 1kwh of solar generated == 1kwh of grid power saved?

Normally homes don't have much reactive power to speak of and residential billing is based on real power (W) and not apparent power (VA). But you would want to check with your local electricity distributor / retailer.

Industrial billing is different and that's where apparent power is more important, due to the use of machinery, motors, pumps etc which have a much greater impact on power factor.

In Australia the newer smart meters are capable of measuring both real and apparent power (and homes installing solar PV are required to have their meter upgraded if it's not already a modern meter). We are billed on real power.

So if I were to make a guess, I'd say the energy measurements supplied by your solar inverter (production) and your retail meter (energy import and export) will both be based on real power and so you can use them in simple calculations to derive an amount of self consumption.

 

[jt_retro]

So if I were to make a guess, I'd say the energy measurements supplied by your solar inverter (production) and your retail meter (energy import and export) will both be based on real power and so you can use them in simple calculations to derive an amount of self consumption.

Unfortunately, my inverter has no measurement interface or app. My readings come from my SCC (Epever) and a Victron BMV 712 on the batteries.

My solar system is totally off grid so my comparisons are to work out how much equivalent grid power I've saved.

 

[wattmatters]

My readings come from my SCC (Epever) and a Victron BMV 712 on the batteries.

Ah, I was confused by what you said earlier about the readings being on your solar inverter:

I've been taking the kwh readings from my epever SCC and Merross smart switches (which are on the solar inverter)

but you mean measurement are on the DC side.

Hard to know what your actual AC output is without an AC meter since there will be losses in the conversion from DC to AC. It might be 80-90% of DC ends up being supplied on the AC side.

 

[Warpspeed]

Domestic power meters only record true power, and that is what you are charged for.

The current actually completely reverses each half cycle anyway, its surging back and forth at 50/60Hz.
Actual average net current flow will always be zero !

When you feed power back into the grid from a grid tie inverter, its the relationship between the voltage and current that changes, so your domestic Kwh meter MUST be able to detect which way power flows, as well as separate true power from reactive power.

Your solar inverter on the dc side will also indicate only true power. Power measurement on the ac side of an inverter could measure either true power or more likely reactive power, depending on what type of watt meter you use to make the measurement with.

All the measurements should add up and agree, provided you are measuring true watts, and allowing for slight inaccuracies due to measurement error and efficiency drops.

 

[Jordi]

On another post I am digging into more or less the same topic and here I am not sure If I understand the conclusions.

Conclusions are (?):
- Households pay only true power = KWh.
- Households receive also reactive power which is mostly borrowed and returns to the net.
- Households consume a very little reactive power through appliances having a power factor close to 1 + system losses.
- The small reactive power consumption from households is not billed by the supplier because it is so low that "they don't care". (see image below from energy supplier Iberdrola)



Unclear parts:
- If households only pay for true power, what happens to the reactive power produced by a grid-tied inverter?

Additional question:
- If a grid tied inverter advertised with a power factor of 98%, only has a power factor of say 80%, is that a reason to claim a partial refund? This because the true power is lower and so its potential use and financial return.

 

[wattmatters]

The small reactive power consumption from households is not billed by the supplier because it is so low that "they don't care". (see image below from energy supplier Iberdrola)

This is very much locality dependent.

In Australia, kVA / kVAh are used for billing business/industry.

They are not used for households though, at least not directly.

Household energy tariffs here are made up of a number of costs. Only ~ 1/3rd of our tariffs are the actual energy production/supply, most of it is network and transmission costs and one of the those costs is a distribution loss factor (where I am it's about 7%) applicable to each kWh supplied. The DLF accounts for a range of losses including those relating to household level reactive and inductive factors.

So we are paying for it, it's just spread out across all households.

 

[Warpspeed]

Households only pay for true power
Yes.

Households receive also reactive power which is mostly borrowed and returns to the net.
No, you never "sent" reactive power, power is something the load draws depending on the nature of the load.
If the load is switched off for example, power drawn from the grid will be zero.
If its a resistive load, it will draw only true power from the grid.
If its a complex load, there will be a mixture of true and some proportion of reactive power drawn from the grid.
The grid only supplies what the load demands.

Households consume a very little reactive power through appliances having a power factor close to 1 + system losses.
Yes, appliances are designed to have minimal reactive power.

The small reactive power consumption from households is not billed by the supplier because it is so low that "they don't care". (see image below from energy supplier Iberdrola)

That is pretty much it. They only get upset when reactive power becomes very large, and that should not normally be the case in a typical home situation.

If households only pay for true power, what happens to the reactive power produced by a grid-tied inverter?

A grid tie inverter does not create reactive power, it just supplies the load. As with the grid, if the load is resistive the grid tie inverter only supplies true power. If its a complex load the grid tie inverter may be able to cope with a small amount of reflected power. The specifications will tell you the maximum. Any more than that may stress out the inverter which may be a problem. Usually the grid will cope with a horrible load sufficiently to protect the inverter.

If a grid tied inverter advertised with a power factor of 98%, only has a power factor of say 80%, is that a reason to claim a partial refund? This because the true power is lower and so its potential use and financial return.

You are misunderstanding the 98% power factor limit. What the specification says is the inverter will tolerate a power factor load of 98%, and anything more (lower percentage) might damage the inverter. Its like saying 2Kw maximum load limit. If you connect up a 2Kw load it will be fine. If you comnnect up a load that is way beyond 2Kw the inverter may be damaged.

You cannot go back to the supplier and say that I connected up a horrible load that only had 80% power factor and the inverter blew up. Its the users responsibility to ensure the specifications are not exceeded. You cannot complain later that the inverter did not do what you expected if that expectation is excessive beyond the published specified maximum.

Power factor is something you can test, just like maximum load power is something you can test. But what happens if you exceed the rated maximum by a lot will probably end in tears.

 

[Jordi]

This is very much locality dependent.

In Australia, kVA / kVAh are used for billing business/industry.

They are not used for households though, at least not directly.

Household energy tariffs here are made up of a number of costs. Only ~ 1/3rd of our tariffs are the actual energy production/supply, most of it is network and transmission costs and one of the those costs is a distribution loss factor (where I am it's about 7%) applicable to each kWh supplied. The DLF accounts for a range of losses including those relating to household level reactive and inductive factors.

So we are paying for it, it's just spread out across all households.

Thank you wattmatters! So while the supplier I found is a real life case, there are differences among countries in the world. Indeed, If I was to look in Spain's energy taxes, there might be something accounting for reactive power losses.

Households only pay for true power
Yes.

Households receive also reactive power which is mostly borrowed and returns to the net.
No, you never "sent" reactive power, power is something the load draws depending on the nature of the load.
If the load is switched off for example, power drawn from the grid will be zero.
If its a resistive load, it will draw only true power from the grid.
If its a complex load, there will be a mixture of true and some proportion of reactive power drawn from the grid.
The grid only supplies what the load demands.

Households consume a very little reactive power through appliances having a power factor close to 1 + system losses.
Yes, appliances are designed to have minimal reactive power.

The small reactive power consumption from households is not billed by the supplier because it is so low that "they don't care". (see image below from energy supplier Iberdrola)

That is pretty much it. They only get upset when reactive power becomes very large, and that should not normally be the case in a typical home situation.

If households only pay for true power, what happens to the reactive power produced by a grid-tied inverter?

A grid tie inverter does not create reactive power, it just supplies the load. As with the grid, if the load is resistive the grid tie inverter only supplies true power. If its a complex load the grid tie inverter may be able to cope with a small amount of reflected power. The specifications will tell you the maximum. Any more than that may stress out the inverter which may be a problem. Usually the grid will cope with a horrible load sufficiently to protect the inverter.

If a grid tied inverter advertised with a power factor of 98%, only has a power factor of say 80%, is that a reason to claim a partial refund? This because the true power is lower and so its potential use and financial return.

You are misunderstanding the 98% power factor limit. What the specification says is the inverter will tolerate a power factor load of 98%, and anything more (lower percentage) might damage the inverter. Its like saying 2Kw maximum load limit. If you connect up a 2Kw load it will be fine. If you comnnect up a load that is way beyond 2Kw the inverter may be damaged.

You cannot go back to the supplier and say that I connected up a horrible load that only had 80% power factor and the inverter blew up. Its the users responsibility to ensure the specifications are not exceeded. You cannot complain later that the inverter did not do what you expected if that expectation is excessive beyond the published specified maximum.

Power factor is something you can test, just like maximum load power is something you can test. But what happens if you exceed the rated maximum by a lot will probably end in tears.

I am wowed! You are a bless to solar noobs. Very clear statements; I was mislead by a product review stating the buyer found a much lower power factor from its inverter and that as result, he wasn't able to get the expected yield. His low power factor must then be related to what he connected to that inverter; when this is the grid, power factor measured should be close to 1.

I am happy to hear that the inverter will mainly produce real power equal to the solar panel output minus conversion&cables losses.

In these regards I just saw yesterday an experiment from a guy running 2 different bulbs (the first with power factor 0,63 and the second with power factor 0,99) from a set of capacitors. The results were that the bulb with power factor 0,99 remained 5% more time on; something the author related to higher losses from lower power factor bulbs. So reactive power only had an effect on the energy loss through the cables and load itself because lower power factor translates into higher currents through cables (current true power + current reactive power); but the power consumption from both bulbs is the same; 5 W each.

In the true/reactive power analogy I see it as a person walking to the bar more times to drink the same amount of beer just because he likes beer with a thick layer of foam. More reactive power, more loss to consume the same amount of true power.