Battery Usage Question

Grouch1980

Senior Member
Location
New York, NY
On an AC or DC Coupled System, say it's a cloudy day, or nighttime. The PV array isn't getting any sun. Is the ESS / battery bank providing power to the house, or is the grid, or both? I'm assuming the battery bank would be powering the house first until it depletes... and then the grid would kick in. Is this correct? The battery and the grid would not be BOTH powering the house. For argument sake let's just assume it's many cloudy days with no sun, so the PV array is not providing any power. and the battery bank had a few days of autonomy.
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Typically entirely configurable in the ESS. When and how to charge and discharge the ESS in a grid tied system is generally made for economic reasons based on the cost of the electricity in time-of-use electrical rate plans. Sometimes ESS is used for backup only, though.

And if the house demand exceeds the ESS inverter output, then certainly both the grid and the ESS can be powering the house.

Cheers, Wayne
 

Grouch1980

Senior Member
Location
New York, NY
Typically entirely configurable in the ESS. When and how to charge and discharge the ESS in a grid tied system is generally made for economic reasons based on the cost of the electricity in time-of-use electrical rate plans.
So this is something that can be programmed in the multimode inverter / the inverter that feeds the battery?
And if the house demand exceeds the ESS inverter output, then certainly both the grid and the ESS can be powering the house.

Cheers, Wayne
Basic question... since i'm not too familiar with batteries... wouldn't the house just draw more amps from the batteries, thereby lowering the hours of use of the batteries (using the amp-hours relationship)? For example, the house could use (just for argument sake) all the battery amps in one hour. and THEN, after the batteries are depleted, the grid would kick in. OR... is there an upper limit / max on how many amps the batteries can deliver?

EDIT: actually... there SHOULD be a max... the output of the batteries are connected to a breaker? therefore it's not possible to go above and beyond a certain amperage, or else you could burn out that inverter... so the grid would then come in?
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
With a grid tied inverter, the inverter decides how much AC current to pump out. It has to follow the grid voltage. Any current demand (by the house) in excess of the current produced by the inverter will be drawn from the grid. And any excess current production will flow back into the grid.

So one common technique with ESS is to use a pair of CTs to measure the house load at each point in time, and for the inverter to provide current that matches the house load (up to the inverter's maximum rating). That keeps the house load from drawing any current from the grid. And if there is a simultaneous production from a PV inverter, then all of the PV generation goes to the grid, which can be economically advantageous during Peak electrical rate times.

The above applies to either the single inverter in a DC coupled system or to both the PV inverter and the separate ESS inverter in an AC-coupled system.

Cheers, Wayne
 

Grouch1980

Senior Member
Location
New York, NY
Understood on all fronts, thank you! Except for this one piece:

So one common technique with ESS is to use a pair of CTs to measure the house load at each point in time, and for the inverter to provide current that matches the house load (up to the inverter's maximum rating). That keeps the house load from drawing any current from the grid.
How does this keep the house from drawing from the grid? Wouldn't the CT's always be measuring the load? or it's when the CT's kick in, that's what prevents the grid from feeding the house?
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
How does this keep the house from drawing from the grid? Wouldn't the CT's always be measuring the load?
So the CTs are located so that all the loads are on one side, and the grid and the inverters are on the other side. That way the CTs can measure the current required by the loads regardless of what that current source is. Then that data gets fed to the inverter controls so that the inverters can react by providing the same current. That makes the net grid current zero.

This repeats every so often (multiple times per second?), so that the typical grid current is zero. Between measurements and the inverters reacting, the load current could change, causing some small current flow to or from the grid.

Cheers, Wayne
 

Grouch1980

Senior Member
Location
New York, NY
So the CTs are located so that all the loads are on one side, and the grid and the inverters are on the other side. That way the CTs can measure the current required by the loads regardless of what that current source is. Then that data gets fed to the inverter controls so that the inverters can react by providing the same current. That makes the net grid current zero.

This repeats every so often (multiple times per second?), so that the typical grid current is zero. Between measurements and the inverters reacting, the load current could change, causing some small current flow to or from the grid.

Cheers, Wayne
Thanks. And as you mentioned in Post #2, all this is also configurable? Can it be adjusted so that some of the grid current can flow in as well?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
So one common technique with ESS is to use a pair of CTs to measure the house load at each point in time, and for the inverter to provide current that matches the house load (up to the inverter's maximum rating).
Most residential solar PV systems are simply grid tied, and residential grid tied inverters cannot do that; the amount of power they produce is governed by the amount of insolation on the array, though SolarEdge inverters can with the addition of some hardware. Many inverters can throttle back output in response to frequency munging, but one obviously cannot change the frequency as long as the inverter is connected to the grid.
 

Grouch1980

Senior Member
Location
New York, NY
Typically entirely configurable in the ESS. When and how to charge and discharge the ESS in a grid tied system is generally made for economic reasons based on the cost of the electricity in time-of-use electrical rate plans. Sometimes ESS is used for backup only, though.

And if the house demand exceeds the ESS inverter output, then certainly both the grid and the ESS can be powering the house.
Just to expand on my original question (Post #1), during the daytime when there is sun... it's the same thing correct? Now you can have the PV array, and the batteries, AND the grid each providing power? All three at the same time? it just depends on how you configure the settings on the inverter(s)?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Just to expand on my original question (Post #1), during the daytime when there is sun... it's the same thing correct? Now you can have the PV array, and the batteries, AND the grid each providing power? All three at the same time? it just depends on how you configure the settings on the inverter(s)?
The adjustments are to the battery management systems, not to the inverters. Most residential grid tied inverters have nothing to adjust.
 

Grouch1980

Senior Member
Location
New York, NY
Another question... can the grid be used to charge the ESS, or is that function solely for the PV array to do?

EDIT: actually I think post #2 answered this. so the grid can charge the ESS as well?
 
Last edited:

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Most residential solar PV systems are simply grid tied
Right, my comments were with respect to ESS. In theory you could build a non-export residential PV inverter that curtails its output to keep production below the measured house load; is that not commercially available?

Cheers, Wayne
 

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Just to expand on my original question (Post #1), during the daytime when there is sun... it's the same thing correct? Now you can have the PV array, and the batteries, AND the grid each providing power? All three at the same time? it just depends on how you configure the settings on the inverter(s)?
Unmanaged house loads will always try to draw the power they want. If the house is grid-connected with a sufficiently large service and feeders, they will always get the power they want. So with that as a baseline, any power that a PV inverter or an ESS inverter puts out just goes to reduce the grid power drawn by the house loads. And if that inverter power put out exceeds the house demand, the excess power will flow to the grid.

How each inverter decides how much power to put out is determined by its design, configuration and programming. As Ggunn points out, most residential PV inverters (without batteries) are designed just to always put out the maximum power available from the connected PV panels. ESS inverters are more configurable.

Another question... can the grid be used to charge the ESS, or is that function solely for the PV array to do?
That's another design and configuration issue. An AC coupled ESS is certainly capable of charging from the grid, but it may be set up to instrument the PV production and keep its charging rate not greater than the PV production. In that way it can be said to charge from PV only. And for regulatory reasons you may want it to charge from PV only. Utility generally don't permit ESS based time of use arbitrage, where you charge from the grid at low rates and discharge back to the grid at higher net metering rates.

For a DC coupled ESS, I imagine there could be an architecture where AC -> DC conversion is not supported, so the batteries can only charge from the PV. I'm not familiar with the products in that market segment.

BTW, I think in some my answers I've failed to sufficiently distinguish between what is technically possible, what is commonly available commercially, and what is allowed by regulation. Just something to bear in mind.

Cheers, Wayne
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Right, my comments were with respect to ESS. In theory you could build a non-export residential PV inverter that curtails its output to keep production below the measured house load; is that not commercially available?

Cheers, Wayne
The only one I know of is SolarEdge, and the inverter alone cannot do it; you must install their energy meter.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Most residential solar PV systems are simply grid tied, and residential grid tied inverters cannot do that; the amount of power they produce is governed by the amount of insolation on the array, though SolarEdge inverters can with the addition of some hardware. Many inverters can throttle back output in response to frequency munging, but one obviously cannot change the frequency as long as the inverter is connected to the grid.
The adjustments are to the battery management systems, not to the inverters. Most residential grid tied inverters have nothing to adjust.
The only one I know of is SolarEdge, and the inverter alone cannot do it; you must install their energy meter.

Perhaps I'm the odd one out, but I think you should stop saying these things, or think about how to say them more accurately. What you say is true for the majority of legacy inverters, but it is and will be increasingly untrue for newer ones. As you acknowledge, Solaredge can follow a metered load, with the addition of the meter accessory, the last point being a rather minor detail in the context of this discussion. The Enphase IQ8, hitting the streets this month, will also be able to do it with an appropriate meter setup (Envoy and CTs). (I believe IQ7 already has some ability to do it, but I haven't used those grid profiles.) Those two manufacturers represent about 90% of the US residential market share, so it is more or less now a fact that most new residential solar inverters can 'do that' [i.e. in Wayne's words "use a pair of CTs to measure the house load at each point in time, and ... provide current that matches the house load (up to the inverter's maximum rating)"]. Also, the comment about frequency munging leaves out the point that interactive inverters can be connected to non-grid primary sources, e.g. a battery inverter in an off-grid or temporarily islanded setup, which is also an increasingly common setup I'm sure our OP will want to eventually understand.

The statement "The adjustments are to the battery management systems, not to the [solar] inverters" is, well, only correct usually. In the case of SolarEdge, there is only one inverter for both, and the behavior (with meter installed) depends entirely on the programming. If you want a zero export or limited export system, you can set that up, and this has been true for more than 4 years, with or without battery. This will soon be true for new Enphase systems as well. Just because all of this isn't done that often (yet) does not mean that solar inverters can't do it. It's never been impossible in theory, and increasingly it will be possible with most off the shelf products.
 

rainwater01

Member
Location
Greenwood Indiana
Occupation
Electrician
Got it. Thanks all for the feedback and help.

Here’s some info from Generac you might find helpful.
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