Mitigation of demand from utility with solar

[phx]

Hi. I'm from Phoenix, AZ with 12.96kW grid-tied system up and running. It has 5 strings of panels and two SMA grid-tied inverters working in parallel. They are AC coupled before solar meter. It's a standard design recommended by SRP (utility company) published on their Web-site

I'm on "Customer Generation" E-27 plan from SRP with peak time 2pm-8pm where demand fees apply. Monthly bill consists of three parts:
1. Monthly NET amount "Per kW/hr" (separately for "off peak" 8pm to 2pm and "on peak" from 2pm to 8pm)
2. Flat monthly "Service fee"
3. "Demand fee" which is calculated for highest peak of electricity consumption "on peak" time from 2pm to 8pm during billing month

My system covers almost 100% of daily electricity needs. In some months I have even negative net balance of kilowatt-hours consumed. Between 2-5pm "on peak" time consumption and peak demand is almost fully covered with solar generation and thus not countable

However from 5pm to 8pm sun gets weak or there is no sun. SRP charges harsh fees for demand during peak time in summer: $17.51 for each kW below 10 and $33.59 for each kW above 10. Hence demand charges jump to $150-180 per month for occasional 10-12 kW spikes happening even one time per month. So demand fee is about 90% or more of my monthly electrical bill and I'm looking for technical solutions to mitigate it

The biggest energy hog is central 5 ton A/C unit consuming about ~7kW of electricity. It's running ~12 hours daily around o'clock to beat the triple digits heat in Arizona. Other large appliances are: laundry dryer, oven/stove and several smaller A/C units in different rooms

I looked at Demand Control systems "Smart Panel 3000" and "Brayden Automation Energy Sentry". They are very expensive. However basically they can help only with periodic turning off A/C and other major appliances when demand is high. This is not a possible solution for my situation. I cannot allow my house to get hotter by adjusting A/C for higher temperature even for less than hour due to health issues. Also it's very hard to control who in the family wants to cook something in the oven, dry laundry or turn on their own bedroom A/C units during peak time

So I'm looking for possible solution to add some battery storage to my system. Possibly it can work like this:

* 8pm-2pm - batteries charging during off-peak hours as electricity if free during that time. It's either produced by solar or drawn from utility. Net balance is fully offset by solar generation and always negative in my bill for these hours. Even if there is no sun in night time, electricity is still free as daily generation creates reserve with net metering to be used in night time
* 2pm-5pm - "On-peak" time when solar generation typically covers my entire demand. It's not reasonable to charge batteries during that time and more beneficial to sell excessive energy back to utility for higher "on-peak" cost
* 5-8pm - "On-peak" time when solar generation is not sufficient to cover 100% of demand. But it covers from 100% of it at 5pm to zero when sun goes down. At this time house should consume still produced solar electricity first and then the rest of energy should be supplied from batteries avoiding expensive utility-provided energy with demand charges. However utility shouldn't be disconnected via transfer switch as starting current of A/C unit is about 140Amps and inverters don't have sufficient power to provide it even for a couple of seconds. So those starting A/C peaks should be covered from utility

I was thinking about adding AC coupled hybrid inverter with battery backup and possibly another solar string with it to my system. That should be a third inverter. I looked at several models of such inverters from SRP-approved list (mainly "Schneider Electric"). And they all work different way than described above. All of them have built-in transfer switch and loads should be connected after it. In such case loads use solar energy and then switch to battery energy if it's not sufficient or grid is down. Inverter and batteries capable to produce 140Amps/240V AC for starting HVAC unit and running it completely from batteries will be very expensive

So ideally system should operate such way:
*Hybrid inverter/charger be AC coupled with other two existing inverters before solar meter
*Batteries charged only at scheduled time 8pm-2pm. It can be either from its own solar string or from AC side - doesn't matter. It might even NOT have its own solar string and be used only as purely battery charger/inverter
*No transfer switch. Loads should still go from main electric panel where utility power is combined with solar power
*System should measure demand from utility at any particular moment on-peak time and supply the balance between demand and solar generation to common connection by using batteries

In such case solar energy would always have priority for consumption and demand from utility stay "zero" unless batteries fully exhausted ans loads start working just from utility. HVAC starting current will be covered from powerful utility connection not overloading inverters

I couldn't find any model of hybrid inverter capable for such operation. Can anybody suggest one?

The only possible solution I see is DIY with 200-550V battery bank hooked up directly to MPT channel of grid-tied inverter. Or same DC voltage provided from 48V battery bank via another "off-grid" inverter/charger and rectifier to be fed into MPT chanel of grid-tied inverter. However in such case I need a demand-measuring sensor on utility line, PWM to control power supplied from battery bank to MPT of inverter and computer to control power supplied by PWM based on momentary demand and time of day. Otherwise the MPT will always try to draw the maximum power that battery can supply quickly exhausting it

Any ideas?

 

[Deleted member 165]

Are those prices really correct? Must be a typo?

 

[phx]

That's a real typical cloudless day May 27th in Phoenix, AZ (just as an example) with real prices and my real electric bill from May. Only PII was erased for security reasons. Attached below is a booklet from SRP (utility) with description of solar E-27 billing plan

SRP demand charges might be pretty hard to understand from the first look. They don't charge $17 or $33 for each kW/hr consumed on peak time. This is what they do to calculate those charges:
*Look at all 30-minutes intervals during peak time 2pm-8pm for entire month
*Find only one - the highest. In that bill for May it was 5/27/20 between 7:30p-8pm
*Ignore all other demand values
*Charge $7.89 per kW for first 3kW of peak demand discovered above (please see E-27 description for "Summer" demand)
*Charge $14.37 per kW for number of kilowatts between 3 and 10
*Charge $27.28 for each kW above 10

So that single 9.1kW spike for half-hour on 5/27/20 cost me $111.33 according to calculations above as reflected on SRP bill. The rest of demand intervals are not countable for that month

In Summer Peak (July-August), demand charges are even higher

 

[Ampster]

Any ideas?

If i understand the attachment it looks like the window for measurement of peak loads is a 30 minute window. I could not read the chart clearly but it looked like your peak went from 2 kW to 8kw during that time. That is a critical assumption to even begin to think about a strategy so give me more detail if you have it?

There are some hybrid inverters that have grid zero modes that can set maximum grid support which might mitigate your issue. If my assumption is correct, you would need 12 kWhrs of capacity to shift just one hours worth of consumption.
Sometimes there are solutions on the consumption side that may also be cost effective. Inverter driven Air Conditioning compressors, heat pump water heaters, induction cooktops, and others.
You may need to spend some money on gathering more data to find where the sweet spot is between energy conservation and energy storage to shift those peaks.

 

[svetz]

...[I have a ] 12.96kW [array]...
...My ...[array]... covers almost 100% of daily electricity needs. ..
...I couldn't find any model of hybrid inverter capable for such operation. Can anybody suggest one?

The 12 kW Sol-Arc handles 16kW of panels. The AC is only pulling 7kW, so that leave 5 kW for everything else. It's also programmable for TOU rates.

 

[svetz]

You could probably also go Enphase with Encharge or a Tesla Powerwall. That would allow you to both "right-size" and have scalability in small increments.

 

[phx]

Thank you all for all responses with ideas

Sol-Arc inverter suggested by svetz is close to what I'm looking for. As I figured out from its description, I need to look for following features while shopping for inverter:
"Meter Zero Mode" - that's the most important feature to completely wipe out demand fees
"Time-of-Use Mode" - can help to mitigate demand charges and high "On-peak" rate if battery capacity is not sufficient to cover entire peak demand

Exactly that particular model might not be the best choice for my needs as it's very expensive ($6,850) and can be installed only indoor where I don't have room. My existing inverters are outdoors and certified to operate at ambient temperatures above 100F, which is critical for Phoenix

But I'm definitely looking for something with those features described above. Ideal (for me) model should be battery charger/inverter AC coupled with two already existing grid-tied inverters. It might or might-not have input for solar array. It can can be just purely grid-tied battery charger/inverter with ability to feed back from battery into greed when needed supporting "Meter Zero Mode"

Tesla Powerwall is pretty far from my requirements. I looked at its wiring diagrams. It has transfer switch. So it doesn't support "Meter Zero Mode". Tesla Powerwall is "all or nothing" switching either to solar+grid (still with demand charges) or to 100% battery-based energy off-grid via transfer switch

Amster suggested optimization of energy consumption as one of possible solutions. That's a good idea in general and can be recommended for both solar and non-solar households. Unfortunately big savings from demand fees are not achievable in that direction. I already have pretty highly efficient HVAC with Nest thermostat, all LED lights in my house and gas water heater. Spending big money on even more efficient appliances perhaps can reduce overall energy consumption by 15-20%. But It won't eliminate demand spikes when A/C unit needs to run at the same time with oven and dryer

Trying to convince anybody in family about saving electricity during peak time and explaining them demand charges fails with arguments from them that we already spent big money for solar and don't want to live with any restrictions and especially in hot house due to A/C throttling. So solution should be purely technical not touching human habits or making any restrictions

SRP (utility) measures demand by 30 minutes windows during peak time. E.g. 2pm-2:30pm A/C was running (7kw), Dryer was running (3kw) and few computers, TV and lights were turned on for entire 30 minutes (another 2kW). Also 11kW was produced by solar at that time. So billed demand will be 7+3+2-11 = 1kW. But if same situation happens e.g. at 7:30 pm, it will be 7+3+2=12kW demand which comes to about $180 demand fee during July-August summer peak even if it happened just one time for entire month

At next 00 or 30 minutes of each hour demand calculation resets to zero and starts accumulating again for current billing window until it hits 00 or 30 minutes of hour and window closes. At 8pm all demand charges become non-countable since it's already "off-peak" time. They are shown on chart with blue

Entire 12kw/hr battery capacity is not needed. If demand from grid doesn't exceed 3kW at any moment during peak time for entire billing month, demand charges are still reasonable and can be consumed. Based on my more detailed analysis 7-8 kW/hr battery bank can provide me what I need in order to mitigate high demand spikes. However the question is how to connect it all together?

 

[svetz]

... as it's very expensive ($6,850) ...

When comparing it to other inverters keep in mind the Sol-Arc is an all-in-one, so has the MPPT and other stuff needed to hook them together (e.g., breakers, disconnects). For example, a 8K pre-wired Outback Radian with the same parts is $7575. All that little stuff adds up fast.

Tesla Powerwall is pretty far from my requirements. I looked at its wiring diagrams. It has transfer switch. So it doesn't support "Meter Zero Mode". Tesla Powerwall is "all or nothing" switching either to solar+grid (still with demand charges) or to 100% battery-based energy off-grid via transfer switch

You have to have anti-islanding to have power while the grid is down, so you'll need a switch of some sort. I believe it can do TOU, something to check on. The Encharge definitely can.

...Trying to convince anybody in family about saving electricity ...

You're preaching to the choir brother! See RF and the Smart Solar Home, I embarked on that adventure to see if I couldn't turn stuff off not in use automatically. I suppose you could install a hidden timer behind the drier that only allows it to run in off-peak hours. When the wife complains it's broken tell her you'll look at it in the morning. ;-)

...the question is how to connect it all together?...

Each vendor has a few twists here and there, but each should have a schematic that lays it out.

 

[phx]

Based on information collected so far, it looks like TOU/Meter Zero functionality with hybrid inverter is possible only if load distribution panel hooked up after inverter. The latter should work as a hub distributing energy between solar, grid, batteries and loads. Considering maximum household demand 10-12 kW and starting current for A/C 140 Amps, that requires very powerful and expensive inverter capable to handle such load

I'm still looking for grid-tied battery charger/inverter without transfer switch that can be AC coupled with existing grid-tied system and load distribution panel like in regular grid-tied systems. It should provide TOU Management by feeding as much energy from batteries to the grid as needed to cover load demand thus achieving "Meter Zero" mode. It doesn't need to be solar. Just battery charger/inverter is enough. However it looks like such model just doesn't exist. If anybody can suggest one, I'll be glad to take a look

So if search of such device is unsuccessful, I'm thinking about trying to piggyback one of MPT channels on existing solar inverter for that purpose. Battery power can be converted to 480V DC with cheap off-grid inverter and rectifier with doubling voltage. Then the main DIY problem will be building a DC power controller to feed that energy into MPT inverter based on current demand. Utility bill will look like sun continued shining every day until the end of peak time. This will make DIY "Meter Zero" mode

I'm considering PZEM-016 MODBUS devices with current transformers for measuring solar production and power coming to/from grid, I have few of them and did some testing. They are pretty accurate in readings and look promising. Raspberry PI can be used as controller to read those PZEM-016 and control power via PWM. The only item I still can't find is 240V/30A (AC) or 480V/15A (DC) PWM with computer interface. It should accept value of power from computer and provide that power to the load (MPT channel of grid-tied inverter)

Can anybody recommend particular model of PWM with parameters above available for purchase. E-bay mostly offers low voltage PWM's or something for 120V and no computer interface. So it's not an easy find

 

[Xerxes]

I do not understand those $17.51 for each kW below 10 and $33.59 for each kW above 10.

That is the most ridiculous rates ever. In the USA no less and in Phoenix Arizona?

Is anyone else in the USA at this forum paying such high rates?
I am curious to know if I am really fortunate (for the first time in my life) to live in California.

Is it to punish those on solar so they pay more than if they were on grid?
What is the normal Joe on grid KW-hr rate?

I think because I am an energy hog at 15KW-hr per day I am in the incremental $1 per KW-hr for those last few KW each day.
At $30 per KW-hr X 15Kw, I would be paying $450 per day and $13,500 per month!

Are you sure about those rates? That is exorbitant and something I have never heard of.

If those rates are true I would ditch the solar and get in a normal rate plan and save tons of money!

Then secretly install one of those on grid solars to supplement your electricity.

 

[Ampster]

I'm still looking for grid-tied battery charger/inverter without transfer switch that can be AC coupled with existing grid-tied system and load distribution panel like in regular grid-tied systems.

Let us know what you find? I do not know how one could accomplish AC coupling without a transfer switch? Do you have a one line diagram?

 

[Ampster]

do not understand those $17.51 for each kW below 10 and $33.59 for each kW above 10.e

That is demand pricing which is fairly common in industrial and commercial rates. This is the first example in residential.

 

[gnubie]

Similar tariffs are available here in Queensland Australia. The thing to remember is that a typical household bill is in kWh, 11c / kWh. This is in kW, ie continuous draw. You pay $17.51 to draw up 10000 watts all day, every day for some number of days (which might be 1 day).

 

[svetz]

... it looks like TOU/Meter Zero functionality with hybrid inverter is possible only if load distribution panel hooked up after inverter.

Do you mean a critical circuits sub-panel? If so, they just show them that way as it's a cheaper install, doesn't have to be that way. Different solutions for different vendors, but here's Ensemble with and without a critical circuits panel as an example:

​

Here's an example post of how it might be done with Sol-Arc.

 

[phx]

I do not understand those $17.51 for each kW below 10 and $33.59 for each kW above 10.

That is the most ridiculous rates ever. In the USA no less and in Phoenix Arizona?

I agree that's demand charge is a total rip-off from utility defeating all solar benefits. That's why I'm looking for technical solution how to beat demand during peak time and get rid of those stupid fees. That's a goal of this discussion

I already explained in my message above on 6/26/20 at 10:43am how those demand charges calculated. This is more detailed description. I took May 27th as example as a typical cloudless day in Phoenix with the highest demand peak 9.1kW for entire billing month. Demand charge included in my bill (see very first posting at the top of this thread) was $111.33

This is how it was calculated:
*SRP (utility) billing system looked at all 372 of 30-minutes demand billing intervals occurred during 31 billing days in peak time 2-8pm
31days x 6 hours of peak time daily x 2 = 372 of 30-minutes intervals for billing month
*Found the highest peak 9.1kW among those 372 intervals. In that bill for May it was 5/27/20 between 7:30p-8pm
Only that (the highest) peak was countable for demand calculation
*Ignored and forgave all other 371 of remaining 30-minutes intervals with lower demand
They all were NOT countable as the highest demand interval was already found above
*Charged $7.89 per kW for first 3kW of peak demand discovered above (please see E-27 description for "Summer" demand and chart below). That's $23.67
*Charged $14.37 per kW for number of kilowatts between 3 and 10. It was 9.1-3=6.1kW. Charge was 6.1kWx14.37=$87.66
*Charged $27.28 for each kW above 10. Thanks God I had nothing in this tier for May
But It's very likely it will happen in July-August when it's much hotter

So that single 9.1kW spike for half-hour on 5/27/20 cost me $111.33 according to calculations above. It's $23.67 for first 3 kW of demand + $87.66 for next 6.1kW of demand between 3 and 10 kW as reflected on SRP bill. The rest of demand intervals are not countable for that month

In Summer Peak (July-August), demand charges are even higher. Please see E-27.pdf attachment above and demand chart below for full description of SRP billing plan

This is a chart of SRP demand charges extracted from E-27 billing plan:
Summer - May, June, September and October
Summer Peak - July and August
Winter - November, December, January, February, March and April

 

[ghostwriter66]

I agree that's demand charge is a total rip-off from utility defeating all solar benefits. That's why I'm looking for technical solution how to beat demand during peak time and get rid of those stupid fees. That's a goal of this discussion

I already explained in my message above on 6/26/20 at 10:43am how those demand charges calculated. This is more detailed description. I took May 27th as example as a typical cloudless day in Phoenix with the highest demand peak 9.1kW for entire billing month. Demand charge included in my bill (see very first posting at the top of this thread) was $111.33

This is how it was calculated:
*SRP (utility) billing system looked at all 372 of 30-minutes demand billing intervals occurred during 31 billing days in peak time 2-8pm
31days x 6 hours of peak time daily x 2 = 372 of 30-minutes intervals for billing month
*Found the highest peak 9.1kW among those 372 intervals. In that bill for May it was 5/27/20 between 7:30p-8pm
Only that (the highest) peak was countable for demand calculation
*Ignored and forgave all other 371 of remaining 30-minutes intervals with lower demand
They all were NOT countable as the highest demand interval was already found above
*Charged $7.89 per kW for first 3kW of peak demand discovered above (please see E-27 description for "Summer" demand and chart below). That's $23.67
*Charged $14.37 per kW for number of kilowatts between 3 and 10. It was 9.1-3=6.1kW. Charge was 6.1kWx14.37=$87.66
*Charged $27.28 for each kW above 10. Thanks God I had nothing in this tier for May
But It's very likely it will happen in July-August when it's much hotter

So that single 9.1kW spike for half-hour on 5/27/20 cost me $111.33 according to calculations above. It's $23.67 for first 3 kW of demand + $87.66 for next 6.1kW of demand between 3 and 10 kW as reflected on SRP bill. The rest of demand intervals are not countable for that month

In Summer Peak (July-August), demand charges are even higher. Please see E-27.pdf attachment above and demand chart below for full description of SRP billing plan

This is a chart of SRP demand charges extracted from E-27 billing plan:
Summer - May, June, September and October
Summer Peak - July and August
Winter - November, December, January, February, March and April

View attachment 16284

I see this all the time in industrial and commercial solar platforms we deal with BUT first time I have actually seen it for residential -- and of all places - Arizona ...

 

[ghostwriter66]

Let us know what you find? I do not know how one could accomplish AC coupling without a transfer switch? Do you have a one line diagram?

Like @Ampster if you find one let us all know -- i would be interested in at least looking at how it works ... I mean by its very nature it would need a transfer switch of some sort ... but the guy looking over my shoulder as I type says he has heard of these without transfer switches but their costs are in the 10's ... and uses software and blah blah blah - (which in my mind is till a transfer switch of some sort)

 

[Ampster]

I see this all the time in industrial and commercial solar platforms we deal with BUT first time I have actually seen it for residential -- and of all places - Arizona ..

I would be curious what strategies are deployed. I think at depends on a lot of factors including rates and the nature of demand peaks and their duration.
I think Tesla is using PowerPacks to reduce demand charges at some of its Supercharger locations. Those same type of PowerPacks have been installed at commercial locations for that purpose. Often those are done by integrators who automate load shedding and other strategies into a plan to reduce demand charges.

I own an apartment building that is subject to demand charges 24/7. I never had a demand charge until I put it a plug for a tenant to charge her EV. She only used a 20 Amp EVSE but that ended up being 4.8kW draw and I got hit with a demand charge. The tenant moved on but I am still looking for a solution other than passing the demand charge on to the next tenants that want that benefit.

The upshot is there are a lot of solutions to the OPs dilemna in the commercial and industrial marketplace where the saving can be in the thousands of dollars. The trick will be finding something that is reasonable in cost and has apayback in less than 10 years.

 

[phx]

I see this all the time in industrial and commercial solar platforms we deal with BUT first time I have actually seen it for residential -- and of all places - Arizona ...

SRP (Utility) has another payment plan E-13 for residential solar customers. Please see attachment. It doesn't have demand fees. However it's even worse than E-27 with demand charges. On Summer peak (July-August) SRP sells energy for 24 cent/kWhr during peak time 2-8pm while it always buys energy from solar for 2.8 cent/kWhr regardless of season or time of day. It's a total rip-off on customers making 854% profit from them. So E-13 is a total waste of resources and money

 

[Ampster]

Don't be too quick to dismiss that rate plan. Have you done the math about going on that rate and buying a smaller battery system that would allow you to self consume your solar and not buy anything at that peak rate?
You can also add solar panels a lot less expensively than inverter and battery capacity. However there is an optimum system design for that rate plan, just as there is one for your existing rate plan.