Author Topic: Behind-the-meter solar storage  (Read 9666 times)

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DamonHD

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Behind-the-meter solar storage
« on: July 09, 2018, 11:22:41 AM »
I have an off-grid PV system with some storage, and its main job is to keep my Raspberry Pi server running year-round off-grid.  If there is enough spare energy in the system then the network gear (Internet router and FTTC box) are also taken off grid.

What I'd really like next is to take all my night loads off grid, especially but not only during the summer.

My grid-tie system has no storage (yet).

One thing that has been bugging me is that obviously my house draws a bit of power from the grid at night.  Not many vampire loads, but the fridge is there all the time.  Maybe ~80W average, or 1kWh-ish per night, less in summer.

It would be nice to get to the stage where outside winter where the PV only generates ~1.5kWh/day we never import from the grid, but that's still too pricey to think about.

The Enphase AC coupled storage looks as if it might work for the night loads (so lighting and maybe TV also in winter):

https://enphase.com/en-uk/products-and-services/storage

~260W max (which covers all of those non-cooking loads easily) and just over 1kWh effective storage.

And small enough to fit beside the consumer unit that it would wire into.

Still spendy, but I'm thinking hard about it.  I can take it with me if we're forced to move, which may yet happen, since the area we are in is being redeveloped.

I can add other stuff in parallel later such as another of the same unit, or something like the Sunamp heat battery.

Rgds

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Ungrounded Lightning Rod

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Re: Behind-the-meter solar storage
« Reply #1 on: July 09, 2018, 09:14:10 PM »
Does it work with a low-power inverter for off-grid?  Or does it require the hard-voltage, hard-frequency line power to be present?

As I read their company hype, this is about optimizing how much power you pull from the grid, especially in situations where your smart meter gives you different peak and off-peak rates or part of your bill is related to your maximum peak draw (so you're ahead to charge the storage when rates are low or your solar panels provide more power than your appliances draw).  It's not clear to me whether it could handle power outages or running with a local inverter for a frequency reference.

Grid-feeding, including grid-supplementing, systems use small frequency offsets to detect "islanding" in a grid outage, shutting themselves down to avoid electrocuting power company linemen.  This system measures line-side current and in principle could avoid backfeeding the line and triggering anti-cheating circuitry.  But in the normal connection scenario it would have to shut down if your utility power went away.

Not saying it won't do your job.  Just that the online hype doesn't make it clear exactly what it's up to.

If it DOES do your job, that 98% round-trip efficiency and long battery lifetime is very attractive - providing the system cost is low enough not to eat it up.

DamonHD

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Re: Behind-the-meter solar storage
« Reply #2 on: July 10, 2018, 02:20:58 AM »
Hi,

1) Not doing this to save money: I would be trying to minimise grid flows (and maximise self-consumption) as an experiment for minimising carbon emissions.

2) This does not provide power when the grid goes down.  The grid is very reliable here, and I have the 2kWh off-grid system if we need light (etc) if the grid does go down.

Rgds

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Bruce S

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Re: Behind-the-meter solar storage
« Reply #3 on: July 10, 2018, 08:56:10 AM »
Damon
Here's a possibility. I was inspired by OperaHouse's fridge setup, but have been too busy at work to build a similar system ;-(.
Is your fridge's insulation good enough to merely put install a timer?
This I've already tested and done.
We have a commercial grade temperature gauge. I checked it in several different configurations.
Uses a whole lot less power overnight.
I know the timer will be offset for the defrost timer I can't disconnect , but even with getting a cold water out of the fridge at night it's still well below the danger zone for the foods in it, even freezer is still below freezing (including ice cream).
 We don't have metered power, but when the smart meters are installed, I'll be using this information for day time runs while no one is at home to open the fridge.

Cheers
Bruce S
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OperaHouse

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Re: Behind-the-meter solar storage
« Reply #4 on: July 10, 2018, 09:11:18 AM »
An advanced fridge control would be an interesting project and you should not have any difficulty doing it.
 
First, use the fridge for energy storage. Run the temps lower when sun is abundant. I run my chest fridge  only during the day and only when the battery is fully charged. While this is not possible with a fridge, it can delay the fridge in the evening. Mine won't even start till the battery reads 13.8V. That that would keep the battery ready for night. This could be done with an auxiliary temperature control. I run at 33F and it seems to have no effect on the food.

Disable the defrost at night.

Run the fridge during the day on solar when conditions are met. Don't transfer power when compressor is running.

DamonHD

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Re: Behind-the-meter solar storage
« Reply #5 on: July 10, 2018, 01:35:53 PM »
My current fridge energy storage scheme is simpler and less likely to cause food poisoning if I forget...  I load up the fridge with new beer and other drinkables when we are generating, to absorb the energy from the PV and store as cool!

BTW, starting to write this up here:

http://www.earth.org.uk/electricity-storage-whole-household-2018.html

I can see an interesting regulatory wrinkle coming up given that I actually meter my exports rather than using the alternative scheme that 'deems' them to be 50% of generation.  Anyway, I'll write that one up when I have a clearer idea.

Rgds

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MattM

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Re: Behind-the-meter solar storage
« Reply #6 on: July 10, 2018, 09:14:17 PM »
I remember your aerogel write-up.  Did you save some aerogel for the fridge?

OperaHouse

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Re: Behind-the-meter solar storage
« Reply #7 on: July 11, 2018, 12:00:43 PM »
This guy SolarTrap has been working on this a couple years, I don't think it has ever fully worked.  https://www.youtube.com/watch?v=IuhiuUjg1yM   I tried to help him on some other forums, but he wasn't interested.  Had ideas about running 130A thru a circuit board and PWM the main supply of an inverter on for a soft start.Also trying to do elaborate calculations of battery charge using very drifty sensors.  A lot you can do with a fridge if made one of a kind.  Maybe wifi fridges will be better.  I doubt it given what people have tried to do with the communication link of heat pump water heaters.

DamonHD

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Re: Behind-the-meter solar storage
« Reply #8 on: July 11, 2018, 05:32:34 PM »
No aerogel on the fridge!

Rgds

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Re: Behind-the-meter solar storage
« Reply #9 on: July 12, 2018, 11:10:38 PM »
Run the temps lower when sun is abundant. I run my chest fridge  only during the day and only when the battery is fully charged. While this is not possible with a fridge, it can delay the fridge in the evening. Mine won't even start till the battery reads 13.8V  ...

I note that the 40F or less of fridges (closer to, but not quite at, 32F is better) and the 0F of freezers is not arbitrary:

Freezers run at 0F because that's where water is solid even if it is saturated with salt.  Eliminating liquid motion substantially retards chemical reactions (and toxic lifeforms).  Going above it shortens shelf life of frozen foods.

Refrigerators are 40F maximum as a tradeoff between food spoilage and not accidentally freezing something.  Running lower is better - making your food last longer and reducing your risk of food poisoning.

Above 40F things spoil - and get toxic - very rapidly (something the energy-saving nazis that try to get you to save energy by raising your fridge temp a few degrees really don't get).  Also:  Once the bugs get going their life processes can make extra local heat, so they can KEEP going for a while once the temp is back down.

Quote
I run my chest fridge  only during the day and only when the battery is fully charged. While this is not possible with a fridge, it can delay the fridge in the evening. Mine won't even start till the battery reads 13.8V  ...   I run at 33F and it seems to have no effect on the food.

33F is good.  Gives you some margin when letting it warm a bit in the day.  Not starting until evening and/or when the voltage is 13.8 or better, even if it gets over 40F, is a recipe for a trip to the hospital.

I'd consider having a fridge regulate at 40F and a freezer at 0F even if the voltage is low, and go for 35F / -10F if the sun is up and the voltage recovered.  (34 or 35F to avoid a daily freeze-thaw in spots if the stuff is packed in tight.)  That would use your surplus afternoon power to cool things down to where it wouldn't turn on at night or earlier in the day unless you opened the door a lot or the ambient temp got high, and protect you from food poiioning as well as or better than a fridge running at 40F and preserve frozens as well as or better than one set at 0F while still conserving power when it's precious.

Quote
Disable the defrost at night.

Better yet, do it only in the afternoon once the batteries are up and the fridge/freezer temp down to the low setting.

« Last Edit: July 12, 2018, 11:16:21 PM by Ungrounded Lightning Rod »

OperaHouse

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Re: Behind-the-meter solar storage
« Reply #10 on: July 13, 2018, 08:26:56 AM »
I have about 2.5KW of panels just to run a chest fridge. It works on the worst of days. A lot of the food you buy at the store isn't even at 40F.   There is a guy on youtube saying a fridge can run with a 100W panel and just 3 hours of sun, he is going to die.

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Re: Behind-the-meter solar storage
« Reply #11 on: July 13, 2018, 09:13:01 AM »
Luckily , I have a wife that is a Chef  :), they have to be Serve Safe certified every two years.
Once you see the videos and go thru the class on what could happen in a very short span of time; food safety becomes paramount.
We run our fridge at 34F and freezer at 0F. The freezer at 0F is also how I separate my alky into higher proofs ( when there is room in the freezer).
A freezer is also more efficient running at near capacity than not.

I also have an over-temp alarm that is set at 38F, if the fridge gets up to that point it loud enough to wake the dead.
I then bypass the timer and let it run until I know what happen to set off the alarm.

Damon;
We seem to have drifted off course again. Apologies!!

Bruce S
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DamonHD

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Re: Behind-the-meter solar storage
« Reply #12 on: July 13, 2018, 09:53:42 AM »
No need for any apologies.  All interesting!

BTW, having difficulty getting a critical bit of info for my purposes.

My evening/night loads will only usually be around 70W, eg for the fridge/freezer while it is actually running, or maybe the TV and some lights in the gaps.  One brand/product publishes its minimum load threshold at which it will start to inject energy to reduce imports from the grid at 175W, and when I've followed up with the manufacturer they have said that it cannot be set lower.  OutBack's threshold in GridZero mode seems to be 240W (1A).  I can't find the numbers for the other products I'm looking at, but clearly it's rather important if I'm going to get the effect I want!

Write-up continuing here:

http://www.earth.org.uk/electricity-storage-whole-household-2018.html

I'm also trying to shovel a little more load off-grid anyway, since the two approached should play nicely together:

http://www.earth.org.uk/low-voltage-drop-out-circuit-design.html#Reuse2018

Rgds

Damon
« Last Edit: July 13, 2018, 10:04:02 AM by DamonHD »
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OperaHouse

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Re: Behind-the-meter solar storage
« Reply #13 on: July 13, 2018, 11:54:28 AM »
I think that minimum threshold  is due to them not knowing which way the current is flowing at those low levels due to how few bits it has to calculate and drift. If your max loads are fairly small compared to what the maximum for the unit is, an extra turn or two thru the sensor will help.  The results will then have to be multiplied.

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Re: Behind-the-meter solar storage
« Reply #14 on: July 13, 2018, 01:47:18 PM »
My router died this afternoon, and my ISP itself was down with a fairly major fault, so probably not much more fiddling around on this one for now, and my Web site will be down until next week...

There is hope on the cut-in threshold though.  The Enphase unit seems to have a lower threshold of 5W, which would be excellent if I can confirm.

The Enphase is expensive per kWh, and the peak power is only ~260W per kWh of storage capacity.  But if it really can manage to absorb the small loads then I have the prospect of fitting something beefier in parallel later.

There also seems to be a possibility of getting enrolled in some kind of product trial with another manufacturer, but I don't know anything more about that yet.

This can of worms seems to have been worth opening!

Rgds

Damon

(edit) PS: The router recovered somehow after a night off.  I have a new RPi 3 to try to do its job as router and WiFi access point as well as run all my services, which I will work on slowly...
« Last Edit: July 23, 2018, 05:01:21 AM by DamonHD »
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Re: Behind-the-meter solar storage
« Reply #15 on: July 23, 2018, 05:02:12 AM »
I've put down a deposit on a modest solution which should get installed in a couple of weeks.  I shall be very interested to see how it performs from now into mid-winter.

Rgds

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Re: Behind-the-meter solar storage
« Reply #16 on: July 30, 2018, 03:40:55 AM »
I've created a simulation to understand better how the battery is likely to work in detail:

http://www.earth.org.uk/electricity-storage-whole-household-2018.html#Simulation

This simulates every minute of the day, and uses my real PV generation numbers (for Dec and Jun) and some real (or nearly real) consumption numbers for a couple of the main appliances that tend to be running overnight.

Rgds

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DamonHD

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Re: Behind-the-meter solar storage
« Reply #17 on: August 07, 2018, 10:56:27 AM »
The new battery is in, and behaving nicely:

http://www.earth.org.uk/Enphase-AC-Battery-REVIEW.html

Yesterday evening the house basically ran off battery after sunset until we went to bed.

It'll be a couple of days before I start to get access to stats.

Rgds

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Re: Behind-the-meter solar storage
« Reply #18 on: August 13, 2018, 12:11:05 PM »
The data is arriving and the storage appears to be working as expected:

http://www.earth.org.uk/electricity-storage-whole-household-2018.html#Data

It was good to import 0.2kWh out of 2.2kWh demand even when roughly half the demand happens when the PV was not generating to cover any of it.

However, the effects are generally more subtle since this system is not speced to prevent the big loads hitting the grid, only the small stuff.  Bigger loads only have the corners knocked off, ie 'smearing' of demand.

Rgds

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DamonHD

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Re: Behind-the-meter solar storage
« Reply #19 on: August 29, 2018, 10:16:48 AM »
I've added some new logic to my off-grid storage to try to get better performance out of the combination of it and the on-grid storage.  Basically, if the house is spilling/exporting to grid, and the off-grid system would normally take some load off-grid but the off-grid storage is not full, that load is now *not* taken off grid.

The aim is to effectively keep that now-unused off-grid storage for later, to reduce grid imports when the grid-tie is no longer covering house load.

The effect is small, maybe ~30Wh by mid-afternoon today, and never likely to be be more than ~100Wh/day, but for about 10 lines of extra code, gaining that much effective storage and reducing grid flows, is good.

Rgds

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Re: Behind-the-meter solar storage
« Reply #20 on: September 23, 2018, 05:32:24 PM »
Hi Damon
How has your experience with Enphase been? I'm a little concerned with the history of the company, but see that some of the major pv module makers are using Enphase micros for their AC modules, so I have to think that they looked very carefully at Enphase before shaking hands.

I'm just starting to build the rack for my ground mount array of 10 Trina 320 W modules. Enphase micros,  I'm looking at IQ6, would add a lot of flexibility to the system, and if the local building official accepts it, would make grounding and bonding easier since they are double insulated (they say no bonding / grounding is required per the NEC).  Also since they will be ground based, they will stay cooler (some claim that heat on roofs was the main cause of their earlier inverter failures), and will make changeouts easy if some fail, ASSUMING they honor their warranty. Their newer micros have a lot of very sophisticated capabilities.

So to my questions.
How is your system holding up?  Have you dealt with Enphase - how is their customer service / support?
Any news from others about dealing with them?

BTW  Enphase has a couple of tech support bulletins about AC coupling their micros with battery inverters / chargers made by other companies.  That is what I will probably do if I use them.
Thanks in advance.
Pete

 

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Re: Behind-the-meter solar storage
« Reply #21 on: September 24, 2018, 02:10:38 AM »
Enphase seems OK in my dealings with them so far, which have all been via my installer.

My system seems well-engineered, though for example, I am currently seeing a warning on the MyEnlighten Web interface which has a broken link, so there are some rough edges.

Note that Enphase has yet to turn a real profit, though has just come very close.

Rgds

Damon

« Last Edit: September 24, 2018, 04:26:02 AM by DamonHD »
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petect

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Re: Behind-the-meter solar storage
« Reply #22 on: September 24, 2018, 06:22:34 PM »
Thanks Damon

Pete

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Re: Behind-the-meter solar storage
« Reply #23 on: September 25, 2018, 12:09:44 AM »
A few of the (very clever) people I've worked with over the years either have or do work for Enphase here in Christchurch NZ. They have a development centre here where the clever stuff like 3 phase micro inverters, battery storage and dual panel inverters are developed.

A lot of brains have gone into the products, certainly better since they moved development from the US to NZ ;) so I'd recommend then technically. As to whether they are good value for money - dunno as I've never bought one as I use an EnaSolar high voltage GTI. They are just across the road from Enphase (about 400m apart!!).
Robin Down Under (Or Are You Up Over)

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Re: Behind-the-meter solar storage
« Reply #24 on: September 25, 2018, 09:02:22 AM »
frackers
thanks for the information.
 I am probably going to go with Enhase. They have had their ups and downs, and seem to be getting their stuff together,  but what inverter company hasn't had problems?

If anyone wants to read about something I thing is amazing, Google Enphase IQ8. IF they pull this off, things are going to change in pv land.
Pete

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Re: Behind-the-meter solar storage
« Reply #25 on: September 30, 2018, 03:40:26 PM »
To quote from:

http://www.earth.org.uk/Enphase-AC-Battery-REVIEW.html#Self

Quote
Self-consumption, ie (Net - Import) / Net, where Net consumption = Import - Export + Generation, rose from 48% in August 2017 to 72% in August 2018 (~0.5kWh/d reduced imports). The Enphase was only installed on the 6th, and August is a special low-consumption case.

September 2018 roughly halved daily imports compared to September 2017 (1.6kWh/d vs 3.2kWh/h), with self-consumption up to 69% from 37%. Some behaviour change and better PV generation may have helped.

So, it seems to be doing what it should.

Rgds

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Re: Behind-the-meter solar storage
« Reply #26 on: September 27, 2019, 01:41:57 PM »
I'm now working to add more behind-the-meter storage, though this time heat, and "cross-vector" ie saving gas by using some locally-generated electricity:

http://www.earth.org.uk/note-on-solar-DHW-for-16WW-UniQ-and-PV-diversion.html

I don't expect it to be effective before February (ie not for Nov/Dec/Jan, and with ~4 week lead-time minimum), so will be working on the details for an install by then.

Rgds

Damon
« Last Edit: September 29, 2019, 11:26:48 AM by DamonHD »
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Re: Behind-the-meter solar storage
« Reply #27 on: September 29, 2019, 10:36:39 AM »
Interesting, the pic looks a lot like an immersion coffee/tea cup heater.

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Re: Behind-the-meter solar storage
« Reply #28 on: September 29, 2019, 11:25:24 AM »
It is!  That was the only immersion heater pic that I have...  B^>



I'm waiting for a better pic, eg a sketch of the proposed system!

Rgds

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DamonHD

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Re: Behind-the-meter solar storage
« Reply #29 on: October 28, 2021, 11:54:18 AM »
This is not quite the right thread, but never mind...

As of today I have gone from ~1.25kWh of ac-coupled Enphase storage ("AC Battery") in early September to ~5kWh today.  (Well, configuration isn't complete, but the physical installation is done.)

This is for science in the sense that I'm pretty sure that this is larger than optimal, especially in winter, but I want to see if that is evident from the stats.

Rgds

Damon
« Last Edit: October 28, 2021, 12:16:39 PM by DamonHD »
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Re: Behind-the-meter solar storage
« Reply #30 on: October 31, 2021, 10:18:23 AM »
When you say "larger than optimal" do you mean you may have more storage than can be effectively be maintained by solar input?
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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Re: Behind-the-meter solar storage
« Reply #31 on: October 31, 2021, 03:29:46 PM »
Hi,

1) That (eg we're already generating less than gross consumption in Oct/Nov), and,

2) The overheads of running the storage may exceed any (carbon) value to the grid in reduced flows.

Rgds

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Re: Behind-the-meter solar storage
« Reply #32 on: November 01, 2021, 07:55:04 AM »
If it charged at the optimal time of day would that skew results in any positive way?  Or do you simply figure buying grid electricity at peak hours?