On-grid domestic battery storage

jcontest

mbeast wrote: »

sorry to jump on this thread but somehow seems appropriate...

I'm looking at changing my standard inverters to something like the lux hybrid as i have 40 x 285w panels to fit and want to be able to set it to zero export as i wont get paid for any export.

I use around 70 kwh per day with a base load of 2.5kw per hour in summer and around 1.5kw per hour in the winter, so i'm wondering if it is worth getting 2 or 3 x lux hybrid inverters with 2 or 3 pylontech US3000 batteries per inverter, and maybe even charge them with economy 7 in the winter or would i be better to invest the cost of the batteries in something else as that might make more than i'd save on my power bills?

if anyone has a spreadsheet too that i could use then that would be great.

thanks

Are you getting FIT payments for this system?.
The way it works now is that you need to have your inverters grid tied, then run your battery storage separately. That's not nearly as efficient as charging directly from the panels (DC) but if you want to keep your FIT payments (If it's On-Grid) then your stuck.

zeupater

jcontest wrote: »

... Mixing that with solar/wind power will help those people consume 80-90% of what they generate. Throw in smart meters and your export will go down to nothing - but I expect that change to happen regardless.

Hi

Really? ... 80-90% ? ... perhaps you could expand on how that possibly happens over a typical year as many on here would be quite interested !!

HTH
Z

jcontest

zeupater wrote: »

Hi

Really? ... 80-90% ? ... perhaps you could expand on how that possibly happens over a typical year as many on here would be quite interested !!

HTH
Z

How you can use 80-90% of what you generate?... You just have a battery system that takes all incoming power you generate, then run from that at all times, Econ 7 charge at night, else just shunt to mains when needed. It's a little more complicated than that



Bear in mind I am saying you can consume 80-90% of your generation. Not that you can generate 80-90% of your consumption.


Why I say 80-90%. "Tested" round-trip efficiency for a few battery systems out there is just above that 90% mark. Those are tested with the specific voltages and current ranges that are exactly what the inverter and charger demands. That perfect pairing is not something you will ever get at home.

zeupater

jcontest wrote: »

How you can use 80-90% of what you generate?... You just have a battery system that takes all incoming power you generate, then run from that at all times, Econ 7 charge at night, else just shunt to mains when needed. It's a little more complicated than that



Bear in mind I am saying you can consume 80-90% of your generation. Not that you can generate 80-90% of your consumption.


Why I say 80-90%. "Tested" round-trip efficiency for a few battery systems out there is just above that 90% mark. Those are tested with the specific voltages and current ranges that are exactly what the inverter and charger demands. That perfect pairing is not something you will ever get at home.

Hi

So, let's test that in the most simplistic of ways ...

An average household uses 3300kWh of electricity per year, and let's say we're considering a 4kWp system generating 3600kWh/year, which is pretty reasonable ....

3300kWh/year consumption equates to 9kWh/day which is 2/3 of the capacity of a Powerwall2, however ...

In the main months for generation the daytime consumption hugely outweighs that at night, therefore of the daily 9kWh, the load on the battery is likely to be <3kWh which is all that is needed to 'top-up' storage in a reasonable generation period no matter what battery size is actually installed, so on a 15kWh generation day, 6kWh is likely to be used directly, 3kWh to the battery & 6kWh still exported ...

In a case where 90% of generation is self consumed the limit to not use would logically be 360kWh/year (3600*10%), which in terms of 15kWh generation day described above equates to less than two months of necessary export .... additionally, the daily losses through the battery at a round trip efficiency of 85% would result in a further daily average loss of 450Wh of the 3kWh diverted to the battery, so a further 3% of unusable generated energy(0.45/15) in the case above.

... then again, there are the days where the PV system is generating between 15 and 28kWh/day where over 2/3 of energy is being diverted ... each two to three reasonable days in this generation band reduces the amount of self consumption with the above battery scenario by a further ~1% ...

Of course, we can all argue the case for batteries based on a set of undisclosed/unknown variables, however, in order for a hypothesis to be supportable, the variables are extremely important!

On this board there is much support for battery storage and, of course, if these batteries were really cheap the question of economics & affordability wouldn't apply and we'd all be installing them tomorrow, however, as they're not then the need for careful performance consideration becomes important!


HTH
Z

Martyn1981

jcontest wrote: »

How you can use 80-90% of what you generate?... You just have a battery system that takes all incoming power you generate, then run from that at all times, Econ 7 charge at night, else just shunt to mains when needed. It's a little more complicated than that

Hiya. It's a lot more complicated than that though when you add in actual, real figures.

For instance, we generate around 4,500kWh pa, and consume around 1,500kWh's of that, plus about 1,500kWh's of import.

Now, let's look at the summer months, for the best 3 months, my system average is 20kWh per day. My consumption is about 7.5kWh of which about 2.5kWh is import, so I export approx 15kWh per day.

Now, let's say I get a big 15kWh battery, so could consume 5kWh of PV directly and store the other 15kWh in the battery, that's great for day 1. Then day two I start with 12.5kWh's in the battery already (15kWh minus the 2.5kWh I normally import), now I enter a cycle of consuming 5kWh's, storing 2.5kWh's and exporting the remaining 12.5kWhs.

If I get an EV and charge that from home, either directly from the PV, or from the battery, then that improves things. We do about the national average, so 8,000 miles pa, so 2,000kWh's so 5.5kWh's per day, but we are still exporting 7/20ths of our generation.

In the winter months when generation is down to about 3-4kWh's per day, then the batt will help us to reach 100% consumption.

Like Z, I'm not trying to be difficult, but matching PV, batts, demand and seasonality are not straightforward nor simple. For best results (more consumption, less import) you need a big battery, but for economic results (cost of batt v's savings) you need a small battery. Great fun.

In Spain and Australia, they are seeing that PV can reduce demand by 40% and PV + battery reduces it by 90%, but they have a far smaller seasonal variation, perhaps 1.5 or 2:1 (summer v's winter), whereas we have about a 4:1 ratio.

Edit - I should add that my battery plans also include installing 4 more panels, steep pitched (winter optimised), south facing. Hopefully will add around 1,250kWh pa.

zeupater

Martyn1981 wrote: »

... In Spain and Australia, they are seeing that PV can reduce demand by 40% and PV + battery reduces it by 90%, but they have a far smaller seasonal variation, perhaps 1.5 or 2:1 (summer v's winter), whereas we have about a 4:1 ratio.

Hi

It must also be appreciated that much of the difference between 40% & 90% being down to the battery supporting air conditioning throughout summer evening & nights in considerably warmer climates!

HTH
Z

jcontest

Martyn1981 wrote: »

Hiya. It's a lot more complicated than that though when you add in actual, real figures.

For instance, we generate around 4,500kWh pa, and consume around 1,500kWh's of that, plus about 1,500kWh's of import.

Now, let's look at the summer months, for the best 3 months, my system average is 20kWh per day. My consumption is about 7.5kWh of which about 2.5kWh is import, so I export approx 15kWh per day.

Now, let's say I get a big 15kWh battery, so could consume 5kWh of PV directly and store the other 15kWh in the battery, that's great for day 1. Then day two I start with 12.5kWh's in the battery already (15kWh minus the 2.5kWh I normally import), now I enter a cycle of consuming 5kWh's, storing 2.5kWh's and exporting the remaining 12.5kWhs.

If I get an EV and charge that from home, either directly from the PV, or from the battery, then that improves things. We do about the national average, so 8,000 miles pa, so 2,000kWh's so 5.5kWh's per day, but we are still exporting 7/20ths of our generation.

In the winter months when generation is down to about 3-4kWh's per day, then the batt will help us to reach 100% consumption.

Like Z, I'm not trying to be difficult, but matching PV, batts, demand and seasonality are not straightforward nor simple. For best results (more consumption, less import) you need a big battery, but for economic results (cost of batt v's savings) you need a small battery. Great fun.

In Spain and Australia, they are seeing that PV can reduce demand by 40% and PV + battery reduces it by 90%, but they have a far smaller seasonal variation, perhaps 1.5 or 2:1 (summer v's winter), whereas we have about a 4:1 ratio.

Edit - I should add that my battery plans also include installing 4 more panels, steep pitched (winter optimised), south facing. Hopefully will add around 1,250kWh pa.

Yea, I did say that it's a little more complicated than that.
The matching process will be difficult. I have no idea how much I export, I know how much I import, and I know how much I produce. How do you measure your "consumption"?


Some charging systems are a lot smarter than others. Some are able to take your expected consumption, expected weather conditions, and calculate just how much to charge the battery on the low-rate (econ-7) to make sure you get through the next day with minimal wastage. So I cant say it's simple, but at the same time you cant sell this idea as something that only a tenured math professor would be able to work out.


The statement that someone can consume 80-90% of their production is really something that cant be denied. To say that someone could efficiently calculate the exact battery you need to match Solar + Wind + Econ7 for Winter and Summer along with EV Charging would just be wrong, and I am not trying to say or imply that.


So if I was you, (and depending on how accurate your "consumption" value is) I would personally opt for something near that 15kW capacity. Your "consumption" value would also need to be separated into Econ7 times and Peak times to work out the size of the system. The key to maximizing profit would be down to you not charging the system too much during the night during the summer, and not too little during the night in the winter, and I would probably even work off of a 3-4 day usage cycle. (UK weather and all that).


Dont get me wrong, I get your point.
You make too much in 1 day to use over the next few days, so it's wasted. I am a little surprised your yearly import is only 1,500. My meter shows I import 5,600+ a year (gas hot water, one gas fireplace). But we had (and will probably get another soon'ish) a EV, so for me it's dead easy to consume everything, for you it's not. I agree it's complicated. I would probably even say that your PV system was too large for your consumption, but the FIT payments really pushed us (and probably you?) to buy it also as a investment option and not to just fit exactly what you need.


I don't think most people would have their exact export/import values to work out the most cost effective solution. SMETS2 could help, but that data is going to be locked away from us regular users ;(....

Zarch

jcontest wrote: »

I have no idea how much I export, I know how much I import, and I know how much I produce. How do you measure your "consumption"?

https://openenergymonitor.org/

Something like an emonPi with a clamp for your usage and a clamp for your generation. The hardware/software works out the rest.

Love mine. Not cheap mind, but its data geek heaven.

zeupater

jcontest wrote: »

... I don't think most people would have their exact export/import values to work out the most cost effective solution. SMETS2 could help, but that data is going to be locked away from us regular users ;(....

Hi

The data registers in the SMETS2 meters don't record at a frequency which would be useful to calculate what you need to calculate ... there's a recent article looking at this very issue regarding battery performance modelling using 30 minute data available from smart meters here ...

Home Solar Batteries: Modelling Energy Demand - Hourly & HHM
...

Regarding Martyn's electricity imports ... considering that the average UK home uses approx 3300kWh/year it's not unreasonable to expect that one with higher than average efficiency measures would use less, with one generating 4500kWh of it's own electricity being left with imports around 1500kWh/year .... if you think that that's low, there are a number on these boards that would disagree as we import even less ... we've not imported anywhere near 1500kWh since 2013!

Anyway, 5600kWh/year imported on top of solar self consumption is huge amount of electricity which would suggest that (unless you have a huge array!) you may already consume the majority of what you generate, leaving little to divert to batteries anyway .... can you give us an idea why it's so high & what PV system you have in place??

Z

jcontest

zeupater wrote: »

Anyway, 5600kWh/year imported on top of solar self consumption is huge amount of electricity which would suggest that (unless you have a huge array!) you may already consume the majority of what you generate, leaving little to divert to batteries anyway .... can you give us an idea why it's so high & what PV system you have in place??

Z

Thanks for the links.


I think the usage in my house comes down mainly to 2 desktops and multiple televisions (they are not on all the time, but still). Our Washer and Dryer also run quite a bit (we do laundry for in-law as well). Sometimes I wonder if the lekky meter is recording usage correctly. Floor Heating in the kitchen probably adds a bit on top but I have no easy way to measure that at the moment, it's never turned up that high, but in the winter I am sure it counts quite a bit.



Roof faces exactly south, pitch isn't great, no shading.

Sunways NT 4200 Inverter
16 x SANYO HIT 240SE10 solar modules 3,84kW System


(Should have been a 4kW System, but the installer screwed that up and it was short notice just before the 2011 announced FIT change/closure so I took the risk, and it didn't end up happening)



Inverter has gone to sleep for the night so cant pull data from it.
Export Meter (not actual exported) Yearly Totals (1st April - 31 March)

(2012-13) - 3440
(2013-14) - 3738
(2014-15) - 3663
(2015-16) - 3723
(2016-17) - 3577
(2017-18) - 3428
(2018-19) - 3766


Cant find all of my historical electric consumption, but...

2007 - 5617 kWh
2011 - 5300 kWh

2017 - 5250 kWh (Had a Nissan Leaf part of this year)


Meter was replaced/certified in 2008 and has a 30 year lifespan. I doubt that it's faulty, we just use too much juice ...