On-grid domestic battery storage

jcontest wrote: »

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"?

Hiya, regarding consumption, that was an easy guess, going back, based on pre-PV consumption of around 7.5kWh summer months, 8.5 Spring/Autumn, and 9.5 Winter. So simply compare your new import to old, and you have your consumption and export.

But for the last two years I've been able to monitor export accurately* thanks to a new SolarEdge HD Wave inverter. With import, export and generation, you can quickly calculate consumption.

*The SolarEdge system gives an exact match on import to my import meter, so I assume, the export figure will be similarly accurate.

jcontest wrote: »

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.

Hiya. A small, or large battery will reduce my 1,500kWh import down to 600kWh. The figure comes out the same since, as explained, I can only store about 2.5kWh's in the summer due to low import, and about the same in the winter due to low generation. Smart charging makes not a jot of difference to this as they are immovable, brick wall, obstructions.

I don't consider E7 in any of my battery calculations yet, as the leccy price + battery usage cost (cost/life cycle kWh's) makes it more expensive than import.

jcontest wrote: »

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.

I agree that the statement that "someone can consume 80-90% of their production is really something that cant be denied." can't be denied, but that is more a specific 'someone' not an average 'someone'.

jcontest wrote: »

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).

If I was me, I would go for about 4-5kWh* not 15kWh, as the carefully calculated impact on my import and savings comes out pretty much the same, for a much smaller battery cost.

Again, at this point in time E7 is not a factor as that results in leccy unit costs higher than before after including battery usage costs.

Not sure why the summer nighttime charging needs to be minimised when as explained, no import would be needed (small, medium or large battery).

*To make better use of ASHP, and future proof for an EV, I would actually consider a much larger battery in the future as costs come down. But for now 2/3rds of the battery (and cost) would be of no benefit.

jcontest wrote: »

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.

1,500kWh doesn't seem low to me at all. My consumption was approx 3,000kWh before PV, a slightly lower than average (then ~3,600kWh) figure. It then went down a bit, but with the addition of a small ASHP has now gone back up to 3,000kWh, not far off the current UK average.

I'd think, assume, that many or most PV'ers with decent sized systems have reduced their import to 1,500kWh or better.

Not sure why you think my PV system is too large. The fixed costs of PV are very high, so limiting the amount of panels is a false economy. What I generate, be it offset or export reduces national FF generation by approx 4,900kWh pa (4,500kWh generation plus reduced distribution losses of 8%), or approx 9,800kWh of gas burnt (plus extracted and transported).

I was an early adopter, but have over the last 7yrs or so helped calculate costs and figures for 100's of people on here, and at no time did going small ever make sense financially, either for the individual nor the environment.

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 ;(....

They will have exact import values (well within the 1% tolerance allowed for import meters), and again, whilst exact export and therefore consumption figures may not be possible, a reasonable calculated guesstimate is simple. Also, if you refer back to threads over the last decade, you will see myself, Z and others suggesting folk asking about PV, make notes of their import (if they don't already) for later comparison.


I have no real problem with what you are saying, it seems very helpful and informative, but please don't judge me, and don't confuse your assumptions with facts. At this point in time, with battery costs high, and warranties relatively short (v's payback periods), the economics are weak, and therefore whilst you don't need to be a tenured maths professor, I'm certainly not, you do need to take a somewhat deeper dive than you have as there is far more to consider - considerations that have been well detailed on this thread over the years.