On-grid domestic battery storage

ASavvyBuyer

chamelion said:

I've visited and revisited battery storage probably every 6 months or so and each time get put off by the math. Doing a seriously quick back of the envelope calculation here's a scenario:
7.2kwh install for £3500 = 6.5kwh usable based on DoD. 

Optimistic outcome: 

• I charge it 100% through my PV array, so cost to charge = 0
• I use it exclusively during my agile octopus peak when it's let's say 22p/kwh. For sake of this optimistic math, I use up the full 6.5kwh in this period during electric peak.
• So my electric saving is £1.43/day
• break even point = 6.7 years

Balanced / more realistic outcome:

• Charge blended through PV and cheap overnight rates of 5p blended through the year (over winter won't generate enough to charge fully). Let's call it 50-50, that's 18p a day to charge after assumed round trip losses
• Cost savings based on my average per kwh price rate from agile. This week was 6.22p/kwh based on daily usage. Savings here for a full cycle then would be 40.4p. 
• So, savings here is 40.4p - 18p = 22.4p per day, or 81 quid a year. 
• break even point = 43 years, and this assumes no maintenance cost / inverter replacements etc.  

Is my math just completely wrong or am i not considering something important here? Sure, i'm assuming i'll always get cheap electricity, but at the same time i'm not considering drops in pricing of this technology, or it being eliminated entirely in favour of EVs charging houses during peak periods (imagine what a 75kwh tesla battery would do versus a 7.2kwh one in your closet!)

I think your conclusions are the same as mine. I would like a battery, but every time I try to work out it is worthwhile I come to the same conclusion.

With the average cost of ALL the electric we import being just under 8p/kWh because of being on the Octopus Go faster tariff of 5.5p/kWh for 5 hours every night, the maths don't work out for having a battery.

In fact, we think it is better to just import electric from a company that invests in renewal sources, as the CO2 and costs benefits work out better.

Exiled_Tyke

chamelion said:

I've visited and revisited battery storage probably every 6 months or so and each time get put off by the math. Doing a seriously quick back of the envelope calculation here's a scenario:
7.2kwh install for £3500 = 6.5kwh usable based on DoD. 

Optimistic outcome: 

• I charge it 100% through my PV array, so cost to charge = 0
• I use it exclusively during my agile octopus peak when it's let's say 22p/kwh. For sake of this optimistic math, I use up the full 6.5kwh in this period during electric peak.
• So my electric saving is £1.43/day
• break even point = 6.7 years

Balanced / more realistic outcome:

• Charge blended through PV and cheap overnight rates of 5p blended through the year (over winter won't generate enough to charge fully). Let's call it 50-50, that's 18p a day to charge after assumed round trip losses
• Cost savings based on my average per kwh price rate from agile. This week was 6.22p/kwh based on daily usage. Savings here for a full cycle then would be 40.4p. 
• So, savings here is 40.4p - 18p = 22.4p per day, or 81 quid a year. 
• break even point = 43 years, and this assumes no maintenance cost / inverter replacements etc.  

Is my math just completely wrong or am i not considering something important here? Sure, i'm assuming i'll always get cheap electricity, but at the same time i'm not considering drops in pricing of this technology, or it being eliminated entirely in favour of EVs charging houses during peak periods (imagine what a 75kwh tesla battery would do versus a 7.2kwh one in your closet!)

I would suggest that even the optimistic outcome is overly-optimistic. If you were going with this approach (and not charging from off-peak grid power) then you'd change to a cheaper tariff than 22 per unit.   

That said I bought a 6.3kWh battery system for £3000. My calculation (not a payback) but spreading the cost of the system over the estimated number of units it should store over it's life is that the unit price is cost effective, although it will take many years to pay back so it may not last that long! If the price of electricity does increase (a possibility perhaps if we reduce dependency on gas for heating and move to EV's) then it may turn out to be a decent investment. For now I live with: 1, it may pay, 2, it's good for the environment 3, The great pleasure of not exporting most of my PV but getting to use it myself. 

1961Nick

The break even point for my 12 kWh system (£4500) is around 12 - 15 years. In the first full year I saved £297 but I didn't switch to Octopus Go until mid November so there's another £25 - £30 to be had. 

Round trip losses are high when you're fully charging/discharging the batteries every day. I've fitted a meter to the battery controller & I'm losing 20.7% grid charging - For anyone using the Solarman app, the actual charge is 4.6% higher & the discharge 6.6% lower.

There's no need to part charge batteries in Nov/Dec/Jan as the batteries will have been powering your house for 5 hours before there's any meaningful solar output. By then the SOC will be low enough to accept whatever surplus pv energy there is.

4 batteries (9.6kWh) might be the sweet spot for ROC but it depends on your circumstances.

There are also hidden savings - you become energy efficient when there's surplus solar rather than looking for something to 'waste' it on. You can also run energy hungry appliances - WM/TD/DW/ASHP - during the off peak period. I did suggest she did the ironing between 00:30 & 04:30 ... but that didn't go down too well!

I've only got 2 full months of grid charging for comparison but it should give you some idea of the saving ...

Dec 2018: £99.88
Dec 2019: £50.02 

Jan 2019: £86.51
Jan 2020: £42.16

(I've adjusted the figures to remove the 3kWh/day of (off peak) immersion heater use during the previous 2 months.

mickyduck55

Good morning just looking at my data, anyone know why I would import very small amounts of electricity from the grid when I still have battery power  available.  Very obvious peaks when Kettle fired up!

EVandPV

mickyduck55 said:

Good morning just looking at my data, anyone know why I would import very small amounts of electricity from the grid when I still have battery power  available.  Very obvious peaks when Kettle fired up!

Hi Micky,
The system isn't 100% perfect and I occasionaly see small amounts of import/export too with our Lux and it's normal.
Having said that, that's a fairly sizeable import you've had there just before 4am.
Keep an eye on it and if you continue to see more than just the odd few watts of import, I would contact your supplier and ask them to check the settings.
Our supplier pushed a firmware update through for us which improved the sampling and reduced the amount of import to pretty much zero.
You can check your firmware version under the configuration tab and inverters.
Our is now BAAA-0E0E after the update.

mickyduck55

EVandPV said:

mickyduck55 said:

Good morning just looking at my data, anyone know why I would import very small amounts of electricity from the grid when I still have battery power  available.  Very obvious peaks when Kettle fired up!

Hi Micky,
The system isn't 100% perfect and I occasionaly see small amounts of import/export too with our Lux and it's normal.
Having said that, that's a fairly sizeable import you've had there just before 4am.
Keep an eye on it and if you continue to see more than just the odd few watts of import, I would contact your supplier and ask them to check the settings.
Our supplier pushed a firmware update through for us which improved the sampling and reduced the amount of import to pretty much zero.
You can check your firmware version under the configuration tab and inverters.
Our is now BAAA-0E0E after the update.

Im a couple of versions behind you BAAA-0B0A... is it possible to update firmware without contacting installer?

mickyduck55

Same thing at 4 am yesterday and a very large spike early afternoon !

EVandPV

mickyduck55 said:

Same thing at 4 am yesterday and a very large spike early afternoon !

Your grid in your graph is certainly a lot more erratic than ours so I would definitely ask for a firmware update. Unfortunately, it has to be pushed by Lux but they did ours within a couple of hours of me contacting our supplier.
Our graph from the 13th shows a couple of large spikes when the kettle or electric oven was on (we only have 4.8kwh so 2.4kw output) but apart from that the grid is fairly smooth .....

Doobedoo

This is pretty much what I’ve been experiencing my Powervault will constantly draw power from the grid all day. It can be anywhere from 0 watts up 350 watts and also spike when I turn the kettle on etc. It doesn’t show on the portal by my energy monitor reports it.





Trying to get Powervault to acknowledge it is a exercise I futility. It has taken them 4 months for me to have full capacity of 8kw rather than the 6-6.7 that I was getting.

All in all this possibly could go down as the single worst purchase I have ever made.

Quick question to you guys with different systems. Do you have a battery capacity % indicator?

EVandPV

Doobedoo said:
Quickquestion to you guys with different systems. Do you have a battery capacity % indicator?

The Pylontech batteries have a row of green LED's on the front which indicate remaining capacity and the Lux ac controller web portal displays remaining capacity in % as well as telling you how much has been charged/discharged each day, battery state of health and a ton of other data/graphs. 
Here's a grab from the one of the portal screens, the data can also be exported to excel.