In early 2020 I purchased 4.8kWh battery system from Big Green Beard which became the basis for my original Battery Storage blog article.
There are three home battery related blog posts on the website.
https://energy-stats.uk/lux-batteries-and-octopus-agile-integration/
https://energy-stats.uk/battery-storage/
https://energy-stats.uk/greenlinx-battery-upgrade/
The battery system comprised of the Lux Inverter (the bit that connects to your mains electricity and also talks to the battery modules, seen on the left in the photo below) and the two 2.4kWh battery modules (bottom right), which are manufactured by Pylontech.
When compared with similar sized systems from Moixa and PowerVault I personally found that the Lux / Pylontech combo was a cheaper option. But it wasn’t just down to cost, I went with the Lux because it was a modular system that I could easily add single 2.4kWh batteries down the line if I wanted to increase capacity. Many other systems have set capacities that can’t be expanded after purchase.
Its hard to compare my system directly to the Tesla Powerwall battery due to the excessive cost of the Tesla. Not everyone needs such a big system, or has £7500 plus install costs to spend on a Tesla 13.5kWh battery system. I can assure you, the Lux / Pylontech combo was a bargain compared to that.
Being a tight Yorkshireman, value is almost always top of my priority list. And initial outlay can form a big part of the repayment period of something like this. ie, how long will it take to recoup that outlay, this is also referred to as Return on Investment (ROI).
Note: Pricing for battery system installs can be quite individual based on a variety of requirements; capacity, charge/discharge rates, physical size of the equipment, physical space available in your home, location of installation, distance from consumer unit etc. This is why you rarely see standard pricing listed as there are lots of variables. So ensure you explore all available options and get various quotes to find the manufacturer with the system that best suits your needs and circumstances.
I have developed a very good relationship with Jake at Big Green Beard since the installation of my system. He has quizzed me tons about Agile pricing and I have taken a lot of info and knowledge from him about battery storage.
If you mention energystatsuk, Jake will give you a discounted quote for a new Lux Power Pod battery system. If this introduction results in the installation of a Power Pod system I will get a small referral reward. That is it.
I paid for my battery system with my own hard earned and given the choice again, I would pay for the very same Lux / Pylontech combo again.
In the article below I am going to lay the historical Agile and Go pricing facts on the table which could be used against ANY battery system from ANY manufacturer.
I’m really looking to help nail down the principles we are after with battery storage.
If you then decide to go ahead and get a battery system, great. Does it matter to me which system it is? No, as long its the right system for your circumstances.
After reading everything you may decide not to get a battery system after all, that’s great too, you’ll have made your own mind up using all the data available.
I just wanted to be 100% clear to everyone about this referral/affiliate/recommendation situation.
Over the past few months I have been helping Lux trial a new feature of their interver; embedded Octopus Agile Integration.
Moving forward this feature will only be available on new installations as part of the Lux “Power Pod” suite of packages available from Big Green Beard. It is not something that will be retrofitted to existing Lux installations.
The way Lux have approached the integration is very simple. You tell the battery system how many 30 minute slots you want the battery to select and it goes away, picks the cheapest Agile slots each day and automatically fills the batteries for you at those times. These choices stay in place on the system day after day until you either amend them or delete.
Lux have split the day in to two parts; overnight (00:00 to 06:00) and daytime (12:00 to 16:00). You can independently choose the number of slots you’d like to charge during these two time frames as you can see in this screen grab from the Lux web control panel.
So what they are thinking here is that overnight you might want to completely fill the battery, especially as its usually overnight when the cheapest Agile prices are available.
With the afternoon slot, they are thinking that you might want to give the batteries a quick cheaper top-up before the 4pm-7pm higher priced peak.
This chart shows the average prices of all the daily time slots in the Yorkshire region over the past 365 days.
At the time of writing (Sept 2020)
You can see from the average cost graph the rationale behind choosing cheap slots during those two periods. Overnight (00:00 to 06:00) and daytime (12:00 to 16:00).
You can also grab this graph (and more Agile stats) from the dedicated Yorkshire Agile page at https://energy-stats.uk/octopus-agile-yorkshire/
And all 14 UK regions are linked from here: https://energy-stats.uk/octopus-agile/
In the future I think the functionality of battery systems and software that controls them will evolve greatly. They will add in usage pattern detection algorithms, base decisions based on weather forecasts, take into account any onsite Solar PV and EV etc etc.
But for now, Lux have stripped things back to basics; how many cheap slots do you need to fill each day. With this aimed primarily at an audience that want little on-going interaction with the system and just fire and forget.
If you wish to take more control of the Lux, then I’d suggest reading my original Battery Storage article where I make reference the open source Octolux set of scripts. These allow for more fine grain control based on Agile pricing but do require a little more technical expertise.
If you’ve read my previous blog entries you’ll know that I have a Solar PV system and I love it. Adding a battery storage system was the next logical step for me to store and then make use of the excess generation that would otherwise get exported back to the grid. The batteries have made a massive difference to the amount of generated energy that I now use.
In the 6 months since having my batteries installed I’ve increased the amount of generated energy that I use in my house from 25% up to around 75%. So very little is now being exported back to the grid. During the summer though, there are still days where the Solar generates more than I can use, even after filling the batteries. Maybe I do need that extra battery!!
Solar and Batteries are a great combo, but you don’t actually need Solar to have a battery storage system installed.
Since the generation element of the original Feed in tariff (FIT) was removed in March 2019 and the export element replaced by the Smart Export Guarantee (SEG), the payback times (in years) of a solar system has increased dramatically.
Note: You can read about the Smart Export Guarantee on the Octopus Outgoing pages.
Much of the outlay for a Solar system is the installation; the scaffolding, the days it takes to install etc, the manpower.
With a battery system you don’t have those expensive elements. Hooking up a battery system to your fusebox/consumer unit is a very simple and quick procedure for a qualified electrician. The only physical space you’ll need for a battery system as you can see from the photo above, is room for a couple of boxes the size of briefcase. No worries about which way your house is facing and how many panels you can fit on the roof.
When you buy Solar, your system size is fixed, which is based on the number of panels, the panel rating and inverter size. But with a modular battery system like Lux / Pylontech you can easily add battery capacity and inverters to grow your system.
Another downside of Solar is that unless you have an EV, heating diversion or indeed batteries, you’ll end up exporting lots of energy back to the grid (as I initially did). This might sound obvious, but you are dictated to by the sun and when you are generating, lots of the time you are generating way more than you can actually use. One day you might generate 20kWh, the very next day there is little sun and you’re only generating 6kWh.
With batteries, if you choose the right energy supplier (like Octopus Energy), you can be using 100% renewable electricity without the need for Solar panels and you won’t be wasting any of that energy.
The final bonus of ‘batteries only’ setup compared to Solar is that you can take the inverter and batteries with you if you were ever to move house.
Put simply, buy electricity as cheaply as possible, store it, then use it that stored electricity at times when it would be more expensive to buy it. So in Agile terms, this means covering that early morning 6am to 9am sort of time and also the very expensive 4pm to 7pm peak when prices can be as high as 35p per unit.
Before we get into the actual nitty gritty of electricity pricing we have to talk about the size of your battery system. This is very important because these are the facts and figures we need to generate accurate historical electricity pricing.
The Pylontech batteries come in 2.4kWh modules. So you can create a battery system with total storage of 2.4kWh, 4.8kWh, 7.2kWh, 9.6kWh etc up to 19.2kWh (8 batteries) on a single inverter.
How many Pylontech batteries and the number of inveters you have dictates how fast you can charge them.
| No of Batteries | Max Charge / Discharge Rate per hour |
| 1 | 1.2kW |
| 2 | 2.4kW |
| 3+ | 3.6kW |
Note: If you link 2 x Lux inverters together you can get upto 7.2kW per hour charge and discharge in parallel. You can also have up to 8 batteries per inverter, so anyone fancy a 38.4kWh battery system with 7.2kW per hour charge and discharge? 2 inverters X 8 batteries on each = 16 x 2.4kWh batteries.
Because I’ve currently got 2 batteries and a single inverter, I’m limited to 2.4kWh both into the battery and coming back out from the battery.
In this image below, you can see I set up the new Lux integration to choose the cheapest two slots overnight. You can see it chose 2:30am and 3:30am.
The graph shows I’m limited to 2.4kWh charging (but its peaking nearer 2.8kWh because overnight baseload of the house is included there as well).
If I were to upgrade to a third battery I would be able to charge and discharge at 3.6kWh.
When it comes to charging, its obvious that being able to pull 7.2kW per hour off the grid is better than 3.6kWh. You don’t need as many 30 minute slots to pull the same amount of energy. So it puts you in a great position if there is only a small number of cheaper slots each day.
It’s discharge though where having a 7.2kWh rate comes into its own. But this depends on your individual usage patterns. Think that a kettle could be around 3kWh, your fridge might be 200W, TVs, lights etc might be on. All of a sudden you’re very close the maximum discharge rate of a single inverter set-up (3.6kWh) and if you go over then you’ll have to import anything over 3.6kWh and pay for that energy off the grid. If this is between 4pm and 7pm on Agile, ouch!!
So it might be worth considering a second inverter if you are a household with high concurrent demands; washing, drying, cooking etc. With that second inverter you’re then able to discharge up to 7.2kWh straight off the battery.
Alternatively, sticking with a single inverter and 3.6kWh charge/discharge you change your approach by running appliances individually and spreading the load throughout the day.
So lets say you have gone and bought a Lux Power Pod 7.2kWh battery system with a single inverter. This means you have a charge rate of 3.6kW per hour. So simple maths shows that to fill 7.2kWh at a rate of 3.6kW per hour would be 2 hours. So in Agile speak, that 4 slots x 30 minute pricing slots.
OctoChargeCalc is a web tool that I created to interrogate historical Agile pricing and show you how much it would have cost to charge devices daily.
Be that device a battery storage system or even an Electric Vehicle (EV).
Access the tool via https://octochargecalc.energy-stats.uk/
So we put all the relevant information into OctoChargeCalc about our system and hit ‘submit query’.
Note: The Lux inverter allows 90% of the total battery size to be used for charge / discharge, hence the % usable field. Different battery manufacturers have different usable capacities, some lower than 90%, so be sure to check that.
Once we have hit submit we are quickly passed to the results page.
| Agile average per unit cost | 4.08p |
| Agile average per day cost | £0.27 |
| Agile total cost | £96.78 |
| Expensive alternative (14.4p) total cost | £340.58 |
| Percentage saving between total costs | 71.6% |
So this table quickly shows you how much filling those batteries up every day on Agile would have cost you using the cheapest available slots each day over the past year. That number of slots based on the information you provided.
It also shows you the price saving against an alternative (usually more expensive) tariff of your choosing. I’ve chosen a unit price of 14.4p as that is around the middle ground of regional Go tariff pricing and also around Octopus 12 month fixed price tariff pricing.
From these figures you can then start to work out the return on investment for your battery purchase. You might even want to put a slightly higher figure in the ‘alternative’ slot if you’re comparing against Agile Peak period (4pm to 7pm). Its up to you.
You could also put in 17p or something similar in there to see what a current British Gas tariff would cost.
The tools are here to help you decide if batteries are right for you. Just have a play with the numbers.
To save some time, I’ve created a summary table for different battery capacities on a both single and double inverters showing various pricing and costs using Aug 2019 to Aug 2020 Agile data and 14.4p as the unit price for comparison.
| Batteries | Capacity | Inverters | Charge Rate | Slots Req | Average Unit Cost | Av.cost per day | Total Cost Year | Alt.Cost at 14.4p | Saving | % Saving |
| 1 | 2.4 | 1 | 1.2 | 4 | 4.09p | £0.09 | £32.26 | £113.52 | £81.26 | 71.6 |
| 2 | 4.8 | 1 | 2.4 | 4 | 4.09p | £0.18 | £64.52 | £227.07 | £162.55 | 71.6 |
| 3 | 7.2 | 1 | 3.6 | 4 | 4.09p | £0.27 | £96.78 | £340.58 | £243.80 | 71.6 |
| 4 | 9.6 | 1 | 3.6 | 5 | 4.18p | £0.36 | £131.68 | £454.13 | £322.45 | 71.0 |
| 5 | 12.0 | 1 | 3.6 | 6 | 4.25p | £0.46 | £167.64 | £567.65 | £400.01 | 70.5 |
| 6 | 14.4 | 1 | 3.6 | 8 | 4.39p | £0.57 | £207.71 | £681.16 | £473.45 | 69.5 |
| 7 | 16.8 | 1 | 3.6 | 9 | 4.47p | £0.68 | £246.82 | £794.71 | £547.89 | 68.9 |
| 8 | 19.2 | 1 | 3.6 | 10 | 4.54p | £0.79 | £286.59 | £908.23 | £621.64 | 68.4 |
| Batteries | Capacity | Inverters | Charge Rate | Slots Req | Average Unit Cost | Av.cost per day | Total Cost Year | Alt.Cost at 14.4p | Saving | % Saving |
| 4 | 9.6 | 2 | 7.2 | 3 | 4.00p | £0.35 | £126.04 | £454.13 | £328.09 | 72.2 |
| 5 | 12.0 | 2 | 7.2 | 3 | 4.00p | £0.43 | £157.55 | £567.65 | £410.10 | 72.2 |
| 6 | 14.4 | 2 | 7.2 | 4 | 4.09p | £0.53 | £193.55 | £681.16 | £487.61 | 71.6 |
| 7 | 16.8 | 2 | 7.2 | 5 | 4.18p | £0.63 | £230.44 | £794.71 | £564.27 | 71.0 |
| 8 | 19.2 | 2 | 7.2 | 5 | 4.18p | £0.72 | £263.37 | £908.23 | £644.86 | 71.0 |
| 9 | 21.6 | 2 | 7.2 | 6 | 4.25p | £0.83 | £301.75 | £1,021.78 | £720.03 | 70.5 |
| 10 | 24.0 | 2 | 7.2 | 6 | 4.25p | £0.92 | £335.28 | £1,135.30 | £800.02 | 70.5 |
| 11 | 26.4 | 2 | 7.2 | 7 | 4.33p | £1.03 | £375.24 | £1,248.81 | £873.57 | 70.0 |
| 12 | 28.8 | 2 | 7.2 | 8 | 4.40p | £1.14 | £416.23 | £1,362.36 | £946.13 | 69.4 |
| 13 | 31.2 | 2 | 7.2 | 8 | 4.40p | £1.24 | £450.91 | £1,475.88 | £1,024.97 | 69.4 |
| 14 | 33.6 | 2 | 7.2 | 9 | 4.47p | £1.35 | £493.65 | £1,589.43 | £1,095.78 | 68.9 |
| 15 | 36.0 | 2 | 7.2 | 9 | 4.47p | £1.45 | £528.91 | £1,702.94 | £1,174.03 | 68.9 |
| 16 | 38.4 | 2 | 7.2 | 10 | 4.54p | £1.57 | £573.17 | £1,816.46 | £1,243.29 | 68.4 |
There is another way to approach this if you don’t want to be on the flexible Agile tariff. You could also go about this being on Octopus Go tariff.
In the Lux web admin pages there is a very simple scheduling tool for fixed time charging. Just put in the start and end time of when you’d like to charge.
So you could set to charge at the 00:30 to 04:30 cheaper Go slots (5p per unit)
And the use the batteries to cover the higher priced 20 hours (around 13p to 14p depending on geographic location).
https://energy-stats.uk/octopus-go-tariff/
As you’ve only got 4 hours to play with on Go, you would only need a system capable of filling a maximum of 8 x 30 mins charging slots. So the tables below have been restricted to show that.
| Batteries | Capacity | Inverters | Charge Rate | Slots Req | Average Unit Cost | Av.cost per day | Total Cost Year | Alt.Cost at 14.4p | Saving | % Saving |
| 1 | 2.4 | 1 | 1.2 | 4 | 5p | £0.11 | £39.42 | 113.52 | 74.10 | 65 |
| 2 | 4.8 | 1 | 2.4 | 4 | 5p | £0.22 | £78.84 | 227.07 | 148.23 | 65 |
| 3 | 7.2 | 1 | 3.6 | 4 | 5p | £0.32 | £118.26 | 340.58 | 222.32 | 65 |
| 4 | 9.6 | 1 | 3.6 | 5 | 5p | £0.43 | £157.68 | 454.13 | 296.45 | 65 |
| 5 | 12.0 | 1 | 3.6 | 6 | 5p | £0.54 | £197.10 | 567.65 | 370.55 | 65 |
| 6 | 14.4 | 1 | 3.6 | 8 | 5p | £0.65 | £236.52 | 681.16 | 444.64 | 65 |
| Batteries | Capacity | Inverters | Charge Rate | Slots Req | Average Unit Cost | Av.cost per day | Total Cost Year | Alt.Cost at 14.4p | Saving | % Saving |
| 4 | 9.6 | 2 | 7.2 | 3 | 5p | £0.43 | £157.68 | £454.13 | £296.45 | 65 |
| 5 | 12.0 | 2 | 7.2 | 3 | 5p | £0.54 | £197.10 | £567.65 | £370.55 | 65 |
| 6 | 14.4 | 2 | 7.2 | 4 | 5p | £0.65 | £236.52 | £681.16 | £444.64 | 65 |
| 7 | 16.8 | 2 | 7.2 | 5 | 5p | £0.76 | £275.94 | £794.68 | £518.74 | 65 |
| 8 | 19.2 | 2 | 7.2 | 5 | 5p | £0.86 | £315.36 | £908.19 | £592.83 | 65 |
| 9 | 21.6 | 2 | 7.2 | 6 | 5p | £0.97 | £354.78 | £1,021.71 | £666.93 | 65 |
| 10 | 24.0 | 2 | 7.2 | 6 | 5p | £1.08 | £394.20 | £1,135.22 | £741.02 | 65 |
| 11 | 26.4 | 2 | 7.2 | 7 | 5p | £1.19 | £433.62 | £1,248.74 | £815.12 | 65 |
| 12 | 28.8 | 2 | 7.2 | 8 | 5p | £1.30 | £473.04 | £1,362.25 | £889.21 | 65 |
| 13 | 31.2 | 2 | 7.2 | 8 | 5p | £1.40 | £512.46 | £1,475.77 | £963.31 | 65 |
Note: all prices shown are unit price only, but do include VAT. No standing charges are included or shown.
What I’m doing with the calculations above is absolute best case each day, especially with Agile. Are you likely to always use the cheapest 6 slots each and every day? Probably not, especially if you top up during the afternoon. The 1pm to 4pm slots might not be the very cheapest slots each day compared to overnight. Remember, OctoChargeCalc works by pulling out the required number of slots from each day, in price order, cheapest first, regardless of time of day.
Also think about the higher price you are comparing to. If you are currently on a tariff that is 14.4p per unit, then rightly use that. But if you are already on Agile the average unit price you are paying may well be much less than 14.4p. But then again, you might want to only cover the ‘peak’ with battery storage?
So please bear all this in mind when doing your calculations. I don’t want people to be disappointed when they don’t achieve the savings noted above. There are lots of variables in play from household to household that need to be understood and factored in.
The round trip efficiency is the charging and discharging efficiency or losses during use. Unfortunately due to the laws of physics the transfer of energy from one form to another results in some losses.
The Pylontech’s are quoted to lose around 6% in this battery comparison review.
If you charge and put 1.00kW onto the battery, you’ll only get 0.94kW back.
So it is essential that you factor round trip efficiency into calculations.
Well if we all had a crystal ball or Biff Tannen’s Sports Almanac we’d all be millionaires. So whilst its great that we have tons of historical data to look at, we don’t always know what the future of electricity pricing looks like. So please bear that in mind during your investigations.
For reference, here is the Maximum, Average and Minimum pricing for each day of the last 18 months on Agile in Yorkshire.
So just like the old stocks and shares adage: “The past performance of energy pricing is not necessarily a guide to the future”
A battery storage system is not a get rich quick scheme. You’ll be in this for the long haul if your sole ambition is finanacial return.
But some people hold their green ambitions and the future of planet higher than any return on investment.
Whenever you charge up on cheaper Agile or Go electricity from Octopus these cheaper times are invariably periods of lower carbon intensity. So you’ll be doing your bit towards shifting national load away from peak carbon periods by storing energy in your battery.
Remember, Octopus Energy are one of the only energy providers to provide electricity from 100% renewable sources. You can read about Octopus Energy’s green credentials on their What makes us so green? article.
I hope you found this summary useful. Let us know your thoughts via Twitter (@energystatsuk) and continue the discussion.
If you find this information useful and are thinking of joining Octopus Energy, please consider using my referral code or ‘buy me a coffee’ to help support running costs of the website.
Using this referral code will gift you £50 of free credit after signup: https://share.octopus.energy/linen-pearl-869
If you’re considering owning part of a wind farm through Ripple Energy, get £25 of free credit (if investing more than £1000) using my Ripple referral link.
We can also be found on Twitter, please follow us @energystatsuk for daily Agile tariff pricing graphs and summaries.
Also, be sure to check out our Dashboard, Download Historical Data and OctoChargeCalc features to help you decide whether these tariffs are the right fit for you.
Note: The past performance of energy pricing is not necessarily a guide to the future.
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