Category Archives: Electricity storage

Here’s a little Tonik..


Today my energy supplier Tonik wrote to me inviting me to consider solar panels, a car charger, or a storage battery – all of which I already have.  However on their website I found a wider vision of the future home which they thought could halve energy consumption. I thought it would be interesting to compare their vision with my status.

As you can see from the table below the content is quite similar, although I have more ambitious use of solar and more sophisticated smart heating management.

Tonik's VisionMy status
Switch to Tonik for lowest cost renewable electricity.Done.
Smart meterWaiting on Tonik
Connected thermostat (whole of house device)Connected thermostats (individual room temperatures and schedules)
LED bulbsDone.
Smart tariffWithout a smart meter on nearest equivalent (Economy 7)
Solar PV Done.
Battery storage.Done.
-Surplus solar electricity diverted to charge electric car.
-Surplus solar electricity diverted to heat water.

Battery storage – the first 6 months

I’ve now completed 6 calendar months with the battery storage system.  Those six months cover January – June so might be considered representative of the year as a whole.  I have real-time monitoring of solar panel output and house consumption / export so I have clear visibility that the battery is working and storing energy, and then discharging energy through the evening as I see the house with near zero electricity consumption sometimes through into the early hours of the morning (less if we’ve cooked our evening meal on electricity).  So, environmentally it’s doing its stuff, but what is it saving financially?

There are at least three ways to assess that, so let’s see how they look.

Firstly, from the capacity of the battery (4 kWh), and assuming that it’s filled daily one can calculate a benefit.  That benefit would tend to overestimate benefit in winter when there may not be enough energy to fill the battery, but equally could understate saving some days when the battery goes through repeated periods of charging and discharging during the day.  One could imagine the heating cycle of the dishwasher, for example, perhaps causing some discharge of the battery during the day if the washing machine load isn’t met from the solar panels, but then then re-charging before the evening and thus its daily throughput being higher than it’s capacity as some of that capacity is used more than once per day.

So, if the battery stores 4 kWh and there are 365 days in a year where each kWh not bought is worth 11.5 p/kWh then the saving might be 4 kWh/day x 365 days/year x 11.5 p/kWh =  £167.90.

Secondly, let’s look at electricity savings.   If I compare the first 6 months of 2017 (with a battery) to the first 6 months of 2016 (without a battery) then electricity purchase has reduced by 824 kWh (38%).  Thus the saving could be 824 kWh/six-months x 2 six-months/year x 11.5 p/kWh =  £189.52.

Of course, as my chart shows, there are other changes that potentially impact electricity use between those 2 time periods, so that might be an overestimate.

The third and final way that I’ve analysed this is to look at the data about how the output of the solar panels has been used.  In 2016 I used 44% of the output of the panels to replace bought electricity, and a further 28% of output to replace gas consumption.  In the first half of 2017 however I used 63% of the output of the panel to replace bought electricity, but only 24% of output to replace gas consumption.  The reduction in use for water heating reflects the prioritisation of the battery over water heating as a kWh of electricity purchase avoided is much more valuable than a kWh of gas purchase avoided.  Against 2016’s full year generation of 4,192 kWh that gives a saving of only £87.84 which is much the lowest of the 3 figures.  However this only accounts for more efficient use of the solar panel output, and not winter savings from shifting energy purchase from day to night time when it’s cheaper.

Whether any of these figures represents a saving over the life of the storage system entirely depends on the lifetime of the system, the life of the batteries inside it, and the replacement costs of those batteries

Battery profile through the day

January 5th was a good day to demonstrate the battery’s contribution to the home on a day on which 9.75 kWh was generated. The battery at this point in the year is configured to charge on cheap overnight electricity as well as store excess PV electricity (when there is any). The day also included evidence of water heating and car charging after I returned home at lunchtime.

Over all only 2 kWh of power was purchased during the day at full price, while 12 kWh of cheap night time electricity ran the dishwasher and washing machine, did some car charging (I charge for an hour which leaves headroom for later solar charging), and contributed 5 kWh to charging the fixed battery.

During the relatively sunny December day, as PV output rises, the battery starts to recharge and then, once the battery is charging at its maximum rate, remaining PV output is used to heat water (although described as ‘Grid Power Out’ in the chart above).  At around 12:30 I arrived home and put the car on charge (the blue spikes) and made lunch (the brown spikes).  The blue spikes of car charging occur as the chargers turn off intermittently to allow available power to be assessed.  As PV output falls EV charging starts to require limited support from the battery (the rising brown line through the blue spikes) until eventually all EV charging stops and the system returns to a combination of battery charging and water heating.

As the sun sets the brown line rises again as the battery takes on the load of the house. Green spikes indicate boiling the kettle before 18:00, cooking dinner from shortly after 19:00, and a further kettle boil at around 23:00.  Apart from these import events totalling less than 2 kWh, the house continues running on stored energy from the battery through into the next day when the cycle restarts.

At some point I assume that it will be necessary to curtail the overnight charging of the battery so that it doesn’t miss out on day time charging from PV as a result of being full. That may be responsible for some of the blue cycling with the car charging associated with the rising brown line – if the battery is already full then the car charger will be enabled at lower levels of power generation potentially causing the battery to partially support the car charging.

Battery capacity

As you may recall my storage battery has a usable storage capacity of 4 kWh which, with the quoted round trip efficiency of 80%, would mean it takes in 5 kWh of energy to make 4 kWh available for output later.

We’ve been away for a couple of days, leaving the house drawing a low amount of power in our absence, however yesterday the battery reports having discharged 5.9 kWh despite its 4kWh capacity.

I’ve already had some days when throughput was greater than capacity, but that’s generally been when the battery charged fully overnight and then partially recharged from solar PV during the day. However this was not the case yesterday where, although there’s a dip in demand associated with limited daytime generation on a dull winter’s day, there’s no evidence of battery charging.

There is a characteristic of battery behaviour that may describe some of this which is described by Perkert’s law. Peukert’s law describes a relationship where battery capacity reduces as current increases according to a power relationship. Since my battery is discharging at about 350 VA that much less than its 1200 VA maximum capability so it may appear to have more capacity.

However it seems to have both greater input and output energy than its rated usable capacity suggests (input energy remains higher than output energy) so it would seem that there’s something else going on here; whether that’s an early life effect, a genuinely larger capacity than nominal, or some other effect is unknown.

The battery has a nominal capacity of 8.8 kWh to provide is usable capacity of 4 kWh, so it’s possible that some characteristic of the control software is making more of the nominal capacity available than would normally be the case.

 

 

Best use of generated power (cont.)

In order to illustrate how the combination of battery and immerSUN distributes generated electric power at different levels of generation I created this chart.

For different levels of power generation across the bottom, the chart shows how the power is divided between battery charging (and occasionally discharging), electric car charging, and water heating; which are generally prioritised in that order. My prior post explained the rationale for the 500 Watt switching threshold for the vehicle charger – based on 1.4 kW of mid-value car charging being better value than a mix of 800 Watts of high value battery charging and 600 Watts of low value water heating.

Alternatively you might like to consider that the horizontal axis represents passing time after daylight comes and that the chart shows how diversion changes as the sun reaches its zenith.  You might then view the end of the day as a mirror image of this as the output of the panels ramps down in late afternoon, although at the end of the day there’s the greatest possibility that one or more of the storage devices is/are full and thus the greatest chance of electricity being exported.

Of course all of this assumes that the storage devices aren’t already full, and indeed that the electric car is present at all. As storage devices fill, or indeed if the car is absent, the system automatically switches to the next best value alternative:

  • Battery full – more car charging and/or water heating.
  • Car full or absent – more battery charging and/or water heating depending on power output.
  • Hot water at maximum temperature – this is the lowest priority electricity use so when this is full we don’t currently have another use to divert power to. However there is an unused output on the immerSUN so it would be possible to drive another load. The underfloor heating in the kitchen would be a possibility, although there’s unlikely to be much overlap between days when there’s enough surplus to reach this point and days when kitchen heating is required so it may never repay the cost of fitting the cables.

Best use of generated power

Over the last few days I’ve been rethinking the best use of generated power.
The prioritisation of battery charging over water heating is clear due to the significant cost difference between day time electricity and any time gas, but the situation on car charging is more complex. It occurred to me that there could be times when prioritising battery charging and water might not always be the lowest cost solution since car charging avoiding mid-price nighttime electricity might be a bigger saving than a lesser amount of high value battery charging combined with low value gas-replacement.

For example, if we look at the lowest level of EV charging that amounts to about 1.4 kW. With our night-time rate of 7.87 p/kWh, 1.4kWh of solar power used for car charging saves 11.0 p of night time electricity.  If the battery is maxed out at 800 VAh that saves 7.34 p of later day time electricity. The water heating using the balance of 0.6 kW saves a further 1.76 pence of any time gas. Thus the total save from 1.4 kWh used for a combination of battery charging plus water heating is 9.1 p, compared to 11.0 p from car charging – so it would appear to be better value to do 100% car charging when a 1.4 kW surplus exists.

A bit of further analysis aimed to establish the point at which it became better value to charge the car, rather than combine battery charging and water heating, even if that involved a small level of mains import. The answer is that, with my energy costs, it makes sense to enable 1.4 kW of charger when 1.3 kW of export would have existed thereby potentially importing 0.1 kW. In practice this 0.1 kW may be supplied by the battery.

Given that the battery has priority by the way it’s wired, and takes up to 800 VA, then I intend to try a 500 W export threshold to start the car charger since 800 VA + 500W ~ 1.3 kW.

A sunny December day

Today as expected was well above average for December with 8.2 kWh generated so it was a good opportunity to confirm correct prioritisation of battery charging and water heating.

The graph above from the immerSUN clearly shows the green area of electricity being generated from the solar panels, the purple line of the ‘house’ electrical load (including battery charging) rising first (i.e. the priority load) within the green area, and then the blue line representing the water heating rising second. The purple line being at about 1kW is consistent with 200 Watts of house load with the 800 VAp battery charger on top.

Water heating starts to reduce first with battery charging around 14:00. Shortly after 14:00 the house switches to running from an increasing amount of stored energy which lasts until shortly before 20:00.

From the battery’s perspective you can see ‘device power in’ i.e. battery charging rising first, followed by ‘grid power out’ which is actually power available to the immerSUN and should correspond with the immerSUN’s blue line. As the sun goes down power diversion for water heating is reduced first, following by power for battery charging; until finally from about 14:30 the battery switches from charging to depletion ‘device power out’ running the house until shortly before 20:00 by which time it’s supplied around 3kWh.

System data from battery and immerSUN perspective

Today (briefly!) I watched the battery storage and immerSUN operating alongside each other. As you may recall things are arranged such that the battery should charge first (at up to 800 VA) and then once the battery is drawing full power (in the absence of a car) the immerSUN makes hot water.

From the immerSUN’s perspective as PV output rises the load of the house (which is actually the battery charging) also rises. Once the battery is charging at its maximum 800 VA power then any remaking PV is diverted to making hot water by the immerSUN. Later as PV output drops the battery continues to match the zero PV output discharging the battery to make the house load zero according to the immerSUN until the battery is exhausted – 2 hours of house load in this illustration.

From the battery’s perspective between 10 and 12 it can be seeing hitting its maximum 800 VA, while a varying amount of excess power is seen to be exported. In reality this isn’t true export, its power available to the second priority device (the immerSUN) and corresponds to the immerSUN’s blue line for diversion.

Overall self-use was 96% although admittedly on a low base. Thursday should be interesting as that’s sunny all day on the forecast.

Why I did buy a battery (almost)

Well, after consideration going back months, I did finally buy a battery (almost). Conceptually I’ve long thought that a time would come when I’d have a storage battery to store excess solar electricity from when I couldn’t use it all, and then use it at a later time; but until now I’ve been put off by various technical constraints and the cost. However a few weeks ago I saw an appeal for homeowners to participate in a battery trial sponsored by a DNO (the companies who own local substations and the cables down the street). After initially being told that the scheme was full, I was subsequently offered a place – presumably because others have dropped out. In exchange for giving the DNO 18 months of usage data (for which a data logger is installed in the house) I get a discounted storage system to keep.

img_0637The battery that I’m being offered is a PowerVault with 4kWh usable capacity. It’s connected on the AC side of my inverter, and can be charged either by the surplus on my solar PV or, when there’s insufficient PV output, on cheap night-time power for later daytime use. It also has a UPS function that could usefully maintain power to my PC, internet, phone etc during a power cut. On the downside it has a relatively low output power so it can’t fully power larger consumers like the oven, car charger etc; although it can sustain lighting, TV, the fridge and a host of smaller loads that make up modern electricity consumption.

This means I now have multiple means to save surplus free power when it’s available and avoid buying power at the time of need which are, in order of financial attractiveness:

  1. Charge the storage battery avoiding later day time electricity purchase.
  2. Charge my electric car avoiding later night time electricity purchase.
  3. Make hot water avoiding later gas purchase.

The system itself is about the size of a slimline dishwasher and is designed to sit under a worktop. I however now have my battery installed in the study, under my desk, a location chosen because of its proximity to the consumer unit and because any waste heat from the battery will (along with my PC) help to heat an otherwise unheated room. It could alternatively have gone in the adjacent utility room, but I’d have needed to sacrifice a cupboard and the utility room has no need of any more heat as it’s home to the boiler already.

One of my concerns was whether the storage battery control system and immerSUN might interact as both sought to divert the same power to their respective storage devices, but I think I’ve worked out how to ensure the storage battery always has priority while allowing the immerSUN to work simultaneously. One could imagine that, for example, at the height of a summer day the maximum 800VA would be going into the battery while the immerSUN managed nearly 3kW into car and/or hot water.

img_0638When it comes to viewing system activity then there’s opportunity for me to see what’s happening or has happened on any day through the cloud servers for both the storage battery and the immerSUN. I’ve not got to grips with the storage batteries’ data logging yet. However, from the immerSUN’s perspective, battery charging looks like additional household load up to 800 VA (but not to the point of exceeding PV output), while battery discharging looks like reducing the household load by up to 1200 VA (but not beyond zero).

And why did I buy a battery (almost)? Well, ignoring the fact that I have a battery but no invoice so far, I won’t have to pay the full installed price because of the subsidy provided by the trial so I’ve arguably only bought some of the battery.

Why I didn’t buy a battery

This week for the first time a salesman ventured into my home to sell me a battery for my solar PV system. His company were not the first to discuss it on the phone, or make an appointment, but this guy actually turned up which is a first.

The principal of such a battery is straightforward – there will (often) be times when you generate more electricity than you can use immediately, so why not store it for later use?

imageFirstly, let’s think about the architecture of the system that was being presented. This system puts a battery and a charge controller on the DC side of the inverter i.e. between the panels and the inverter. This reduces consumption on the generation meter when you charge the battery, but registers on the meter when the battery is discharged – not in itself a problem just a difference.

This architecture creates a system in which the battery can only charge in daylight and only discharge in darkness but to my way of thinking that’s less efficient than it could be. Firstly during my day there are often load peaks that go beyond what the panels can cover. Events like boiling a kettle, or the heating periods on the dishwasher or washing machine often push electricity demand beyond the immediate panel output, and you you might hope that the battery would cover these peaks and then recharge later – but no this system cannot discharge the battery during daylight so you’d end up buying electricity for these peaks. No discharge in daylight also means that activities like cooking in summer evenings on our electric oven and hob would be on imported electricity (even though the battery has sufficient energy) because it’s still daylight. Finally my Economy 7 meter tells me that I regularly don’t use as much as 4kWh overnight but to get pay back you want to empty and refill the battery as many times as possible.

Of course I could shift electrical load to the nighttime to empty the battery, but that’s contrary to trying to move load to the daytime to maximise use of PV directly. Which is better? Hard for the user to tell since no tools are provided to tell the user how much energy is in the battery, or any history of usage. By contrast my immerSUN produces detailed graphs of usage to show you what it’s achieving on my behalf. If this can be done with one device, why not also with a battery costing 20 times more?

How can I make sure the battery is empty in the morning when my normal night time usage would only be 1-2 kWh? Well you might consider running a heater to warm up a critical room or rooms at the start of the day (once you’ve got the equipment the energy is free of course), or in my case I might want to top up the electric car, is there a switched output or other mechanism to signal when the battery is exhausted and switch off such a device? No there isn’t.

So it can be hard to ensure that the battery is empty. What about filling it, how easy is that? Remember that the proposed battery has 4 kWh working capacity and sits on the DC cables between the panels and the inverter. Well, my 4 kW PV system like many is actually 2 x 2 kW systems feeding dual input inverter. That means that only half my panels would feed the battery so to fill the battery I’d need to generate at least 8 kWh in a day (and not use much of that during the day as immediate usage is prioritised over battery charging). My average daily generation between November and February is well below 8 kWh so there are many days in which the battery wouldn’t be charged fully (and may be not at all).

Then there’s the kicker – the price. If I look at a battery with 5,000 cycles life (that’s over 10 years although the brochure also quotes 10 years as the life) where each cycle is 4 kWh (which it will only be when new) that’s a lifetime throughput of 20,000 kWh. Electricity at today’s money is about 12 p/kWh so those 20,000 kWh are worth roughly £2,400 in savings. And the price of the unit? More than double that.

I did a detailed analysis including my actual ability to charge the battery based on real daily generation and usage, declining battery capacity with age and use (down to 80% of original capacity after 5,000 cycles), and energy price inflation; and concluded that energy price inflation would need to be around 30% annually each year just to get my money back over 10 years – without actually generating a return on the investment. While I do expect energy prices to rise faster than inflation in the long term I think 30% annually is unrealistic.

In the next few years I expect battery prices to fall as more cells are made for other purposes like electric cars, and also as used cells/batteries become available from electric cars at the end of their lives. I also expect revenue opportunities to arise through grid stabilisation and support where some access to the domestic battery is made available to support the local network.

However a battery on the DC side of the invertor which only charges in daylight and only discharges in darkness will be of little use to support the grid. I think a battery needs to be on the AC side of the inventor to flexibly interact with the house and grid based on charging to eliminate export and discharging to eliminate import in microcycles (unless the grid overrides that).

So I do expect to get battery storage eventually – just not now, not at this price, and not this solution.