Category Archives: Electricity storage

Clamp orientation for ImmerSUN and PowerVault

In the last few days I’ve been assisting a reader of this blog who also has an ImmerSUN immersion heater controller plus PowerVault storage battery combination.  Like me, he had the immerSUN first and later added a PowerVault, but had immediately disabled the ImmerSUN to get the PowerVault to work.  Left to their own devices, the ImmerSUN will normally take the surplus power first before the PowerVault has chance to respond since it has a more dynamic control system, however economically it makes more sense to prioritise the PowerVault.

I previously posted on this topic in Prioritising smart loads for self-consumption but wanted to provide more clarity on the orientation of the clamps.

Both ImmerSUN and PowerVault rely on current clamps to get their control signal. Such clamps fit around an electrical cable and measure the flow of electricity through that cable.  Normally clamps for both these devices alone would be around the live in the incoming mains cable, but do that with both and the ImmerSUN will always take the available power first which is undesireable.

The illustration shows my solution, as per the prior post, where the PowerVault clamp surrounds both the incoming live and the live output to the immersion heater.  If the clamps are correctly orientated, this allows the PowerVault storage battery to be prioritised over the ImmerSUN immersion heater controller.  When the PowerVault clamp surrounds the two cables, it is important that outgoing power to the grid and outgoing power to the immersion heater from the consumer unit pass through the clamp in the same direction.  This solution should work for clamp-driven solutions too.

The PowerVault clamp is directional – it has an arrow which should point towards the consumer unit.  That means, if you pair the two cables as I described, so outgoing power flows in the same direction in both cables through the clamp, then the arrow should point in the opposite direction i.e. towards the consumer unit.

Fundamentally it doesn’t matter which way the ImmerSUN clamp faces, as during commissioning the ImmerSUN will work it out by cycling the power several times, but you shouldn’t change it after commissioning.

Daytime charging as quickly as possible

On Wednesday an unusual pattern of vehicle use occurred (at least for me) which showed another way the charger could be used. I needed to take my daughter on a round trip in the morning (dad’s taxi), return home, and then repeat the trip later in the afternoon. Each round trip pretty much exhausted the range of my electric car, so I needed to recharge in between. My normal home charge routine would have prioritised the home storage battery, but that would have left me completing the second round trip partially on petrol which isn’t the optimum solution.

To fully recharge the vehicle to complete the second round trip (and thus avoid using petrol) I disabled the programmable logic controller (PLC) on the car charger using a push button which suspends the program causing the charger to operate at full power. It was a sunny day (at least for March) so the solar panels were producing a similar amount of power to that required to charge the car and, with the fixed battery partially charged during the period I was making the first round trip, the fixed battery was able to manage the difference between power generated by the panels and that required by the home (including car charging) for much of the period.

The result was an almost fully charged car for the second round trip (sufficient to avoid using petrol) for only around 1kWh of grid energy.

Solar PV Installation – 2 years on

The end of 2017 sees 2 full calendar years of output completed (plus a few months at the end of the prior year) so it seemed like a good time to assess performance and return.  Return comprises two parts – firstly the payments received for electricity generation (and export) the so-called feed-in tariff or FiT and secondly the savings obtained from using that free energy myself instead of buying it from the grid.  In my case I can use and store my solar electricity directly, or use it to heat water via the immersion heater thus replacing gas.

I’ve summarised the status in the following table:

 Nominal20162017 
Generated366840564033kWh
FIT£629.36£639.61
Electricity self-use50%41%64%of Generated
183416632596kWh
£159.65£251.12
Gas replacement27%23%of Generated
1095941kWh
£41.20£27.66
Export50%32%12%of Generated
Return in calendar year£830.46£918.40
13.6%15.1%of PV cost
Return cumulative£830.46£1748.86
13.6%28.7%of PV cost

Summary:

  • Output from the panels was slightly reduced in 2017 versus 2016, but still significantly above the performance projected in the quotation.  I put the slight reduction down to year-to-year differences in the weather.  Over time I would expect panel output to decline, but I think it’s too soon to attribute any decline to this.
  • Income from FIT was slightly higher as inflation on the price / kWh has overcome the slight output reduction.
  • Electricity self-use is up considerably from 41% to 64% due to my 4 kWh storage battery.  The costs of that battery are not reflected in the table.  Return would have been 8.8% in 2017 (rather than 15.1%) if the cost of the battery was included.
  • Gas replacement is down from 27 to 23% versus 2016 as more of the electricity from the panels is used for high value activities like charging the storage battery or my electric car, and less is left over for water heating.  The low price of gas per kWh makes gas replacement my lowest priority for self-use.
  • Exported electricity (i.e. what’s left-over that I cannot use myself) is considerably reduced from 32 to 12% largely as a result of increased electricity self-use.
  • Financial return in calendar year 2017 is improved from 13.6 to 15.1% neglecting the costs of the battery, or reduced from 15.1 to 8.8% taking into account capital costs of the battery.
  • It will take approximately 7 years (i.e. 2 past + 5 future years) for the combination of the solar panels and battery storage to pay for the solar panels (neglecting the battery costs), and a further 2 years of system savings to pay for the storage battery.

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.