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.

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Bought Electricity 2016

Yesterday I produced a graph of the number of units of electricity that I’d bought or imported each month since we changed to Economy 7 metering back in October 2015 – a change that I made to reduce the costs of charging my electric car.  I rather surprised myself.

Key features that caught my attention were:

  • If I ignore the months of significant solar generation where significant electricity is available that isn’t bought, then from November 2015 to March 2016 and November 2016 to December 2016 there are 6 months of consecutive month-on-month reduction in bought electricity.
  • Comparing November and December between 2015 and 2016 (the only 2 months available for direct comparison) there’s a reduction of a round a third in bought electricity.
  • Comparing December 2016 with January 2016, bought electricity is down by a quarter.

That seems to be a compelling case for a significant reduction in import having taken place. Such a reduction could be combination of 2 sorts of things, firstly fundamental reductions in electricity use such as through having a more energy-efficient appliance, or secondly shifting use from paid-for electricity to free solar electricity (albeit that the months in question are winter months where less solar is available).

Potential contributors to this effect are as follows:

  • November 2015 to March 2016 – Increasing availability of the garage for car charging. In November 2015 I demolished an internal partition within the garage that had rendered the remaining space too small for a car.  That created an open double garage which allowed a car in for the first time since the partition was created in the 1980s. In March a suspect garage door was replaced making routine access much easier since the old door was badly corroded, required considerable effort to lift, and had an odd locking arrangement.  Thus during this period garage use went from 0 to 100% utilisation for car parking/charging. This might seem an odd item to include but my rationale is that if the electric car is in the garage then it will be warmer than if outdoors and charging will be more efficient as there will be less heating and more charging. Charging also has potential to be at higher current in the garage due to the availability of a Mode 3 charger which I think is also more efficient.
  • December 2015 – gas boiler replacement. Again perhaps not obvious at first sight, but if the new boiler gets the water hotter then less work needs to be done pumping water round radiators to take place to deliver a certain heat output at the radiators.
  • January 2016 – fridge freezer replacement. In January we replaced the former separate fridge and freezer with a single combined fridge-freezer. Although considerably larger in combined volume, the new fridge freezer replaced a freezer that I’d had for over 25 years so I anticipate an energy saving there.  The new fridge-freezer is rated A+ at 496 kWh/annum.
  • April 2016 to September 2016 – charger control project. Development of the charger control project shifts some electric car charging from overnight bought electricity to daytime free electricity even in winter. Previously even on a sunny winter’s day I probably wouldn’t have charged in the daytime due to a risk of import ruining the economics, but now I just leave the car plugged in virtually all the time and let the charger run automatically if and when sufficient free power is available.
  • July 2016 – double oven replacement. In July the failure of the fan on the oven prompted a oven replacement as the old fan was inaccessible for replacement due to corrosion of the surrounding bolts. I’m not sure how old the prior oven was (although I have evidence of the kitchen being remodelled in 2005) , but anticipate an efficiency gain due to replacement.  The new double oven is rated A/B – i.e. the smaller oven is efficiency A and the larger one efficiency B.
  • December 2016 – storage battery. Installed too late in December to have much of an influence here, the ability of the battery to capture electricity that would have been exported as surplus to requirements for later use should reduce bought electricity. The secondary ability of the battery to storage cheap night time electricity for later day-time use is likely to be of interest only in the winter months, and then won’t reduce demand (indeed it will increase demand slightly due to its round-trip efficiency) but it will reduce costs.

It will be interesting to see how this develops through 2017.

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Electricity use 2016

The immerSUN provides a useful app showing electricity use which includes an annual option.  The graph below shows the 2016 annual data:
Although some of the data was only collected from mid-March 2016, the graph still shows useful information. I think that the graph overstates bought / imported electricity in January to March but understates generated electricity proportionately in the same period.

The purple line shows monthly electricity consumption and is broadly consistent month-to-month.

The green line shows the generated electricity from the solar PV system.  Its seasonality is clearly visible.  Solar generation exceeds electricity use in four summer months, and is very close in a fifth.

The red line shows bought electricity.  It’s generally a mirror image of the green line reflecting more purchased electricity in winter and less in summer, but is not zero even in months where generated electricity exceeds used electricity due to time of day issues – cooking and car charging often occur at times when solar output is low such as cooking in the evening and charging at night.

The blue line shows surplus day-time electricity being used to heat water, and thus saving gas.

The turquoise line shows surplus day-time electricity being exported once the water has reached its set point.

It will be interesting to see how this changes in 2017 as a result of a full year of solar car charging in its current mature condition and with the new battery storage that should help get more of the generated electricity used by saving it for evening use.

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Solar PV Generation 2016

The end of 2016 marks the end of the first full calendar year of solar PV operation in this house since the system was installed in late September 2015. In 2016 we’ve generated 4,129 kWh – 12.5% more than the 3,668 kWh annual output estimated for the system.

Income for 4,129 kWh generation with 50% deemed export is £641, with further savings made from the self-use of the electricity rather buying electricity or gas.   Detailed monitoring of these savings only started in March 2016, so there’s less than a full year of data, and my ability to use generated electricity improved through the year – particularly with my electric car charger control project.  From March to December 2016 we used 44% of the generated electricity replacing bought electricity, and a further 28% of the generated electricity replaced bought gas for hot water.  Back in September at the anniversary of the solar PV installation I estimated these energy savings as a combined £200 (link).

The chart above shows the minimum, mean and maximum daily generation for each calendar month since installation.

As one might expect, significant seasonal variation is evident month-to-month and well as day-to-day variation within any month. June 2016 looks a bit disappointing compared to May and July. The last few days of September 2015 immediately after the panels were installed were obviously quite good for September, but within the spread for September 2016. October looks quite similar for 2015 and 2016, while November 2016 is rather better than November 2015.  December again looks quite similar for both years.

Solar panel power outputs deteriorate with time.  My panels are supposed to have at least 90% of the original minimum power output after 10 years, and 80% after 25 years.  Generally mine seem to be performing well with annual generation significantly higher than estimated, and no evidence of performance deterioration in the late September to end of December period for which 2 years of data exists.

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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.

 

 

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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.
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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.

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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.

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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.

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Our hot water cylinder

A year ago when we replaced our gas boiler we also replaced the hot water cylinder. There were various reasons for that:

  1. I wanted to use an immersion heater to maximise use of generated PV electricity
  2. The old cylinder didn’t have an immersion heater and we were advised against trying to open the portal on a 40 year old cylinder.
  3. Ideally a cylinder for such heating would have a low down entry portal for the immersion heater so that the whole cylinder volume is heated – whereas our portal was in the top.
  4. Our old cylinder was uninsulated, although it did have an insulating jacket.
  5. Given that we’re potentially heating day’s hot water up-front with solar PV rather than on demand by gas then we might reasonably want a larger stored volume.

We ended up with the 210 litre Gledhill tank illustrated for which we had to sacrifice an airing cupboard shelf as it’s that much taller than the old cylinder. Both immersion heater and thermostat for the gas are at similar height towards the bottom. The smaller cylinder at the side is to manage expansion of the water in the absence of an expansion tank in the loft.

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