Category Archives: Energy

Electricity Purchase to July 2018

In the last few days I’ve reported our status on electricity generation from our solar panels and our gas consumption, so here comes some thoughts on electricity purchase from the grid.

Starting in late 2015 after the meter was changed to Economy 7, there’s a general downward trend from November 2015 to March 2016, before my car charger project kicks in maximising use of my own solar electricity to charge my car (when available) which causes a significant drop in purchased electricity between march and April 2016.  That seasonal saving gradually drops through the autumn, although it’s interesting that by November 2016 we’re back on what seems to be a continuation of a downward trend from January to March 2016.  Electricity purchased is also significantly lower than 2015 as we enter the second year.

The second significant change is the addition of the storage battery in December 2016.  However from January to August 2017 (yellow) electricity purchased is significantly below the prior year (magenta) – potentially showing the benefit of the battery in saving electricity generated during the day to reduce consumption later in the day.  This benefit largely disappears from September to December 2017, presumably because my increased vehicle mileage after my daughter started school is offsetting the prior savings.

2018 (orange) generally falls somewhere between 2016 and 2017 as it combines both the storage battery and the higher vehicle mileage throughout the year to date.

The August 2018 figure is a projection based on the first few days of the month only, but may yet come to represent the month as a whole being a function of:  (i) record solar outputs, (ii) continuing battery storage availability, and (ii) no school in August leading to reduced mileage.

Mid-year thoughts

Summer is definitely upon us now as we enjoy the glorious summer weather.  Disappointing weather earlier in 2018 has given way to two record months in May and June which yielded the highest monthly outputs for their respective months since the system was installed back in 2015.  Some days we buy no measurable electricity or gas  (given that the electricity meter has a resolution of 1 kWh) depending on what the need to charge the car.  If the car is at home then I can fully charge it from the solar panels, whereas if the car is at away from home during the day then I may need to give it some charge overnight.  When charging overnight I have been tending to charge for the minimum number of hours up to 7:30 AM when I typically leave home on a weekday – that pattern provides for mostly Economy 7 Energy from the grid less whatever comes off the solar panels from the rising sun less whatever might be left in the PowerVault from the previous day as illustrated below:
The green ‘hill’ from around 3:30 to 7:30 AM is created by car charging.  Normally this would be seen as a rectangular block as the car charger effectively runs at a constant 10 Amps (2.3 kW) through the operating period.  However in the illustration the charging event (at least in terms of power drawn from the grid) seems rounded at both the beginning and the end.  At the beginning of the car charging period the mustard ‘Device Power Out’ curve shows the last remaining stored energy from yesterday being drawn from the PowerVault, while at the end of the charging period the ramp down is a result of increasing output from the solar panels reducing the need for power from the grid.  Hence at the moment the car charging ends there’s a sudden switch to charging the PowerVault at full power (the blue line) and some surplus power not used by the PowerVault (the purple line) – suggesting that something around 1 kW is suddenly available.  Although the purple line is described as ‘Grid Power Out’ that’s not strictly true here as much of that surplus is being diverted to make hot water (although this is invisible to the PowerVault).

After that digression, my actual purpose in making this post was to reflect upon relative energy costs and the best use of my solar power to reduce energy cost.

Energy usagePetrolDaytime ElectricityNighttime ElectricityGasSolar Electricity
Battery Storagen/an/a - no economic case to charge battery from grid during dayOptional - need to consider value of saved energy versus cost of 1 cycle of battery cycle-lifen/aSelf-use Priority #1 via PowerVault (daytime electricity -> solar)
Car ChargingManual 3rd backup (typically only used for long journeys when charging en-route becomes impractical) Manual 2nd back-upAutomated 1st backupn/aSelf-use Priority #2 via ImmerSUN (nighttime electricity -> solar)
Water Heatingn/aManual 3rd backup (never used in 3 years)Manual 2nd backup (never used in 3 years)Automated 1st backup for dull daysSelf-use Priority #3 via ImmerSUN (gas to solar)
Space Heatingn/aManual backupManual backupDefaultn/a - a summer solar surplus is a poor match to winter heating demand but could be Priority #4

 
The table above shows columns of energy sources ordered by reverse energy cost versus the major energy consumers in the house: battery storage, car charging, space heating and water heating. Energy consumers are ranked according to the value of displacing the the alternative energy course if not solar:

  1. Battery storage – I currently only charge the battery storage from solar, although there would be a seasonal economic argument to charge from cheap rate electricity if the differential between day and night rates was higher.
  2. Car charging – I generally charge on cheap night-rate electricity when I don’t have enough solar. In summer I program my car charger via the ImmerSUN’s 7-day timer to deliver sufficient charge for the day ahead, but sufficient headroom to make use of any available solar.
  3. Water heating – water heating is my 3rd priority for solar self-use and is automatically based up by the gas boiler which runs for an hour making hot water in the early evening if the tank isn’t already hot from diverted solar power during the day. The gas thermostat is also set slightly colder than the immersion heater – still very usable for a bath or shower from gas but giving some ability to delay water heating from a dull day to a following sunny day.
  4. Space heating – my space heating is generally gas. It would be possible to run a heater (or heaters) such as storage radiators via the ImmerSUN’s third output, but I consider that the cost of the heater(s) and installation is unlikely to be recouped given the major mismatch between surplus solar generally being in summer and heat demand being in winter.

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.

Solar output – 2017 Jan – Jun

2017 H1

Now that we’re half way through 2017 it seemed appropriate to have a look at energy usage from the solar panels – especially as those six months reflect the first six months with the battery storage system.

The graph is taken from my ImmerSUN smart controller which automatically diverts surplus solar electricity to the car charger or immersion heater.  The battery storage system has independent controls but its benefits can be seen via the ImmerSUN.

The purple line shows the consumption of electricity (excluding the immersion heater) and is relatively stable month by month.  Consumption is relatively large due to my electric car and cooking with electricity.

The green line shows the generation of electricity from my solar panels.  Not surprisingly output is lower in the winter, but from April we generate more electricity than we use despite our relatively high consumption.  In principle we could be electricity independent during those months but for the time of consumption not matching the time of generation.

The red line complements the green line as it shows the import or purchase of electricity from the grid, and thus reduces as the generation rises.

The blue line shows the diversion  of electricity to heat water via the immersion heater when neither the battery storage system nor the car charger can absorb the available electricity.

Finally on the graph the turquoise line shows export of electricity to the grid when all smart capability within the house to use electricity is exhausted i.e. battery storage system at maximum power or full, electric car battery full or absent, and water in cylinder is hot.

Among the numbers:

‘Savings” at £80 refer only to the value of the water heating achieved from solar electricity versus buying electricity (although our backup is mains gas).

“Self consumption” at 86% refers to the proportion of solar panel output used i.e. not exported to the grid.

“Green contribution” at 59% refers to the proportion of total electricity consumption (excluding water heating) derived from the panels rather than from the grid.

 

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.

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.

Discounted renewable power

img_0596We’ve recently moved to Bulb as our electricity and gas supplier. Bulb is a new company founded by Hayden and Amit providing 100% renewable electricity and 10% renewable gas and, for us, was the cheapest provider of the same.

Bill logo

Bulb logo

Bulb are currently offering a £50 discount to new customers via this link, so there’s even more incentive to go renewable.

They also have 9.8 out of 10 on TrustPilot, a UK call centre, and are a Living Wage employer.

Energy Unit Costs

We’ve just switched energy suppliers at the end of a fixed rate term.  Unit costs are as follows:

 Oct 2015
- Sep 2016
Oct 2016
- Dec 2016
Jan 2017
- date
Day-time electricity 11.71 p/kWh11.90 p/kWh11.48 p/kWh
Night-time electricity 7.57 p/kWh6.86 p/kWh7.87 p/kWh
PV electricity (when available if metered)4.85 - 4.91 p/kWh4.91 p/kWh4.91 p/kWh
Any-time gas3.01 p/kWh2.43 p/kWh2.35 p/kWh
PV electricity (when available if deemed like mine)0.00 p/kWh0.00 p/kWh0.00 p/kWh

In both cases electricity is on so-called ‘green’ plans where the supplier sources electricity to match my consumption from renewable sources such as wind turbines, solar farms, or hydroelectric.

I’ve included my export rates in the table as for some technologies this will make a difference to the cost-effectiveness of that technology.  My electricity company chooses to deem my export so it pays me assuming that 50% of my generated electricity is exported, rather than metering and paying for my actual export.  That results in the cost to me of using my own solar being zero, whereas if export was metered then using my own solar would cost me the export payment.  Thus for me it’s economically attractive to use excess solar to make hot water rather than using gas thus saving the cost of gas but, for someone with metered export, the lost export payment would outweigh the saving in gas.

Since the table rows are ranked by unit price (higher priced fuels are at the top) then another way to look at this is that it’s financially attractive to replace a fuel higher in the table with one lower in the table, but disadvantageous to replace a fuel lower in the table with one above.

Annual Energy costs

I was reading a newspaper article earlier which highlighted a 3 bed semi with annual energy consumption costs of £500.  Our net energy consumption costs for my early ’70s 4 bed detached in 2015/6 was £400.  That includes charging my electric car.

img_0591

The house has:

  • A-rated gas boiler (Dec 2015),
  • A-rated double glazing throughout (prior owner),
  • C-rated hot water cylinder with bottom-entry immersion heater and all accessible hot pipes insulated (Dec 2015),
  • 7 day timer (prior owner) with Thermostatic Radiator Valves (TRVs)  throughout (except hall and 2 towel rails – Dec 2015),
  • Standard cavity fill and 4″ loft insulation (prior owner, now nearer 10″ but Oct 2016 installation outside 2015/6 data window),
  • Almost 100% low energy bulbs (mostly Sep 2015),
  • 4kWp solar panels (Sep 2015) with energy management system (Sep 2015) with remote monitoring (Mar 2016) directing surplus PV to car charger (Apr 2016) and/or hot water cylinder (Dec 2015), and
  • Economy 7 electricity (Oct 2015).

I spent £1,000 on gas and electric in 2015/6 which was partially offset by £600 revenue from my solar PV giving net costs of £400. Given that some of the above were introduced during the year, a full year’s use should reduce consumption further.

 

 

Making best use of energy

If I want to make best use of my solar PV (i.e. use as much of it as possible) then it seems logical to prioritise replacing my most expensive energy purchases. Examination of my bills suggests that my different sources of energy have the following costs:

  1. Day-time electricity: 12.31 p/kWh
  2. Night-time electricity: 8.02 p/kWh
  3. Any-time gas: 2.87 p/kWh
  4. Solar PV (when available): 0.00 p/kWh

In winter when little PV electric is available our normal practice is to shift as much electric load as possible to night time use saving 4.29 p/kWh by use of timers. Washing machine, dishwasher, and electric car charging all get shifted to Economy 7 by use of timers. The washing machine is the oldest and uses a external timer plug while both dishwasher and car have their own inbuilt timers allowing consumption to be delayed.

In summer the same approach can be used to shift these loads to daytime to make use of solar PV although if the weather is dull and cloudy there’s no guarantee that sufficient PV will be available risking consumption of bought daytime electricity.

My Immersun already minimises exported/surplus PV by running the immersion heater to mop up surplus electricity, although as gas is cheap the savings are smaller than might be achieved by avoiding a similar amount of night-time electricity consumption. In winter we minimise gas use for water heating, while still providing a gas safety net by displacing in both time and temperature. The Immersun naturally heats water in daylight hours only and generates hot water at 60C (not adjustable) as defined by the immersion heater thermostat. The gas is set to complement this being set by timer not to overlap Immersun hours and is set to 50C by the cylinder thermostat. The water temperature difference is not noticeable in practice especially as the showers are thermostatically regulated.

Car charging 1The device that consumes most electric power is undoubtedly my electric car. Its usable battery capacity is about 10.5 kWh and, with charging efficiencies, it can take up to 12 kW of mains electricity to charge (less in summer). The appeal of charging this on solar PV is high, although the cost of doing this if it inadvertently draws day-time electricity is also high (but still much cheaper than petrol or Diesel). It occurred to me that it would be good to use spare channels on the Immersun to control car charging as my largest consumer of my most expensive fuel. It has three channels – two variable power and one on/off – although it can be configured to produce three variable outputs using the on/off to switch between devices.

It seems to me that there are three alternatives for control:

  1. Proportional control.
  2. Stepped control.
  3. On/Off control.

Proportional control would be the most capable. The Immersun has the capability to provide proportional control to multiple resistive loads, and the Mode 3 communications standard to which virtually all electric cars adhere also provides for the charging equipment sending a proportional signal to the car via PWM, so there’s scope for a proportional system. However it seemed that the time to develop this was beyond what I have available, and in practice although the car would receive a proportional control signal it would normally respond in a stepped manner so there’s little to be gained by investing my time to develop a fully proportionate system..

Stepped control is easier to achieve. As it happens my car model is more steppy than some as I believe that it switches between 6, 10 or 15 Amps consumption only. 15 Amps is very close to the full 4 kW output of my panels and, given that there are always other loads in the house, then I don’t think it would ever use this setting, which leaves me with switching between 0, 6 and 10 Amps. Unfortunately of course you can’t switch between 3 states with an on/off signal that only has 2 states, so I’d still need to decode an analogue Immersun output or invest in a second Immersun to get another on/off output but I deemed the latter cost-prohibitive. Generating a variable current signal to the vehicle would also require some logic to reduce the pulse width on the PWM signal on demand – not impossible but still requiring some time to create.

That leaves me with on/off control at a fixed current. This is readily achieved from the Immersun which has a relay output that can be configured to operate when a certain solar power is available (such as 1.5 kW to drive a 6 Amp charger). I decided not simply to turn the whole charger on or off by switching the power as this would cause the mains contactors in the charger and the HV DC contactors in the vehicle to drop out under load which is bad for longevity. Instead I’ll interrupt a low voltage control signal within the charger to trigger stop and restart in a more controlled manner – effectively car and charging equipment will see the signal as the user wishing to disconnect the cable as you might if you wished to drive before charging was complete.

Finally I considered how to connect the Immersun to the charging equipment. Unfortunately the Immersun is in the airing cupboard upstairs (where it drivers the immersion heater) and the garage where the car charges is a separate building alongside the opposite side of house. Neither a second Immersun or a physical cable looked like an attractive prospect on cost grounds so I decided on radio control using around £20 of home automation parts and a total investment of around £40. More on this solution over the next few weeks.