Category Archives: Energy

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.


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.

Solar panels can generate significant amounts of electricity for example on March 25th our 4kW panels generated 22.4 kWh over the course of the day as shown.
ImmerSUN 25-03-2016
This generation typically substantially exceeds the load being drawn by the house leading to export of electricity to the grid – 12.3 kWh in this example or more than half the generated energy.

Moving loads such as the washing machine or dishwasher from night to day can help use this electricity, but done to excess risks increasing day time electricity import if changing cloud cover or other factors reduces generated power. The varying power demand from such devices is also unlikely to coincide the the surplus power available leaving some unused surplus. I was thus pleased to come across the ImmerSUN system.
The ImmerSUN is a device which diverts surplus power to a range of possible consumers such as an immersion heater. It has a current sensor that measures any surplus power being exported and then diverts a similar amount of power into the immersion heater. Normally of course an immersion heater is either on or off, but the ImmerSUN provides proportionate control between 0 and 100%. It can control several devices diverting power to a lower priority device once, for example, a higher priority immersion heater has raised the water temperature to the set point and power for that purpose is no longer required. In the top illustration the blue bars show 4.7kWh being diverted into water heating (and thus reducing my gas bill) that otherwise would have been lost to export.

For further details see ImmerSUN website .

At the time of writing if you order online quoting Referral Code 206505 you will receive £25 in department store discount vouchers or a free system upgrade to remote monitoring.

End of 2012 Gas Analysis

Well like many UK homes our central heating and hot water is fuelled by natural gas. We also use natural gas for cooking on the stove top (the oven is however electric) and we have a gas fire in the living room, but these latter two uses are much less significant.
I’d lived here for 10 years before I started actively trying to reduce my energy consumption. During those 10 years I used an average of 562 units of gas annually.
Since then I’ve reduced my consumption by two routes, firstly to reduce heat loss so that less energy for heating was required, and secondly by replacing gas with renewable sources. It’s however also the case that my circumstances have not been constant during that time so, for example, when I started this I lived alone then 4 years or so ago I married and my wife joined me here; and now my wife is at home during the day with our baby daughter much of the time. Consequently the demand for hot water for washing and heating has increased.
The steps taken to reduce gas consumption include:

  • Having insulation installed in the cavities of the house walls. Houses here conventionally have a double skin of brick or block and, when this house was built, the cavity between the inner and outer layers was usually left as an air gap. We’ve had that gap filled with insulation as you would find in newer homes.
  • Increasing the thickness of insulation in the loft.
  • Replacing the old wooden windows with new PVC ones with higher “A” grade insulation – most easily seen by the larger air gap in the sealed units.
  • Installing solar water heating panels on the roof in conjunction with a larger hot water tank. In the summer these provide almost all our hot water (no heating is required), although at olther times of the heat they don’t provide enough heat alone for hot water they can help to pre-heat the cold water leaving the gas less work to do to achieve a usuable temperature.

We didn’t do this all at once, but if I compare the last 2 years to the original 10 year baseline then we’ve reduced average annual gas consumption by 23%. I think that the reduction would have been higher had our circumstances between constant.