Friday, 2 December 2011

CurrentCost meter updates

Recently, I’ve been leaving my main PC on 24 hours a day to collect the readings streamed out of my CurrentCost meter.

This obviously wastes 100 watts, which is about 25p/day.  Clearly that’s a waste.

So, I’ve decided to take the plunge and build my first ever hardware project, based on an ARM mbed processor that my son who’s doing electronics had sitting around gathering dust.  I’m a software guy, and I hate hardware.

I’m powering the circuit from a DC adapter I had in the loft, generating 6V DC, 600mA, so it shouldn’t cost me more than about 3.6w when I’m finished.


Here’s a bad picture of the breadboard and the device.

The idea is simple, keep the output format of my device similar to the original, but add buffering.

The mbed code captures data from the serial port (Pin 8 on the RJ45 from the current cost meter, and pin 4 ground if I remember correctly) which I crimped together into an RJ45.

I then spool the data into an SD card (writing raw blocks).  The current SD card is 16MB (I had it from an old Canon Camera), but I’m planning to put in a 256 MB card which will give me about 2 weeks of buffering with my existing setup that includes 4 sensors in addition to the main sensor, though there’s no reason a 1GB or larger card couldn’t be inserted instead.

The PC will then poll for data in the mbed by sending a carriage return, and in response the mbed will send out the next LF delimited xml message it has stored, which will be transmitted back to the PC at 921600 baud, and processed by my software.

One pitfall with the current cost data is that there isn’t any way of finding out what date the data was logged on.  To deal with that, I’ve reserved the first 4 bytes of each block in the 512 byte SD card sectors to store the real time clock of the mbed, and the PC can then query what date/time the message was actually collected on.

Friday, 14 October 2011

Electric Cars? I’ll stick with my bike.

My son may soon need a car for commuting to work, and the question is, is electric viable?

I recently looked up the Nissan Leaf’s statistics, and see that it’s rated at the equivalent of over 99mpg by the epa.  Of course the real question is how many miles per kWh?

The answer appears to be around 3.4 miles / kWh.

My bicycle is 62.5 miles per kWh.

Tuesday, 11 October 2011

Programmable Thermostatic Radiator Valves

Just programming up the new valves before installation next week, and thought I’d do the calculations.  The valves shown on the right aren’t the same, but the price is about the same, and the calculations still apply.

I managed to get two valves with bodies for £53.44, and our gas prices at the moment are £0.03182/kWh, so I need to make savings of 1700 kWh to reach break even point.

The plan is to put them in the lounge on the radiators there, which are 600mmx1400mm (1500 W), and 600mm x 1000mm (1000 W).

Currently the radiators have no thermostat whatsoever, and our heating is on from 6:00 until 8:00 every morning and from 15:00 until 22:00 every evening for the 180 heating days of the year.

That equates to (1.5+ 1.0 kW) * (2 + 7 hours) * 180 or 4050 kWh/year to heat that single room, but our total usage doesn’t seem to be anything near that high when scaled up.

Last year our annual usage was 15200 kWh of gas, of which we averaged 15 kWh/ day for heating water and cooking (or 5500 kWh/year), leaving 9700 for heating.

I’d guess that based on the sizes of the other radiators in the house that the heat used in the lounge is about 1/5 of the total, or 1940 kWh a year.

I propose to limit heating in that room to be only between the hours of 17:00 and 21:30, representing a reduction of 50% on the amount of time heated, generating a maximum savings of  970 kWh/year.

Being very pessimistic (by assuming we get only half that savings), it looks like the devices should pay back in under 4 years.

For comparison, last year our gas consumption in the heating season was 60.73 kWh/degree day. Our target this year for kWh of gas per degree day is now: 55.55 representing a 9.5% reduction in consumption if we are to achieve payback in two years.

Energy Saving Halogen Replacements

When we first moved into our house 15 years ago, we installed halogen lighting in our hall.  Back then, electricity wasn’t quite so expensive, and we didn’t use the room that much anyway, and there weren’t that many options for recessed lighting.

We opted for a mains voltage system that could go on the existing dimmer switch, and therefore didn’t require expensive transformers for each light of the eight downlights.


Yesterday, I changed two of the lights over the table for the new Philips MASTER LEDspot PAR 20 MV dimmable LED lights.  I have to admit, I was ready to be disappointed, however the lights are very good.  Nearly as warm as the old lights, dimmable like the old lights, and fit in the sockets neatly as well.

I bought them from in Cambridge.  I did get a quote from someone else for a discount of about £1.60 per lamp online, but I’d have had to buy 6 to get the discount.

Also on Amazon


The new lights have a slightly wider angle (40 degrees) but that was intentional, as the lighting was a bit too focused before anyway.

The downside?  These are really expensive (£26.34 each including VAT).

So, the break even calculations are:

£26.34 / £0.09 kWh = 293 kWh (amount of energy to break even at current prices)

293kWh / (50W – 7W) = 6813 hours (amount of time lights must be switched on to break even)

The old lamp has a life of 2500 hours, and they currently cost about £5.00 each, so it looks like we’d be replacing two of them during the payback period, so we put that back into the calculation:

(£26.34 - £10.00) / £0.09 kWh = 181 kWh

181 kWh / (50W – 7W) = 3693 hours

Now, we would like to leave these lights on in the evening from about 5pm until 10pm in the winter time, which probably averages out at 2 hours per night over the year.

3693 hours / 2  hours/day / 365 days/year  = 5 years.

That’s the payback for two of  the lights, but the 0ther 6 are more difficult, as they won’t be on nearly as often, probably averaging 20 minutes a day at best.  That makes it a 30 year payback, using the current levels of  lighting.

So the plan at the moment is to use up the stock of existing bulbs as they blow in the existing fixtures, and keep our fingers crossed that the prices come down over the next few years.

Friday, 30 September 2011

Please don’t raise the speed limit to 80mph!

There’s talk in the press today of raising the speed limit here in the UK.  While admittedly most people seem to drive at 80 anyway, increasing the limit will definitely affect most people’s pocketbooks.  I also feel it will increase pressure on other people to drive faster.  If the limit is raised, perhaps it should become an absolute limit, removing any margin of doubt.

This summer, we drove to Spain fully loaded with a roof box on top of the car with my wife and two children.  A 2500 mile marathon!


On the journey down through France, we drove at just under the speed limit of 130 kph (~ 80 mph).

We don’t do a lot of driving long distance in the UK, but specifically bought our Kia diesel for long trips like this one, and carting my daughter’s sailboat around the country to various events, and we use the cruise control a lot.

The car is fairly efficient, having averaged 46 mpg with a boat on top.  On the journey to Spain, we averaged only 44 mpg, with some legs in France getting as low as  38 mpg.  Once in Spain at 120 kph and the mountains of the Pyrenees, the economy rose, bringing the average up.

You’d think that would be the end of the story, but on the way home we made another change.  We bought wine; a lot of wine!  About 60 litres in Spain, and then added another 30 in Calais.

When we stopped for dinner, I realized my tyres were looking a bit low, so I dug out the manual while crossing in the tunnel, and decided that they were probably pretty underinflated.

Don’t forget to keep your tyres inflated properly

I decided that despite the very late return home, we had better get some air, so stopped at the exit of the tunnel, and inflated them to the recommended level for the load.

What a difference.  On the journey back through Spain and France, we managed to average only 39.5 mpg.  That’s 11% down on the journey down, just because I didn’t inflate my tyres properly.

To complete the exercise, we drove home in the car in the same state, but this time stayed at 60 mph, as it was very late at night, and we were tired, and the car was very loaded.

Having filled up in Calais, I had a new baseline, and this time with the same full load and roof box, we managed 49 mpg.

You can trade time for money

It looks like driving 20 mph faster decreases fuel economy by 11% as well.  And driving additionally with underinflated tyres decreases by a total of 20%!  I’m lucky the fuel averaged only £1.10 a litre on the continent instead of the £1.40 pretty much balancing out the extra unnecessary expense.

Further mileage tests

I also had the occasion to do two other long trips recently.  To London in the C1, where driving at 60-65 on the motorway increased mileage from the 47 mpg my wife got to my 55 mpg, again a 15% decrease for a 10 mph change.

Finally, I drove to Southampton with 4 people in the car and no roof box.  We had a good journey from Cambridge, and again, drove at between 65 and 70 mph.  This time, we managed to achieve 58 mpg, a record for the car, showing that adding weight and a roof box appears to decrease fuel economy by around 11%.

Thursday, 29 September 2011

It’s Indian Summer here in the UK, 30°C/86°F here today!

The last two days (September 28, 29) we’ve had cloudless skies, so sun all day long.  We also had a very good day on September 23, but the temperatures were much cooler (around 17°C), and there was a bit more cloud.

What better way to find out how temperature affects solar PV output!

Though the total on each of the days was around 9kWh, with the peak both yesterday and today at 1.5 kW, but on the 23rd, we had our record output for a 5 minute period of 1.8 kW!  So it looks like a warm sunny day will reduce output by about 17%. 

The system is 2.1 kWp, and given the sun is pretty much over the equator on the 23rd, we can calculate that at noon on that date, the sun is at 38° (90° – 52°) elevation, and the roof is at 30° elevation, making an angle of incidence of 58°.

Sin(68°) * 2.1 = 1.94, so the panels seemed to be within spec on the 23rd.

Today, the sun was already down to 35° at noon,  therefore
Sin(65) * 2.1 = 1.90, so the difference in solar radiation was 2.1%.

Ouch—15% degredation with temperature!  I wonder if there’s any way to cool the panels more effectively?

Current Cost EnviR shortcomings regarding Power Factor

We recently installed a Current Cost meter

The system allows you to clamp a sensor around your mains cable which broadcasts readings of current to the display.  It also allows connection of up to 9 appliances.  The display aggregates the data into two hourly, daily, and monthly bins.  It also transmits the data on an RS-232 to any listening PC.

The main sensor clamps onto the live output to the main fuse box in the house, which measures all current being consumed by the house.

Our challenge was to try to use this data to monitor where our energy consumption was going, as well as collect information on our solar PV installation.

In order to monitor additional systems in the house I bought a pack of three of the sensors on the left.  One clamps around the live feed from the Solar PV system, measuring Generation.

I clamped another around the live leaving the fuse box going to the underfloor heating system.

The final sensor was intended to be clamped around the mains cable to the cooker.

Current clamps work with a single core only

Unfortunately, my strategy of clamping around a cable in the loft to measure the current does not work.  You must clamp it around only the live cable, or the current on the neutral cancels out the live and you get no reading.

Because I had an extra transmitter, I made a special extension cable where the live passes through the sensor, so I can plug in any set of appliances into this cable.

What’s using all that “power”

When my system eventually was up and running after the initial set backs, and some opening of my fuse box to get the clamps on the circuits, I noticed that my under-floor heating was reporting 35 watts when it was not actually heating.  This struck me as too much.  I wouldn’t leave a light bulb on all the time.  While the thermostats were warm, they weren’t that warm.

So I powered down the entire house, switched on the under-floor circuit only, and found that they were not using any measurable amount of power.

Power Factor strikes!  This happens in several circumstances.  Large magnetic loads (refrigerators, and pretty much anything with motors) only use a portion of the power, returning some of it back out of phase.  The link above tells you more about it.

Now, it’s really hard to monitor “power”

Unfortunately, this makes life difficult as the sensors are describing power that is not actually being billed to you (though it does cause inefficiencies for the power companies, and the amperage actually does flow through the circuits in the house, increasing load).

What else suffers from this? My PC for one appears to show a power factor of 0.7, which means that I have to multiply the reading on my meter by this factor to get real power.

Oh, and the Inverter for the Solar PV as well.  It shows 80 watts at night, supposedly consuming this amount, though in fact it only consumes less than 1W.

There is a solution for individual appliances.   These measure the real power on the device.  Unfortunately they don’t also let you know the power factor, so as a result, you cannot subtract the “phantom” consumption from the total.

They do though let you measure real cost for the individual appliance.

Note that in the UK, consumers are not billed for Power Factor.

Dimmer switches have no effect?

I also tried measuring what happened when I used the dimmer switches on eight 50W halogen lamps.  Apparently nothing, they use almost exactly the same power. 

Of course, that can’t be right, as the dimmer switch isn’t acting like a 500W heater when the lights are dimmed.

While researching the new LED lights I talked about yesterday I was warned by my electrical supplier that they’d had some problems dimming them.

Philips have a document that describes the way these things actually work.  Again, you can see what’s going on with the power factor, as it’s only consuming a portion of the load.

Too many Sensors?

So, now I’ve populated my system with a main meter, 3 additional clamps an an appliance monitor.  The system is supposed to be able to handle 10 inputs with up to 3 readings on each.

Unfortunately, it drops packets as they are colliding, and not being received by the main unit.  There is no protocol to avoid collision.  So we now have lumpy sampling to deal with as well.

Buyer Beware

I hope this has been a useful overview of the shortcomings of the Current Cost system, which while flexible isn’t really good enough for highly accurate measurements, due to the power factor issues, and lost packets.

We’re doing our best to produce some software that alleviates some of these issues, so stay tuned.

Wednesday, 28 September 2011

New energy saving gadgets ordered

Following on from today’s earlier post, I’m still searching for ways to make the house more efficient.


We currently have some 240V, 50W halogen lighting in the hall, which generate 1000 candelas of light.  Now, there are 8 of these, so that’s 400W when they’re all switched on.

Today I ordered two Philips Master LEDlamps (these things aren’t cheap—over £25 each including VAT), but they do only draw 7W.  They’re also dimmable, and should fit in to the existing sockets.  If they don’t work out, I can still return them and get my money back.


In addition, I also ordered two electronic radiator control valves, that allow me to programmatically set the temperature a room should be at various times.  The target is the living room with the gas fire.  While the balanced flue fire appears to be around 90% efficient, it’s still easier to heat the lounge with the main central heating.

The plan is to not heat the room at all until 3PM, and then heat it to a base temperature until 8:30pm-ish, when the heating will switch off.  Normally, if we’re in the room, we’ll put on the fire to make it extra cosy, so if no-one’s in for any evening, it won’t keep heating.  When the fire’s on, it will warm the room sufficiently for the boiler to stop supplying heat to that room, and heat the rest of the house instead.

These devices aren’t cheap either (£25 each), and I need to drain the central heating system to add them, but as the current valves are leaking, I need to get a plumber in anyway.

Fingers crossed.  I can’t wait to see what effect these things have.

Solar PV, 3 weeks in

Our solar panels have now been in for exactly 3 weeks, and in that time we’ve generated 140 kWh of electricity.

It’s been a fairly sunny three weeks, but the panels are generating to specification.  If you want to know more, please contact me.

In order to see what is going on, I also purchased an energy monitor, which I’m developing some software for. The monitor measures the current every 6 seconds; measuring what the house is consuming, what the solar PV is generating, and aggregates the data. You can see our usage since I bought the monitor above. I’ve also got sensors on several appliances around the house as well as the under-floor heating, so more measurements should be coming as we enter the heating season in October.


The green dots at the top are the total kWh used by the house each day, and the yellow dots at the bottom are what was generated by the solar panels.   The bar graph is the consumption for two hour periods each day, the the sample immediately after the date line being from 1-3 AM.  The values above 0 are consumption, and the values below are generation from the solar panels.

The monitored values are currently inaccurate, as you’ll notice the generation in the middle of the night.  This is because the current measurement does not correct for power factor, but that is alas another blog post.

The good news though?  Over the last 3 weeks we’ve used almost no net power during daylight hours, averaging just 1.5 kWh.

Thursday, 4 August 2011

Another way to cut CO2 and save money

While I had been focusing on the house as a way to cut our CO2 emissions, it was becoming apparent that there would be little we could do to find any more significant reductions in usage.

But, there was something we could change lurking outside of the house.  The cars!

We drive an average amount, and had a Renault Alliance convertible that was 25 years old, and got about 22 mpg in town (though it only drove 3000 miles) and a Chrysler Grand Voyager that got about 30 mpg, that was 10 years old, and starting to cost us quite a bit after we had its gearbox refurbished.

Rather than continue to fret about these two cars, I put my foot down and decided we needed to do something about it.

Now the UK has a scheme where cars that put out less than 100 g/km pay £0 road tax, less than 110 g/km pay £20, less than 120 g/km pay £30, and less than 130 g/km pay £90.

Older cars pay £180 per year.  So, right there, a potential savings of £360 a year, if I an get two cars in the lowest band.

Well, that didn’t happen, but we did manage to get a Citroen C1 (or Toyota Aygo/Peugot 107 depending on which brand you went with) that attracts only £20 road tax, and a Kia Cee’d SW 2 that attracts £90 road tax.  We had tried to buy a Skoda Octavia that is in the £30 bracket, but unfortunately had to cancel the order as delivery slipped.

The C1 has now done 4400 miles since we bought it (and with my son learning to drive in it), and averaged 48 mpg, more than double the 22 mpg car it replaced.

The Kia has been travelling all over the country with a boat on top for my daughters sailing, and despite that, has averaged 47.6 mpg over the 7000 miles it has travelled.

So, the Chrysler generated 58% more CO2 than the Kia and the Renault generated 118% more than the Citroen.

How much CO2 have we saved?  A ton!

On the Kia, 647 l * 2.7 kg / l * .58 = 1013 kg
On the C1, 416 l * 2.3 kg/l * 1.18 = 1129 kg

That’s a 55% reduction in CO2 caused by travelling, and that’s assuming we had the same cars.  In fact we’re actually driving less now too.

And how much money since last October?

Kia: £871.26 * .58 = £505 + £90
C1: £519.46 * 1.18 = £612 + £160

Grand total: £1367 in the 10 months we’ve had the cars.

An update on energy use

I haven’t updated the blog for 2 years, so here are the last two years worth of figures and the previous year for reference.

Season Degree Days @17.5C Average Per Day
2008-2009 2134 10.51
2009-2010 2121 10.45
2010-2011 2072 10.21

Our energy usage appears to have stabilized; here’s the daily average usage in kWh.

  Low Rate
High Rate Electricity
Per Degree Day
08-09 7.20 16.30 71.90 95.40 9.08
09-10 7.09 16.18 62.52 85.79 8.21
10-11 7.14 18.46 60.73 86.33 8.46

What happened in 09-10?  My wife was away skiing, and it was also very cold.  I took the opportunity to tape over the open fire in the lounge with cardboard and duct tape.  I had planned to stick a picture of some flames on top too, but never got around to that part.

That simple measure dropped our usage by over 10%.

In November 2010 we replaced our open fire with a balanced flue system, installed additional installation over another part of the loft, and installed new double glazed french doors.  Despite this, it appears that our usage actually increased, though that may have been caused by a large family Christmas last year.  We also actually used the fireplace, and the loung was a little warmer.

It certainly felt like a colder winter last year, despite the number of degree days being lower, and we had long period of snow and sub zero temperatures.

Alas, because of the balanced flue, we began to have some problems with condensation, because air wasn’t being sucked out of the chimney, so we also began using a dehumidifier for some of the year to prevent condensation.  It turned out reorganizing the loft to add additional insulation had blocked the airflow, and that was actually the cause, so we now rarely use the dehumidifier.

Again, I also tracked our neighbours usage (with their permission of course)

  Average Daily
Total kWh
Per Degree Day @ 19.5
08-09 106.70 8.50
09-10 98.93 7.95
10-11 94.74 7.76

This autumn, we install Solar PV, as I mentioned earlier today.  Fingers crossed we can bring down the energy usage even more.  I’ve also started a campaign to hibernate off PC’s when you’re not actually sitting at them.

Solar PV

Earlier this year I wrote:

I keep hearing people tell me that it's crazy not to install PV on our house, especially with feed in tariffs that pay back almost 40 pence for each kWh generated, so I thought I'd look into it at bit.

So, start with the assumption that we fit about 2.1 kWp to the house, that means that on a perfect day, the solar grid would produce 2.1 kW of power. It turns out that where we are, the expected output of that panel would be around 1750 kWh per year. That currently costs us about £150 for that amount. Now, the feed-in-tariff would also pay an additional £720 per year (tax free? maybe let's assume it is).

Now, the installed cost for this system is £8700, which the solar websites tell me would make me £29,000 better off after 25 years, and that I break even after 8 years and 6 months. Really? Where do I sign up, and do you have a bridge you want to sell me too?

Sure, the government currently promises that we'll get the subsidies over 25 years, but I'm not sure I trust promises from governments.

Now, what's the real payoff time?

Well, first of all, the opportunity cost on the money is missing. Let's say that you take your savings out of a nice safe bank account to pay for it, and that it's currently paying you 1% interest.

The lost interest costs you £87 per year, so that now takes you 13 1/2 years to break even.

But I know you manage your money better than that. You can get 3% really easily here in the UK, and interest rates are soon to rise. At that rate it's 13.5 years, but if you think you can pull off 4.5%, it goes all the way out to 18.5 years. Don't even think about taking out a loan at 7.5% to pay for this; if you do, payback goes out to 31.5 years!

If you instead ignore the government subsidy, even at a paltry interest rate of 2% on your money, you're losing £24 a year every year.

Never mind you've invested your capital so if it breaks, you lose your investment, unless you insure which costs you money, making payback worse.

So, for me, I've decided to put this on the back burner again, until something else changes.

What’s Changed?

My old college roommate installed the panels on his house, and they are producing electricity as expected.  Based on the quotes he received I did the above calculations.

However, when I actually came to have someone quote me, the installed system price was just over £6500, giving me a payback time of shortly over 7 years.

The parts are guaranteed for 10, so even if it all goes wrong, it looks like the system should pay for itself, so we’ve given the go ahead, and in just over 4 weeks time, we should be generating a quarter of our electricity.