Just as a little exercise I totted up the relative dirtiness of the various electricity supplies on the market. Using the most recent published fuel mixes for the various supplies churns out the following:

Supplier	Nuclear	Gas	Coal	Renew.	Other	CO2 Intensity (kg/kWh)	CO2 emissions (kg/year)	Cost on bill (annual)	External cost (annual)	Cost including external (annual)
Npower	2%	55%	29%	12%	2%	0.474	1565	£516.71	£63.28	£ 579.99
Brit Gas	27%	50%	14%	8%	2%	0.316	1042	£448.08	£38.78	£ 486.86
EDF / Sainsbuys	69%	0%	28%	3%	0%	0.253	834	£488.27	£60.60	£ 548.87
Eon	6%	49%	30%	11%	4%	0.476	1570	£499.56	£69.23	£ 568.79
Southern / Scottish Hydro / Swalec / Atlantic / M&S /Ebico	1%	49%	35%	14%	1%	0.501	1653	£485.42	£71.09	£ 556.51
Scottish Power	0%	40%	47%	14%	0%	0.568	1874	£483.23	£88.80	£ 572.03
Ecotricity*	2%	20%	12%	64%	2%	0.191	630	£488.31	£28.82	£ 517.13
Good Energy	0%	0%	0%	100%	0%	0	0	£485.20	£1.82	£ 487.02
LoCO2	0%	55%	0%	45%	0%	0.198	653	£ 452.31	£7.90	£ 460.21
Utility Warehouse	2%	55%	29%	12%	2%	0.474	1565	£ 436.02	£63.28	£ 499.30
First Utility	6%	52%	32%	5%	4%	0.500	1651	£ 564.50	£73.76	£ 638.26
Ovo*	5%	39%	24%	29%	3%	0.377	1244	£ 403.79	£56.01	£ 459.80
Green Energy UK*	0%	64%	0%	36%	0%	0.230	760	£ 460.44	£8.89	£ 469.33

Notes:
*These suppliers also offer a dark green tariff that would have the same impact as the Good Energy one.
Prices were the latest standard online dual fuel tariff including imminent price rises where applicable, but not including special offers. For simplicity some resellers piggybacking on big 6 companies like SSE have been lumped together even if their prices differ. It's the relative cleanliness we're interested here.
CO₂ intensity is based on (kg kWh^-1: Coal=0.91, Gas=0.36, Other=0.61. Overall CO₂ figure is base don UK annual average of 3300kWh.
External costs are the total impact of GWP, damage to forestry, buildings, health, waste handling etc expressed as a figure in p kWh^-1 (Ref:
Pearce, D.W., Bann, C. and Georgiou, S. (1992) The Social Costs of Fuel Cycles, Report to
the UK Department of Energy,). Figures used were: Nuclear=0.48, Gas=0.39, Coal=5.4, Renewable=0.055, Other = 6.05). Other estimates of external cost are available, but all follow the same pattern of low costs for renewables, medium for gas and nuke, and high for coal and oil, so the effect is the same.


Some of my thoughts:
Even within the Big 6 energy companies (not known for their greenness) there’s a wide variation in the quality of the supply. When you look at just CO₂, EDF and British Gas are surprisingly good, getting almost as low as some of the smaller specialist green suppliers. However, EDF only gets so low because of all their nuclear plants, and this shows in the overall impact where they fall back into the pack of dirty offenders. In the long run, the green suppliers like Good Energy, LoCO₂, Green Energy UK and Ecotricity all come out well ahead, although British Gas is actually not far off them (and is cheap!).

Npower is the worst of the big six, being both dirty and expensive. But if you want the absolute worst deal in Britain you’ll be heading off to First Utility.

Scotland is a country keen to promote it’s renewable resources but unfortunately the Scottish companies seem to be topping up their decent renewable percentages with a lot of coal, so end up being some of the dirtiest. Oops.

Ovo are the cheapest overall, but LoCO₂ only cost £1 more a year and emit about half as much CO₂. If you go with Ovo, use their proper green tariff, as their standard one is pretty dirty.

This is the average fuel mix, which they're required to publish (annually I think?). So no, it won't take into account variation by time of day.

However, given that EDF are almost entirely coal and nuclear I wouldn't expect their fuel mix to fluctuate by much during the day, as both nuke and coal will be on fixed-price base load contracts AFAIK. So your external impact (ie: the amount that in a perfect world you would have to pay to cover **all** the effects of your energy use, not just CO₂) for EDF is probably pretty close to 1.8p kWh^-1 whatever time of day. The only real reason to time-shift a power supply based on a near-pure coal/nuke mix is things like avoiding having your consumption draw on storage, which isn't taken into account in the "external impact" numbers. So yes, probably some benefit for you to run things overnight, but not a whole lot IMO.

Posted By: djhWhere do the 'cost on bill' numbers come from? They don't match what comparison websites show me.

Standard online dual-fuel tariff direct from each supplier's websites, or the closest analogue. Most companies have lots of different tariffs, special deals for new customers, etc. Plus on price comparison sites you're dealing through a middle man so I'm not surprised you might get different numbers than if you went direct.

I haven't put too much effort into ensuring that column is super-accurate for a couple of reasons:

1) I'm more interested in the CO₂ and external cost figures. These can be applied to any tariff with the same fuel mix from that supplier.
2) The immense variety of tariffs available renders any attempt at total accuracy a bit pointless.

Take the "Cost on bill" as broadly indicative, rather than definitive.

Hmm, I think you'll find there's less difference between those two than you might think:

http://www.goodenergy.co.uk/what-we-are-doing/investing-in-renewables
http://www.goodenergy.co.uk/blog?category=Our+Wind+farms

Good Energy own and build wind farms, they recently refurbished the old one down at Delabole in Cornwall.

From what I can tell, all the proper green suppliers are pretty good and I'd happily go with any of them. What does suck is the Big 6 wrapping their existing renewable obligations up and marketing it as a "green" supply. That's the big con.

Given the physics of it, when I turn an electrical appliance on, I am consuming electricity generated with the entire grid's fuel mix at that moment, not just electricity generated by my supplier.

The granularity of the data used for supplier-level fuel mix calculations is yearly consumption. So, even [substitute supplier's name here] is simply trying to hit fuel mix targets for its customers' aggregate annual consumption.

But electricity is a 'perishable' good, in the sense that it has zero shelf-life (it must be consumed as it is generated, pumped hydro storage aside).

So there isn't any necessary relation between, say [substitute supplier's name here]'s generation patterns and its customers' consumption patterns within the day. In fact, the measurement infrastructure isn't in place yet to make such determinations. Smart meters could allow that to change, depending on how they're implemented in practice.

Thought popped into my head, what is the generation market share of each company. That makes a difference to their CO2e contributions.

Apologies: I didn't intend to single out ecotricity for any opprobrium. I've edited the post to reflect this. In fact I like what ecotricity is trying to do within the constraints of the current system.

An implementation: http://www.realtimecarbon.org/

Another here: http://www.earth.org.uk/_gridCarbonIntensityGB.html

Thanks for posting this table Seret

We are with Ecotricity but I have been concerned about being taken in buy there green wash but they don't look too bad in the table.

I've been over to your site and read your blog http://andyduffell.com/techblog/

I think the most succinct way of responding, Seret, is to point out that because of electricity's (instantaneous) perishability, the supply of electricity is time-and-date stamped -- and two different electricity supplies with different time-and-date stamps are not, in a demand-theoretic sense, "perfect substitutes".

Just curious, but I don't know how the UK grid works now that everything has been privatized. Back when I lived in the UK, there was The National Grid ...

Who is now responsible for deciding what sources are used to put power into the grid to meet the demand? Is there a central body? What guiding principals do they use for the choices they make? Purely monetary?

Paul in Montreal - where we have a monopoly supplier and only one grid operator, but private companies are finally being allowed to supply the grid

Thanks ST - very interesting. I remember something about the nominal voltage being reduced to something closer to 230V a few years ago and I know the frequency tolerance over the long term is very tight. When I was a student I visited Fiddlers Ferry powerstation and remember there being two clocks - one showing the correct time and one showing "grid" time. We were told that grid time and real time had to be in sync everyday but there could be (if I recall correctly) up to 20 seconds of difference. They'd make it up at night when loads were lower. We were there around one of the late afternoon peaks and it was interesting to see the frequency get lower as the load increased. I also remember as a child of 3 or 4 standing inside the under-construction chimney at Drax powerstation (my Dad was working on that I and used to go with him from time to time ... health and safety rules were a bit different in the late 1960s. I can still remember the little circle of light at the top of the chimney when viewed from the inside!

Paul in Montreal

Let's imagine simple scenarios.

Scenario #1:

I am the customer of a green energy company, and I am its only customer.

In the year I consume only 1kWh, and I consume this at a very inconvenient time of the day and year: when the ratio of coal-generated to gas-generated load favours coal (due to relative prices favouring the former), when the wind is not blowing, and furthermore I do it during a particular 'peak hour' during the day. Result: my 1kWh has a relatively high CO2e externality.

Joe Bloggs is a customer of one of the regular electricity utilities, and he happens to consume his 1kWh in the night, when the wind is blowing. Result: JoeB's 1kWh has a relatively low CO2e externality.

Now my green energy company operates wind farms, and let us say that JoeB got the 'green electons', as you put it.

How does me paying the green energy company for 1kWh of electricity reverse the fact that my choice was to consume 1kWh of the dirtiest electricity possible, and that this CO2e is now in the atmosphere? It doesn't; it cannot.

As a general principle, the price system is only effective if externalities are 'internalised' in some fashion -- i.e., the externalities are brought back inside the price system and cease being 'external' to it. The current system does not achieve this; we lack the measurement and pricing infrastructure (smart meters *could* rectify this; the devil is in the implementation).

Scenario #2:

Suppose *everyone* switched over to the greenest of the green energy suppliers. What would be the consequence of this?

In the short-to-medium term, the effect would be virtually zero. The reason? The green energy company would simply have to purchase -- from the existing stock of generators -- more wholesale electricity (enough to meet total grid load). Interestingly, this 'green' energy supplier's fuel mix would converge -- almost precisely, I would add -- to the existing grid fuel mix.

Only in the long run (defined as that horizon at which it becomes possible to vary the amount and type of fixed capital in place) would one begin to notice slight differences, as the moral commitment of the green energy company would cause it to (presumably) make somewhat different capacity replacement&growth investment decisions.

However (and now switching out of Scenario #2) we are already benefitting from any and all current 'green' generation capacity, whether we are customers of green energy suppliers or not, whether the green capacity is actually built, owned & operated by a green energy supplier or not.


Bottom line:

I think we have to be very careful about the green energy companies' marketing/PR line.

I read that paper in 2005 but had forgotten all about it.

One thing about buying green electrons is that it sends a clear message to the generators about how much extra we are willing to pay (actually not much). Be interesting when the Carbon Tax starts to bite, £30/tonnes is not going to worry anyone (at my aggregate rate a 15% increase, I just had a 10% increase for doing nothing).

Those external costs are very real, but not always what we think they are.
I am currently doing a funded Research Masters (a rare beast these days), the grant to the university comes from a central pot of money somewhere that has some of our money in it. One small report I read cost us nearly £80,000, it was a cheap one, there were ones for DECC and DEFRA in there that cost closer to £1m (shall try and find the site that had the stuff on).
Would be interesting to find out how much all this 'backroom' stuff costs us and what, if the money was spent on green generation, would generate.

here we go.
http://blueandgreentomorrow.com/features/uk-renewable-energy-generation-2010-2011-infographic-analysis/
click on image to enlarge

approx. 31% of 2011 renewable energy is from section 1 of the above quote ( from biomass and wastes)