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Discussion Starter #1
I spent an enjoyable 30 minutes yesterday trying to figure out how to get the charging timer on the car to play nicely with my Economy 7 times.

I am with Ecotricity on their 100% green Economy 7 tariff. In summer the cheap period is from 1am to 8am and in winter from midnight to 7am.

The charging timer on the Ampera is quiute flexible as it allows for many different scenarios with cheap rate electricity being one of them. However, the car allows for 3 rates... peak, mid-peak and off-peak. The challenge was to get the timer to only allow charging between 1am and 8am... this is how I did it.

I split the day into 3 sections and allocated them to these rates...

midnight to 1am - mid peak
1am to 8am - off-peak
8am to midnight - peak

I then set the timer to only charge in off-peak. It is possible to set it instead to charge during both mid-peak and off-peak if that is your choice.

The timer can also combine the off-peak charging with a departure time to make sure that your car is charged before you need to depart next day. I set my departure times for all days to 8am.

Last night the timer started charging at 1am and it finished at 4am so that works well.

When winter comes all I will need to do is reallocate the times so off-peak starts at midnight and ends at 7am with mid-peak 7am-8am and peak stays the same as 8am-midnight.

If anyone has any problems with this (let's face it, it isn't easy is it!) then just PM me :)
 

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Paul

Thanks for spending the time both working it out and noting it down in understandable language!

My challenge, although similar, is to optimise the charging so I use the electricity generated by my solar panels to keep the car charged up. In my situation I don't think Economy 7 is appropriate althogh always willing to learn more.

I recognise the panels won't provide all the energy but it must be better to charge the car with any of the "surplus" rather than have it go back into the grid for the miniscule 3 pence export tariff I earn.

I think I'll wait until we see a few more fully sunny days and I get a dedicated charging point fitted before exerting the brain too much, unless anyone has sorted this challenge already.

Big Paul
 

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Discussion Starter #3
Actually Paul, contrary to what you might think... depending on the size of your solar PV it is often better NOT to charge directly from solar in the day, especially if you have Economy 7 or a similar night time off-peak tariff.

I have done a lot of research and considered this issue very thoroghly over the past year. I had our solar PV installed before we got the Leaf and so I had plenty of time to do the sums.

The problem with solar PV is that it is hardly ever able to supply entirely all the power needs of your home and car at the same time. Even if you have a big PV array, say 4kWp, then most days it will be producing a lot less than that and even on sunny days it is normally intermittent with some clouds meaning the the power generated flutuated from minute to minute.

So if you choose to charge the car in the day you will find that unless the sun is continuously shining and unless your solar PV is generating enough power to cover your house and car requirements you will be importing from the grid.

Charging at 6A (about 1400W) will help with this and if you can charge at 6A in the day then you might be able to keep your power requirements to under that generated by the PV. Charging at 10A for me that is always impossible as 10A is about 2300W and my solar only ever generates 2000W max. Even at 6A for me, 1400W, it would require a continuously sunny day and us to keep our house use down to not import.

So, unless you have a large PV array and it is very sunny you will always import from the grid.

If you are on a normal, single rate, tariff then importing at night costs the same as in the day so yes, financially you will be better off using as much as your solar as you can in the day.

However, if you are Economy 7, or other dual rate tariff, though importing in the day is expensive when compared to at night so for us, and I suspect for many, the better option is not to charge in the day at all. Every unit I import in the day I can import 3 at night for the same price and so if I am importing more than 1/3 of the cars needs in the day with solar it would cost me less by not charging in the day and charging at night and paying for every unit but at the lower rate.

These figures depend on your tariff but in principle it applies to anyone on a dual-rate tariff.

There are other, non-financial, benefits to charging only at night if you need to import. CO2 creation at the power stations is normally considerablly lower at night. At night the demand falls and so the most costly power stations are shutdown and it turns out that the most costly are the dirtiest. By always charging at night, regardless of how much solar PV you generate, you are ensuring that you generate the least CO2. Your surplus solar PV in the day goes to the grid and reduces the daytime grid CO2 and at night you use the cleanest electricity yourself. I realise that most people's priority is the financial aspect of all this but if it is financially marginal for you whether you charge in the day or at night then choose at night and reduce your CO2 footprint.

I have written a piece on this aspect here : http://evcharging.blogspot.co.uk/2012/04/charging-your-ev-by-solar-power-at.html

It is a highly personal thing whether you charge in the day or at night. It depends on your tariff, your home useage, you attitude to CO2 and your general lifestyle. It is impossible to say if charging is better in the day or at night as it will vary for each and every one of us. however, it is clear that the obvious "charge in the day and use my solar PV power" approach is not always the best.

We have decided that for us, there will be the occasional day when charging in the day works for us but in general we charge at night unless we need a top-up in the day. It is most definitely cheaper for us to charge the car, even in the day, than to use petrol!
 

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Paul,

I'm afraid my "logic" if you can call it that, did not take the CO2 element into consideration so thanks for you observations/paper on that side of the debate.

I certainly need to look at whether the dual tariff option might be suitable as the Solar array I have is only 2.28kWp nominally and while I have already had good days when it generates more than 8kWh, enough to cover a top-up, but not a charge from empty!

Believe it or not the main thing that prompted my line of thought was that before the regular electric meter was replaced with a digital unit, I quite often saw it going backwards which suggested to me I was not using all the electricity I had generated and therefore when the car arrived I thought it better to use the surplus to keep it charged up rather than see it go into the grid.

As you will be able to tell, while I think I could "do the maths" it was very much a seat of the pants decision that drove my "logic"

Anyway when my current supply contract is up for change without penalty I'll take another look at the dual tarrif rates.

Thanks for the input and I hope you are still enjoying your car now it's more than 24hours old!

Big Paul
 

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Discussion Starter #5
You say you have generated 8kWh in a day but the total amount generated isn't the important part. It is whether it generates enough to continuously supply your energy needs at all times while you charge. If not then when the generation power drops (cloud over sun for a while) or temporary increase in house use (kettle, immersion, oven etc) so at those times it will be costing you money to import.

The total generated in a day in not a figure that is important in this equation unless you can guarantee to completely run your house and car charging all the time from what you generate and with 2.28kWp you won't.
 

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Thank you Paul for posting the clear instructions on how to set up Economy 7 charging from 1am to 8am, it worked 1st time, well impressed with your solution, I can now show British Summer Time again :D I
Will let Trevor at Vauxhall know of this wonder cure :!:
Many thanks, Brian
 

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I completely Agree that you are always better charging at night when A Greater % of the power supply comes from renewable sources. if you take anything during the day then its likely to be from fossil generation.

Push your clean PV power into the grid during high demand times and help them to turn down the coal power, then at night take cleaner wind power back from the grid at cheap econo 7 rates... If your PV supplies less than 73% of what is needed, at the right times, to charge your car, then it'll be cheaper to charge on Eco7 at night too!

From a purely financial side using:
1 unit Export Rate 3p
1 unit Standard rate 14p
1 unti Eco 7 Day rate 17p
1 unit Eco 7 Night rate 6p

If you use 10kwh to charge your car you need to be generating more than 7.3kwh during the charge period, to be in profit using Standard rate and Day time Charging.

If you use 10 but only generate 5 under standard rate with day-time charging (using all you generated) you pay 70p
If you use 10 but only generate 5 under Eco7 rate with night-time charging (exporting all you generated) you pay 45p

If you use 10 but only generate 6 under standard rate with day-time charging (using all you generated) you pay 56p
If you use 10 but only generate 6 under Eco7 rate with night-time charging (exporting all you generated) you pay 42p

If you use 10 but only generate 7 under standard rate with day-time charging (using all you generated) you pay 42p
If you use 10 but only generate 7 under Eco7 rate with night-time charging (exporting all you generated) you pay 39p

Hopefully the attached tables will show you the trend that develops, The magic number being 73%! Also notice which side of that number is steepest, the penalty for missing the magic number on the standard side rises far higher than the penalty for missing on the Eco7 side.
 

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Discussion Starter #8
Parax, Great you have joined us and welcome!

I agree with the thrust of your post except that it doesn't allow for the fact that for the vast majority of us the 3p export is only theoretical because is is an estimate and not the actual export. I think it is fair to say that the vast majority of solar PV owners will get an export assessment of 50% of generation whether it is actually exported or not. Only a very few will have export meters.

Sure, if your one of those with an export meter then you should take that 3p in account but for the rest of us it is irrelevent to the calculation as we get it regardless.

I think you post does highlight though the complexity of the calculation. It is not easy to determine the most financialy benefitial charging regime as everyone is so different.

Nice post though... thanks. :)
 

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I guess I should have expanded that grid to slightly higher usage. to fully charge an Ampera is ~14kWh. (6 hrs at 10Amp on 240V)
so here's the difference chart showing 14kWh.

To benefit from day-time charging on standard rate you'd need to generate at least 10.2kWh per day during the 6 hrs charge window. (that's the 73%)

if you only hit 8 kWh you'd have gained 24p being on Eco7.
if you manage 12kWh You'd have gained 20p being on Standard rate.
 

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I did not know about that 50% assessment that would mean it would not make a difference to the charge time, effectively it would be Zero Difference in my calcs instead of 3p Difference between night & day.
This is the Difference chart with Zero export income: (Magic Number is 58% ie 8kWh of 14kWh - Below Eco7 wins, Above Standard rate wins)
 

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Discussion Starter #11
Am I right in saying that your charts do not take into account house use?

That is important because it isn't what we generate that is important... it is the surplus over and above what we would use in the house that determines whether we need to import or not and that is very difficult to assess as it is intermittent.

I think the charts are of benefit to help us understand what is going on but I am reluctant to suggest that they be used in practice. It isn't that simple as everyone really needs their own chart taking their own figures into account. Perhaps I should write an app!
 

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yes and no... I have not included any daytime Eco7 usage as this would make a 3d table cube... a bit too complex for here. but you can use the standard rate table to include house usage just add it up and read from the correct column.

I have put these tables up just to show the relationship between generating a small amount and using a lot in a car... and that making assumptions about topping up the charging using the little trickle from pv can be more expensive if you are not generating enough..

My house usage is only tiny fraction of what the car will use, so the car is the primary object to think about now. As always do your own figures for your own situation.

Also its worth bearing in mind that the EV charging may not be from totally flat each day.

P.

Yes an app would be useful but as you say the figures vary a lot, so to be accurate it'll need a lot of info, even things like the difference between winter summer PV gen...
 

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Discussion Starter #13
I am sure members reading this thread will get a lot from your hard work and apply your principles to their situation. Many thanks.

As you rightly say, an app would need a lot of variables entered but it might be fun to try! Especially as I have never written a phone app from scratch and that is after 30+ years of programming computers. How I have avoided I can't imagine!
 

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British Gas have a special tariff that you can use:

Standing charge:24.878p per day
16:00 to 20:00 :14.958p per unit
20:00 to 16:00 :09.707p per unit

I can't quite get my head around if this is a good deal or not - any thoughts.
 

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Discussion Starter #15
Depending on how much you use per day and when I would say that for many people that looks like a good. Peak (expensive) rate is only from 4pm - 8pm.

For me though I wouldn't be interested in BG as their electricity is not 100% sourced from renewables. Yes, of course I pay more for that but it isn't a huge amount more and I want to support the renewables industry and be able to say my Ampera is charged from green, 100% CO2-free power!

My rate with Ecotricity is:

Day rate about 16p
Night rate about 6p
 
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