The pros and cons of E7 and battery or Sunamp energy storage

[Jeremy Harris]

In this thread I mentioned that boost charging our Sunamp in winter from E7 off peak rate would give a modest saving in electricity cost: https://forum.buildhub.org.uk/topic/7743-sunamp-container-bulging/?do=findComment&comment=138790

 

Rather than take that thread further off-topic, I've started this one, to try and work through the arguments for changing to E7, and also look at how best to try and optimise our use of PV generated energy.

 

I started from the position of assuming that E7 wouldn't make sense for us, as the increase in standing charge and peak rate unit cost would offset any saving from off peak use.  However, it seems that there isn't now much, if any, standing charge penalty for switching to E7, and that, plus some reorganisation of the time of use of our demand, can shift the balance in favour of E7.

 

The first thing I'd say is that working this out in enough detail, so as to be able to see whether or not switching to E7 would make sense, wasn't easy.  I drew up a spreadsheet, put in all our 24/7 loads (treatment plant, UV water disinfection, fridge freezer etc), then tried to estimate all the other loads in terms of energy used per 24 hours and whether or not that energy was used in peak or off peak times.  I then chucked in the mean daily PV generation for each day of the year and offset that from the peak rate daily usage.  More by luck than judgement, I ended up with a total for electricity usage over the year that seems to tally pretty well with reality, as far as I can tell.

 

From this data I've been able to work out how much electricity we would use at peak rate, compared with that which we would use at off peak rate, and concluded that, with a bit of careful demand management, we can get our off peak rate usage to between 40% and 45% of the total, which is comfortably inside the region where E7 makes sense (the break even point for most households seems to be around 35% of total usage being at the off peak rate, it seems).

 

Having done this, I then looked at the impact of charging my electric car, and also at the impact of adding battery storage.  This is where things get interesting, as it's dead easy to switch all the car charging to the off peak rate during the 4 to 6 months of the year when the chances of charging from excess PV generation are very low.  Even more interesting is that battery storage still looks to make sense, even if that is charged in winter from off peak electricity, rather than excess PV.  The main advantage in winter comes from being able to use battery storage to supplement PV generation during the peak rate period, so offsetting import during this time.  In turns out that this gives a significant saving, and doesn't need a lot of power, but does need a reasonably large capacity battery.

 

So, based on this, I've asked for our meter to be changed to an E7 one, so we can switch to an E7 tariff.  Even without battery storage this makes sense, just by changing some of our winter demand so that it's during the off peak time.  It looks like we could save around £200 a year by switching to E7, probably more if I include car charging energy in the sums. At the moment I don't know how much the car charging demand is going to be, as I don't have enough experience to be able to make a reasonable estimate.

[Ed Davies]

41 minutes ago, JSHarris said:

Even more interesting is that battery storage still looks to make sense, even if that is charged in winter from off peak electricity, rather than excess PV. 

 

That is interesting, as it's traditionally been considered not worthwhile because the cost of the wear and tear on the batteries exceeded the price difference. That was a calculation that needed re-doing, though, given how battery prices have changed. Be useful to say what your assumptions are here.

 

Remind me, how much PV do you have?

 

For the moment are you going to send the first lot of excess PV to DHW? Are you planning to also send it to your car, later? I.e., actually have the car charge rate roughly follow PV generation in some way?

[Ferdinand]

9 minutes ago, Ed Davies said:

 

Remind me, how much PV do you have?

 

 

IIRC, @JSHarris has 6.5kWp of solar.

 

55 minutes ago, JSHarris said:

It looks like we could save around £200 a year by switching to E7, probably more if I include car charging energy in the sums.

 

Can I interrogate this number, Jeremy? I think the last thing I saw from you on Energy Costs was this from March 2017.:

 

Quote

 Even in the winter months we usually generate more electricity than this from the photovoltaic (PV) panels, so, as hoped, it looks like we won’t have any energy bills at all.

 

Am I right to think that your +£200 here is a delta, and that the context is that your balancing financial position (PV revenue - Energy Cost) becomes £200 better, in this case by taking that off the money you are paying out.

 

Cheers.

 

F

Edited January 1, 2019 by Ferdinand

[Jeremy Harris]

15 minutes ago, Ed Davies said:

 

That is interesting, as it's traditionally been considered not worthwhile because the cost of the wear and tear on the batteries exceeded the price difference. That was a calculation that needed re-doing, though, given how battery prices have changed. Be useful to say what your assumptions are here.

 

Remind me, how much PV do you have?

 

For the moment are you going to send the first lot of excess PV to DHW? Are you planning to also send it to your car, later? I.e., actually have the car charge rate roughly follow PV generation in some way?

 

It's surprised me, TBH, as I've been doing some pretty simplistic estimates of the cost/benefit of battery systems for a couple of years or so now, and the return didn't quite match the investment.  Two things have changed.  One is that the price of battery storage systems keeps decreasing, the second is that I'm now in a position to better utilise E7, and that has an impact on the savings from a battery system just from being able to offset peak rate imports.  That offset is significant when looking at our peak rate energy usage pattern, where all year around the peak rate peak is in the early evening, from cooking, watching TV etc.

 

We have 6.25 kWp of installed capacity, facing 207 deg and inclined at 45 deg.  We generate around 5,800 kWh per year, and to simplify the sums I just looked at winter (150 days assumed) and summer (215 days assumed).  I may go back and refine this by month, but doubt that this simplification changes things by more than 10%, if that.

 

Yes, the DHW takes priority for any excess PV generation, as we need DHW all year around, with a near-constant daily demand.  The car charging can't easily utilise excess PV generation (although I am trying to do the best I can) because of the limitiations of the J1772 EVSE protocol.  The latter sets the minimum charge current that can be utilised at 6 A, and most car chargers aren't able to respond "on the fly" to charge point capability changes.  This means that although the available charge current can be changed from the charge point end, in practice it's necessary to shut the charge point off, then change the current that's available, then turn the charge point on again, so that the car can sense the new available current and adjust its on-board charger to not exceed that.  This is a "feature" that units like the Zappi seem to gloss over, with there being an assumption that using such a system can result in all the car charge coming from excess PV generation, which is far from being the case.

 

I still need to do some work on how best to manage car charging during the summer, when there is a lot of excess PV generation available, but it looks for now as if the simplest solution may be to just use a low charge rate that only turns on when export exceeds that rate for a short period of time.  I believe that will fit with my pattern of use, anyway, as much of the time the car will only need around 2 or 3 kWh per day of charge, if that, to cover the sort of journeys I do most of the time.  For longer trips it makes more sense to just charge the car up at the off peak rate, as I'd do in winter.

[TerryE]

We average about 70% off-peak in the winter falling to maybe 45% in summer, so in our case it's an absolute no-brainer. OK, we have no PV thanks to the planners, and we still haven't scheduled the ASHP selection and installation, but E7 is a no-brainer at the moment for us.

 

As J says you do need to tune your lifestyle to optimise E7 usage. We do our bulk UFH, SunAmp, washing and dishwashing during off-peak.  We accept that there is a ±½°C ripple (at most) on the house temp because the bulk heating is done overnight,.

 

An example of one change that I have made this last month where if the average outside temp drops below about 5°C then we need more than 7hrs @ 3kW to keep the house temp in tramlines is that I have a small oil filled electric radiator which I leave on low on a midnight - 07:00 timer in my study adjacent to the first floor landing.  This acts to top up the house heat using E7 rather than peak and also helps rebalance the small ground floor vs upper floor temperature difference that occurs when the outside temp is low.  My house heating algorithm is adaptive and so just compensates for this secondary heating source.

 

[Stones]

When I've looked at E7 previously (and lived with it) I found you had to use 45% of your energy at E7 to make it worthwhile.  A lot may well depend on the tariffs rate in your regional area.  For us it remains marginal at best.  Variable pricing (as and when it eventually comes in) may well be a better option as I would be able to time a lot of our energy use over cheaper rate periods, which I suspect would cover 10pm - 6am, and 10am to 4pm.

[Jeremy Harris]

Just to put some numbers in to give a sense of perspective for our all-electric house, our annual grid import is currently estimated to be 4,305 kWh, for a 130m² house.  Heating is only a fairly modest part of that, about 450 kWh.  Hot water is around 680 kWh.  Car charging I've estimated at around 550 kWh at the moment, but there is a fair bit of uncertainty around that figure.

 

Our heating uses a lot less energy than @TerryE, both because of our particular local microclimate, where we are very sheltered and in a relatively warm spot., and because our ASHP seems to run with a high COP, over 3.5 most of the time.  If we used direct electric heating then our heating energy usage would be around 3.5 times higher, but cooling would remain the same.

 

Splitting out the estimated peak rate and off-peak rate elements of our total energy usage for the year, allowing for PV generated energy self-use, I currently get an off-peak usage of 2,236 kWh and a peak rate usage of 2,069 kWh, so a percentage split of about 52% off peak rate, 48% peak rate.  In cost terms, switching from the tariff we're on now to a basic E7 tariff (which is what we'll have to do initially, as only some suppliers will change the meter, it seems) will save around £100 a year, but if we then change supplier I can get that saving to around £210 a year.

 

[Stones]

Can you quantify in kWh what you use / export from PV ?

[Jeremy Harris]

Just now, Stones said:

Can you quantify in kWh what you use / export from PV ?

 

Roughly, yes, based on actual generation data plus estimated consumption data, the latter having changed a fair bit since we moved in permanently from increased energy usage for hot water and cooking, but decreased energy usage from not charging my old car here every day (the two don't quite balance out).

 

Our total generation comes to around 5,800 kWh/year.  We manage to self-consume around 2,000 kWh/year at the moment, but I'm aiming to increase that significantly if I can.  Our export is way over the deemed 50% that we get paid for, at around 3,800 kWh/year.  Being able to utilise some of that export with battery storage is one thing I'm looking at, plus a bit of it will get used to charge the car.

[Ed Davies]

37 minutes ago, JSHarris said:

Heating is only a fairly modest part of that, about 450 kWh.  Hot water is around 680 kWh. 

 

Does the preheat energy from your heat pump for your DHW come out of the heating or the hot water figure here?

 

I'm thinking it must be from the heating part as 680 kWh/a is about 0.9 kWh/peep·day (assuming two people) which is way less than the usually assumed 3 kWh/peep·day so that 450 kWh is basically the electrical input to the heat pump?

 

Still, quite impressive. Say your heat pump output is 1575 kWh/a (450 kWh times COP of 3.5) and you're actually using 2 kWh/peep·day of DHW that'd be 1460 kWh/a for DHW of which 680 comes from direct electric heat so 780 kWh off the heating leaving 795 kWh going into the slab. At 130 m² that's 6.1 kWh/m²/a which is not terrible. Or am I missing something?

[Jeremy Harris]

Yes, the pre-heat does come out of the heating part, as it's the buffer tank for the heating that's used to pre-heat the water in the heating season.  In the non-heating season there's enough excess PV generation to not need the pre-heat for DHW.

 

I aimed to meet or exceed the performance requirements for a certified Passivhaus, without incurring the additional costs associated with getting PHI certification, so overall I've been pretty pleased with the way things have turned out.  It has been a process of tuning things to reduce energy use though, as initially I fitted a big water filled thermal store, and that had losses that were only a bit less than our DHW usage, even after I'd added a lot of additional insulation to it. 

 

Fitting the Sunamp really improved things as far as DHW goes, as the losses are much lower, which has a significant impact on our DHW energy use.  With the thermal store we'd have been using around 65% to 75% more energy for DHW, much of which would have been lost, as we had severe overheating problems in the service room and the adjacent bedroom, so much so that opening the bedroom window, even in mid-winter, was the only way to get that room to a bearable temperature.

[Jeremy Harris]

Just phoned SSE to enquire about getting the meter changed to an E7 one.  Apart from them trying to put me on a barking mad direct debit, on the basis that their data (which has to be less accurate than my data) seems to indicate that we use more than double the amount of electricity that they, and I, know we do, it was pretty painless.  The chap's coming to swap the meter over next Monday afternoon and we'll switch straight over to E7 when that's done.  The standing charge is just 0.08p/day higher, cheap rate is 6.6p/kWh cheaper, peak rate is 2.78p/kWh more expensive, but I can get a cheaper deal once we have the meter in by shopping around.  No charge for the meter change, either, which was good news.

 

Just need to work at shifting winter and long duration, high power loads, to the off peak period.  Shouldn't be hard, I can change the heating programmer to charge the slab up from the ASHP mainly during the off peak rate, my car has the ability to only accept charge during the off peak rate and the DHW boost is already during the off peak period anyway.  Looks like I can get well over 50% of our demand into the off peak period in winter.

 

Just need to look at battery storage now, with a view to reducing peak rate demand to an even lower percentage.  Not sure it will make sense in terms of the return on the investment, but having the ability to run some critical loads during a power cut will be useful, and add some value for us.

[joe90]

Very interesting thread, I always “thought” that E7 or E10 would be better for us but no data yet (my laptop has fallen over even though steamy tea gave me a few months data from his magic kit). As ours is a heavy house I think the UFH and DHW from the ASHP could easily be timed for cheap rate . @Ed Davies did you manage to make any sense of Steamy,s data?

[Jeremy Harris]

For info, my mean daily grid energy usage projections, before optimising for E7 are:

 

Winter:  Peak rate = 8.563 kWh Off peak rate = 10.583 kWh

 

Summer: Peak rate = 3.650 kWh Off peak rate = 3.015 kWh

 

I can shift a mean daily energy of around 1 kWh from peak to off peak without too much trouble, which will improve the off peak to peak rate ratio a fair bit, especially in summer.  It also means that I can get the mean daily peak rate energy down to zero by fitting a reasonable size battery system; I'm looking at fitting a 9.6 kWh system which should be able to cover the ~7.5 kWh of winter peak rate usage, and would comfortably cover the summer peak rate usage.  This may well result in us having no peak rate grid energy import at all a fair bit of the time, which would give a significant saving over the year, and would, incidentally, help the grid, by reducing peak demand.  Bit of a win-win really, and all without the need for smart metering or 30 minute tariff changes...

[TerryE]

1 hour ago, JSHarris said:

The chap's coming to swap the meter over next Monday afternoon and we'll switch straight over to E7 when that's done.

 

Are they still willing to install old dual tariff meters? 

 

46 minutes ago, JSHarris said:

I'm looking at fitting a 9.6 kWh system which should be able to cover the ~7.5 kWh of winter peak rate usage

 

How are the financials of that playing out?  In your case with PV, this would mean that you in essence operate in one of two modes: (a) (summer) zero grid draw; (b) (winter) E7 low rate only draw.  What is your predicted payback term?  I (and I suspect others) would be interested in your calculations.  Certainly in my case with mode (b) only, I can't make a cost benefit case with current battery technologies.  If the liquid metal / salt technologies come on stream + ½ hourly pricing so I can optimize purchase periods then it might be viable in the next 5 years -- about the same time that we'll switch to EV.

[Ed Davies]

1 hour ago, joe90 said:

. @Ed Davies did you manage to make any sense of Steamy,s data?

 

I won't derail this thread, but I did have a look and did a quick program to see what was happening to the humidities in each direction of the MVHR. Not terribly conclusive as the RH data is so noisy but I'll write a note about it, probably a separate thread here would be best, I guess.

[Jeremy Harris]

6 minutes ago, TerryE said:

Are they still willing to install old dual tariff meters?

 

Yes, it seems so.  I've confirmed that it's a dual rate E7 meter they are fitting, not a smart meter.  As it happens they already knew that we couldn't have a smart meter, as our location is flagged as one with no mobile signal, it seems.  The lady on the phone mentioned that there was no point in asking if we wanted a smart meter as her system was already telling her that one won't work here.

 

6 minutes ago, TerryE said:

 

How are the financials of that playing out?  In your case with PV, this would mean that you in essence operate in one of two modes: (a) (summer) zero grid draw; (b) (winter) E7 low rate only draw.  What is your predicted payback term?  I (and I suspect others) would be interested in your calculations.  Certainly in my case with mode (b) only, I can't make a cost benefit case with current battery technologies.  If the liquid metal / salt technologies come on stream + ½ hourly pricing so I can optimize purchase periods then it might be viable in the next 5 years -- about the same time that we'll switch to EV.

 

 

Tricky one, as it's not very clear that it will make sense in terms of return on investment within its usable lifetime.  I'm betting on energy prices rising over the ~10 year life of the battery system, allowing for some benefit we'll have from having some circuits powered from a big UPS, in effect (we get a lot of power cuts here) and some (fairly pessimistic) estimates of how much we may save from load shifting from peak to off peak using the battery system, plus storing excess PV generation during the summer. 

 

It's a bit chicken-and-egg, in that I won't know whether it makes sense until I've tried it over at least a year, to find out how accurate my estimates are.  The idea at the moment is to always charge my car during the off peak time.  To always use excess PV generation to charge the Sunamp first, then charge the battery system, and to use off peak to charge, or top-up, the battery system to avoid using peak rate energy.  Only time will tell if it works out, but I am pretty sure I'll have fun playing around with it (perhaps I need to factor in the value of having fun with it, too?).

 

[TerryE]

5 hours ago, JSHarris said:

Only time will tell if it works out, but I am pretty sure I'll have fun playing around with it (perhaps I need to factor in the value of having fun with it, too?

 

Ahhhh, you need to factor in the value of having a new project to interest you into the cost-benefit mix.

Edited January 4, 2019 by TerryE

[ProDave]

A solar PV diverter thread made me think of this question, but thought it best to pose it here.

 

If you have a solar PV diverter to dump power to hot water, AND battery storage, who's aim is to dump surplus PV to battery charging.   Which one "wins"  i.e which one will take priority?

 

I guess it all depends on how they are programmed. but if you buy an off the shelf battery system, you won't have control of it's software.

 

This reinforces my view that battery storage needs to be a DIY thing, probably integrated into the same controller that is doing the HW dump control, so the one device can make a decision (your decision) whether to prioritise dumping to HW or dumping to the batteries.

[Jeremy Harris]

Mine will be set for the hot water storage to take priority when using excess PV generation, and the battery charging to only use anything left from that, really because we always need hot water but may not always need all the energy stored in the battery.

 

You have a massive amount of control in the off-the-shelf unit I'm looking at; so much so that setting it up looks quite daunting at first.  I won't pretend to be an expert, as I've only read the manuals so far and not had a good hands-on fiddle withit, but it does look to have a lot of flexibility, including the option to set up off-peak charging as well as using excess PV generation charging, and to set the priorities for energy sources it uses to charge.

[Jeremy Harris]

Just about a month of being on E7 now, and it looks like the split is 59% off-peak, 41% peak, which is slightly better than I'd estimated.  This ratio will change, I'm sure, but probably not by much, as with the longer days our peak rate usage will drop a lot, because of the PV generation.  The off-peak will drop a lot too, and just be the house baseload, at least until I get the battery system installed.  When that's up and running I suspect that I'll be able to virtually stop using peak rate electricity all year around.