How do I calculate the amount of PV needed?

[ToughButterCup]

Building on @Stones and @ProDave 's lead (pun accidental and innocent)  here, it seems that we should put in just enough PV to consume for ourselves. And if, in the same thread , @AndyT agrees as well  @JSHarris, you have to have a really good reason to do something different. 

 

Next question. How much PV is just enough?

Is the calculation as simple as adding up all the likely kWh per annum consumption, and comparing that with the likely  kWh the PV system will produce?

 

There will be days when we produce far more than we can consume, and others when we can't produce enough.

 

It'd be good if we could switch panels on and off at will - bit like varying the pitch of a propeller according to need - use all our panels in winter and switch half off during summer. But then that'd be expensive too.

[Jeremy Harris]

I think the best approach is really to just fit as much PV as you can afford/have room for.  I've never heard anyone complain that they have too much PV, and, providing you can afford it/have room for it there are no problems with having more than you need in summer, as that then gives you a bit more in winter.

 

No need to switch anything on and off, the inverter does all that already, in effect, and will maximise the energy delivered from the amount of sun shining. 

[ProDave]

I think the key to maximum self usage is to try and spread the generation throughout the day.  For instance I plan to start with 4KW arranged as 2 banks of panels.  One will face just a little south of East along that boundary to try and catch the first sun of the day and start useful generation as early in the day as possible.  The second string will face due South to get the main mid day peak.  If I can, I might try a third, probably smaller string facing West to get more late in the day generation, but that is not so easy.

 

Of course if you have the capital (or can DIY it) and have the room, put them all on a tracker.

 

The normal arrangement for a FIT system is put the whole lot facing due South. This gives the maximum yield, but also a much larger mid day peak, that will be harder to self use, and less generation at the ends of the day.

 

Sticking with no more than 4KW means you can fit it and notify later without fear of being refused. Any more than 4KW and you have to get permission first.  All a bit of a moot point if you don't intend exporting anything significant.

[ToughButterCup]

6 minutes ago, ProDave said:

[...]

put them all on a tracker.

[...]

 

Tell me more....

[ProDave]

10 minutes ago, recoveringacademic said:

 

Tell me more....

https://en.wikipedia.org/wiki/Solar_tracker

 

Basically a device to move the panels to follow the sun.

 

I might make a very much easier very basic single axis tracker to flip the E facing panels over to the W for the evening.

[richi]

I read about another idea, supposedly appropriate for climates where it's often cloudy: Just mount the panels horizontally.

 

The energy generated when it's cloudy is maximised, supposedly. Is this so much horsefeathers, or could there be something in it?

[Jeremy Harris]

Getting back to basics, nine times out of ten there are external things over which you have no, or limited, control over, so best to start with the framework within which you can consider options, before going down into rabbit holes about detail.

 

The core external constraints are:

 

1 - What is the maximum power that the DNO will allow to be connected to the grid where you are?  Everyone is allowed to installed up to 3.68 kWp of PV (16 A per phase) without consent from the DNO, but often the DNO will grant consent for more than this (our DNO was happy for us to go up to 10 kWp, for example).  You can only find this out by asking the DNO, as it depends on the local network capacity.

 

2 - Are you able to roof mount and array, or will it have to be ground mounted?  The answer to this leads to follow on questions:

 

   a) If you can fit roof mounted array, or arrays, what inclination and direction would they be able to face, and how much roof area will you have available?

 

   b) If you cannot fit roof mounted arrays, then, without planning permission, you are only permitted to install a ground mounted array of 9m², which isn't a lot, around 1.4 kWp.  If you can gain planning permission you can fit a larger ground mounted array, and have the space, then you have more options.

 

3.- How much money are you prepared to invest in a PV system, and what sort of a return or benefit would you hope to get from it?

 

I appreciate these questions are not instantly answerable, but there's little merit in planning a way forward until you have a feel for the limiting conditions that will post some constraints on what you would like to do.

 

Once these are cleared out of the way, the it will be easier to come up with a range of options that you might be able to consider, and with a bit of luck match one as closely as possible to your needs.

 

 

[pdf27]

SMA have a pretty good online tool called Sunny Design (https://www.sunnydesignweb.com/sdweb/#/Home - registration required but free) which lets you specify a system and will then calculate production, self consumption and the like for given load profiles, heat pumps, etc. I suspect it's a little pessimistic for a well insulated house, but otherwise seems pretty reasonable. You'll need to put in your own electricity cost and subsidy estimates however - that's the one part of the programme where I get the strong feeling that the sales department got involved and the figures look very fishy to me.

 

To give an example, for our (potential) build there is space for an ~8kW system. With electric heating and hot water plus SMA's system for increasing self-consumption, they're predicting ~1,700 kWh would be self-consumed (with 3,000 kWh still imported - this for a 40m² "passive house" as for anything bigger the heating loads start to look silly) with 5,500 kWh exported. On that basis the system would be worth £288 in export tariffs, another £288 in FITs and ~£255 in avoided electricity bills for an annual value of ~£830, or about £100/kW. I think I could probably do a little bit better by playing with the smart grid functionality on a heat pump (e.g. turning up the thermostat a degree whenever the PV system is exporting and using the heat capacity of the house to store energy), but it won't change the value much. At ~£100/kW/year it's worth doing for us, but only within limits - we'd do it if we had the cash, but other aspects of the build would take priority.

[ProDave]

47 minutes ago, JSHarris said:

  b) If you cannot fit roof mounted arrays, then, without planning permission, you are only permitted to install a ground mounted array of 9m², which isn't a lot, around 1.4 kWp.  If you can gain planning permission you can fit a larger ground mounted array, and have the space, then you have more options.

 

That is a valid point. I knew we would not do well with roof panels due to the shading from the trees, so on my planning application I included ground mounted panels. I must admit is was more for completeness rather than thinking we actually needed PP for them. A sort of fortunate accident that they are now included in the PP.

[ToughButterCup]

48 minutes ago, JSHarris said:

[...]

1 - What is the maximum power that the DNO will allow to be connected to the grid where you are?  

[...]

2 - Are you able to roof mount and array

[...]

3.- How much money are you prepared to invest in a PV system, and what sort of a return or benefit would you hope to get from it?

 

3.68 kWp,

Yes,

Just enough for self consumption - since FIT appears to be a waste of money.

 

My thinking starts at what I think might be considered by some as the 'wrong end' of the stick.

I can probably make a good guess at the amount of electricity we are likely to need in kW hours: kWh/a :  lets say for argument 3000 kWh. My question is - 

Given that I want to avoid exporting power to the grid;

 

How little capacity should I install to assure the optimum supply for self consumption?

[ProDave]

Another "wrong end" way to look at it, is you want to bu the kit as cheap as possible, so keep your eye out for bargains. Something may come up as a particular bargain and what is available cheap may end up dictating what size of system you install.

 

For self usage there are 2 basic things that will go a long way. First is use the big appliances like washing machine, dishwasher and fluffy towel machine one at a time in the daytime. The other is excess power diversion to hot water heating. 

 

Another one to keep an eye on but I don't think we are there yet, is battery storage. There will come a time, and I suspect not far away, when that will be viable to store excess power for the evenings.

[Jeremy Harris]

18 minutes ago, recoveringacademic said:

 

3.68 kWp,

Yes,

Just enough for self consumption - since FIT appears to be a waste of money.

 

My thinking starts at what I think might be considered by some as the 'wrong end' of the stick.

I can probably make a good guess at the amount of electricity we are likely to need in kW hours: kWh/a :  lets say for argument 3000 kWh. My question is - 

Given that I want to avoid exporting power to the grid;

 

How little capacity should I install to assure the optimum supply for self consumption?

 

That's a difficult question to answer accurately, but I can have a stab based on our experience.  We have a 6.25 kWp array facing South at 45 deg inclination.  This is pretty good for midday, especially in winter, because of the steep roof pitch.  However, we come no where near to generating enough electricity in winter, and struggle to even generate enough to cover our base load.  We rarely ever export any power for around 3 or four months, and often import power for days on end.  Arguably our 6.25 kWp array is too small, and would need to be around three to four times larger to meet all our daytime loads, all year around.

 

The flip side is that in summer we generate massively more energy than we can use.  Most days recently We've been exporting a few kW, having charged up the Sunamp PV, charged the car, run the dishwasher etc.  We export around 1/3rd of our total annual generation, and all that export happens in around 6 months.

 

I don't think you can ever avoid exporting to the grid, even with battery storage.  The dynamic range between a good generation day and a bad generation day is just too great.

[pdf27]

1 hour ago, recoveringacademic said:

 

3.68 kWp,

Yes,

Just enough for self consumption - since FIT appears to be a waste of money.

 

My thinking starts at what I think might be considered by some as the 'wrong end' of the stick.

I can probably make a good guess at the amount of electricity we are likely to need in kW hours: kWh/a :  lets say for argument 3000 kWh. My question is - 

Given that I want to avoid exporting power to the grid;

 

How little capacity should I install to assure the optimum supply for self consumption?

Insufficient information, unless you're planning on installing a huge battery system. Because electricity (unlike heat) isn't easily stored, then if you want to avoid exporting any power to the grid you need to have the smallest panel which can deal with the always-on loads (standby on the TV, clocks, MVHR, etc.) on a bright, cold day - probably about 50-100W. If you want to claim that you're "net zero" that's actually pretty easy - south facing panels in the UK make about 1000 kWh/kWp so for 3,000 kWh you'd need a 3kW system.

 

If you're feeling a little more adventurous with a spreadsheet and have a decent idea of how your plug loads will vary by time of day and day of the week, take a look at Sheffield Solar (https://www.solar.sheffield.ac.uk/pvlive/) and download the data on it for several years (I think the complete dataset starts in 2015). That provides half hourly PV generation figures along with up to date grid-connected capacity - divide one by the other and you have the output per kW of installed capacity. Mix in your predicted plug loads, have some fun with macros, and you can see how your savings would vary with different sized PV systems and plot system size against ROI. Again, this doesn't actually answer your question but is probably as close as you're likely to get I suspect.

[SteamyTea]

As @pdf27 as said, self consumption is about matching power and not energy.

 

There is a moral argument about claiming FiTs and then minimizing your exports, but that is another issue.

You also have to look at the opportunity costs involved i.e. could you have spent the money on something that would save even more energy i.e. an EV

[ProDave]

7 minutes ago, SteamyTea said:

As @pdf27 as said, self consumption is about matching power and not energy.

 

There is a moral argument about claiming FiTs and then minimizing your exports, but that is another issue.

You also have to look at the opportunity costs involved i.e. could you have spent the money on something that would save even more energy i.e. an EV

It's not our fault we have a stupid system.  I still believe a much better system would be simply to pay you a slightly below retail price for what you export, but that would mean everybody having a meter that measures export separately o import.

[SteamyTea]

7 minutes ago, ProDave said:

It's not our fault we have a stupid system.

" All that is necessary for evil to succeed is for good men to do nothing " Edmund Burke (1729-1797).

[pdf27]

8 hours ago, SteamyTea said:

As @pdf27 as said, self consumption is about matching power and not energy.

 

There is a moral argument about claiming FiTs and then minimizing your exports, but that is another issue.

You also have to look at the opportunity costs involved i.e. could you have spent the money on something that would save even more energy i.e. an EV

I'm a lot more relaxed about it than I once was now that the rates are far lower and there is less coal on the grid, but I still really don't like immersion diversion devices except in very specific circumstances (as a dump load when you would otherwise exceed the DNO export limit). In most cases people are shifting from heat generated by burning gas at 90% efficiency in a domestic boiler to burning electricity which is generated by burning gas at maybe 50% efficiency in a CCGT, and getting paid to do so.

 

7 hours ago, ProDave said:

It's not our fault we have a stupid system.  I still believe a much better system would be simply to pay you a slightly below retail price for what you export, but that would mean everybody having a meter that measures export separately o import.

I'm very curious to know how close we are to smart meters being able to do this. I know the capability is in the SMETS specification, but given how long it is taking everybody to meet this spec in full I'm guessing it isn't available yet. Time of use tariffs are starting to become available (e.g. https://octopus.energy/agile/), and I suspect eventually we'll also see dynamic pricing for export - most likely once batteries/V2G starts to become more widespread as there is a lot of potential commercial value to an aggregator being able to turn on a lot of export to the grid all of a sudden, and they'll need evidence that they've done this.

[Jeremy Harris]

1 hour ago, pdf27 said:

I'm a lot more relaxed about it than I once was now that the rates are far lower and there is less coal on the grid, but I still really don't like immersion diversion devices except in very specific circumstances (as a dump load when you would otherwise exceed the DNO export limit). In most cases people are shifting from heat generated by burning gas at 90% efficiency in a domestic boiler to burning electricity which is generated by burning gas at maybe 50% efficiency in a CCGT, and getting paid to do so.

 

I'm very curious to know how close we are to smart meters being able to do this. I know the capability is in the SMETS specification, but given how long it is taking everybody to meet this spec in full I'm guessing it isn't available yet. Time of use tariffs are starting to become available (e.g. https://octopus.energy/agile/), and I suspect eventually we'll also see dynamic pricing for export - most likely once batteries/V2G starts to become more widespread as there is a lot of potential commercial value to an aggregator being able to turn on a lot of export to the grid all of a sudden, and they'll need evidence that they've done this.

 

I disagree strongly.  Our house has no access to the gas grid and for obvious reasons I didn't want to burn oil, coal or wood, plus we had no where we could safely fit an LPG tank if we'd wanted one.  That left is having to use electricity, and so it makes perfect sense to divert self-generated electricity to the Sunamp PV and use it for our primary hot water system.  By doing that we massively reduce our demand on the electricity grid, and, in summer, reduce the level to which the local grid shuts our inverter down (it's a rural area and the grid voltage regularly hits the 253 VAC upper bound on sunny days).  The same goes for the diverter that feeds my car charger.  It seems far better to use excess locally renewably generated power to charge my car on an ad hoc basis, than it does to charge the car from the grid, perhaps at a time when the grid load is already high.

 

Smart meters are primarily a means to introduce variable spot pricing, and that will be the number one priority as soon as the adoption rate is high enough.  The whole concept of smart meters was, and is being, driven by the need to reduce the high risk that suppliers currently have to hedge, which is setting their supply tariffs based on their best estimate of the mean 30 minute unit wholesale cost that they buy in at.  Smart meters are not aimed at reducing emissions, improving efficiency or anything else that is claimed, they are primarily a means to allow consumers to be charged a flexible tariff, that aligns better with the wholesale cost per 30 mins + profit.

 

Nothing wrong with that from a business perspective, but I do wish that the people "selling" this programme to the public would be honest.  The truth is that once there is widespread rollout of smart meters the first thing consumers will see is a variation of the variable tariff scheme that companies like EDF already use in parts of France.  We have one supplier already offering such a variable tariff scheme, albeit in a crude way, with peak time unit prices at close to 30p and off peak unit prices down to just a few pence.  Right now we know that the off-peak wholesale cost drops below zero at time, and the peak wholesale costs approaches double the average retail price.  This is something that the smaller electricity supplier find hard to manage, as they just don't have the volume of sales an financial security to ride out periods of high peak wholesale cost.  I suspect there may well be some interference in the process by the large energy suppliers, using market forces (nothing dodgy) to make sure that wholesale prices stay high during peak times, in order to put the squeeze on the smaller suppliers.

 

The sad thing is that smart metering has the potential to be a force for positive change, but I doubt very much that will ever happen, as the temptation to just use it to reduce supplier risk and raise profit levels has to be very high indeed.

[SteamyTea]

One problem with variable pricing is that the government has dictated that our energy bills need to be 'simpler', so less opportunity to have variable pricing.  Not sure how this sits with the energy supply companies who probably want to use variable pricing.

We have a secondary problem with our infrastructure insomuch as some supply companies are also generation companies.  Generally this should not be a problem as it introduces an element of competition into the market, except it didn't as the main players have tended to stick to individual technologies i.e. EDF with Nuclear, British Gas with Gas, EON with Coal (but now RE and gas).

Then you have to remember what happened in California in the early 2000s.

California had 45GW of installed generation capacity and demand was about 28GW.  But Enron realised that they could shut down supply to the state, sell it to a neighbouring state, then buy it back again at an increased price to the consumer.

Our Contract for Difference would allow this to happen.

 

Another, more serious problem with variable pricing is the the public will just not understand it.  I occasionally chat to someone that supplies a data service to the power companies.  He raves about 15 minute billing and how it will even out supply and demand.

Sounds great, except no one plans their life to that degree.  You only have to look at Gridwatch to see that we are all individuals, just like everyone else.

 

This is rather going off thread here, but the point about the morals of self use is that most people on the FiTs scheme are getting paid to supply the grid with 50% of their generation.  It is really not right to take that money and not supply the goods.  If it was a business it would be prosecuted for fraud.

 

But back to the main point.  The first thing you need to do is an energy audit of your hourly consumption, then plot it against hourly irradiation.  That will tell you what you can expect to happen.  A hunt around on WeatherUnderground should find a local weather station with that data (you used tt be able to download it but the have changed the webpages so now not so easy).

[Jeremy Harris]

14 minutes ago, SteamyTea said:

This is rather going off thread here, but the point about the morals of self use is that most people on the FiTs scheme are getting paid to supply the grid with 50% of their generation.  It is really not right to take that money and not supply the goods.  If it was a business it would be prosecuted for fraud.

 

We export well over 50% of our generation, so it's us being scammed, rather than the supplier or generation company.  Our meter is an old one, by current standards, but records export internally (as virtually all digital meters do, and have done for years as an anti-fraud measure).  It's dead easy to read out the export register via the IrDA port, I do it all the time on our secondary (unofficial) house meter, as it's the easiest way to obtain good data on house consumption and generation.

 

The fact that the suppliers can't be arsed to use the standard IrDA port to read meters is their problem.  The meters were designed specifically to allow this many years ago, and at the same time the export register was added internally to allow better accuracy in cases of meter fraud, so the suppliers could make accurate claims in the courts for those that defraud meters using the very common reverse phase trick (I won't go into details, but suffice to say it's a very cheap and easy way to stop any meter recording imported energy).

[SteamyTea]

2 minutes ago, JSHarris said:

The fact that the suppliers can't be arsed to use the standard IrDA power to read meters is their problem. 

Yes, they are lazy and the information could be useful to customers and suppliers.

3 minutes ago, JSHarris said:

We export well over 50% of our generation, so it's us being scammed, rather than the supplier or generation company

I think that is why there is the deemed export rate and the total generation rate.  In an ideal world there would be no gain for anyone.

The real problem is that the scheme was a pig's ear from the very beginning.

But if you are exporting over 50%, then your PV system is too large (getting back on topic)

[le-cerveau]

5 minutes ago, SteamyTea said:

But if you are exporting over 50%, then your PV system is too large (getting back on topic)

The interesting question would be over a year what is your import vs export?

CAn you really have too much? there will always be locations that cannot generate the required load (high rise) and those that generate more, the bigger question is smoothing out the supply or buffering the excess.  WHilst battery technology is still improving, domestic smoothing will be limited, the DNO/generators are in the best position to shunt excess generation to a storage facility for latter use, be it battery/hydro/....

[jack]

If the government didn't want people to consume locally, they could just as easily have insisted on proper metering. It wouldn't have cost them anything - the consumer pays for the installation, and the power companies pay for the cost of administering payments.

 

I therefore take the view that the government put the deemed consumption measure in specifcally to encourage local consumption. Maybe they foresaw the rise of the electric car, and wanted to get a head start on installing local capacity to avoid the need to expand the grid to cope with increasing home charging.

 

Hahahhaahhhha, had you going for a minute there.

 

Of course, what really happened is that the government stuffed up, in which case I'm performing my civic duty to punish them whenever they drop the ball. If there're no consequences for their incompetence, how will they learn?

[SteamyTea]

17 minutes ago, le-cerveau said:

CAn you really have too much?

If the local DNO can handle it, then no.  More is better.

17 minutes ago, le-cerveau said:

domestic smoothing will be limited

I often think the scaling is wrong.  We try to self store enough energy to supply our needs on the days of little generation.

But if we accept that we do need grid supplied power, then much small storage can be used.

Taking data from Gridwatch for a random day in March, the peak demand was 20% higher than the mean, or 10 GW.  The peak was in the evening and lasted 3 hours (5PM to 8PM).

This is almost certainly caused by domestic use.  So all that is needed is enough storage for that 30 GWh peak.

Now keeping the maths simple, there are about 26 million houses, so that will be 1.1 kWh of storage per house.

Edited June 20, 2018 by SteamyTea

[Jeremy Harris]

37 minutes ago, le-cerveau said:

The interesting question would be over a year what is your import vs export?

CAn you really have too much? there will always be locations that cannot generate the required load (high rise) and those that generate more, the bigger question is smoothing out the supply or buffering the excess.  WHilst battery technology is still improving, domestic smoothing will be limited, the DNO/generators are in the best position to shunt excess generation to a storage facility for latter use, be it battery/hydro/....

 

 

Over the whole year we export about 30% more than we import, so in winter we are a net importer.  It's not uncommon at this time of the year for us to export four or five times more energy than we consume each day.  Our net export over the year is around 60% of our generated power, but we only get paid for 50% of it.  I don't really have a problem with that, other than the fact that it would be very, very easy to just use the exported energy reading from the meter to pay us, rather than assuming a deemed 50% export, but the supplier can't be arsed to ever issue meter readers with an IrDA reader, even though the standard has been around for 20 years or more.

 

We don't have enough PV, that's for sure.  If we had more we would not have to rely on the grid so much during the winter.  Given that winter is the period when the grid is more heavily loaded, it would make sense if we were better able to support it, but we fitted as many panels as could physically fit on the roof - we have no more space.

 

Our PV array is 6.25 kWp, but I've been thinking of trying to add around another 1 to 1.5 kWp on the wall at the rear of the garden.