What percentage of PV energy is usable?

mickyduck55

Last year I used almost 80% of my generation

2021
3735 kWh generated (not a great year)
768 kWh exported... = 20.56%

2020
3950  kWh generated (from mid Feb at install of batteries) 
868 kWh exported...= 21.97%

I have A/A Heat pump, batteries ... and bake

Exiled_Tyke

Petriix said:

arty688 said:

Also people use the term free electricity usually around the iboost stuff , it's not free its what ever you export rate is say 5p. So if you are comparing against gas and savings you need to use that figure. Having said that it defiantly greener and is a good thing to do.

I don't think it necessarly is 'greener'. Exported electricity almost exclusively reduces gas generation at ~ 60% round trip efficiency. A gas boiler is typically 80% efficient so it's likely 'greener' to burn gas at home and export the electricity.

For those of us on deemed exports, however, it is use it or lose it. 

But apparently about 38% of UK electricity come from fossil fuels (27% gas)

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1032260/UK_Energy_in_Brief_2021.pdf

so the issue is whether your argument for 'almost exclusively reduces gas generation' is valid or not. I can see the logic for this but I'm not convinced it's necessarily the case. It could just as easily reduce imported nuclear leccy from France? 


The argument that each kWh of home useage is money saved does of course need more investigation. I'm sure most homes generating from solar change their habits, some more about convenience rather than pure money saving: putting on a tumble drier rather than hanging out washing on a sunny day, doing that extra load of laundry or dishwasher cycle because the electricity is free etc.. 

JKenH

Exiled_Tyke said:

Petriix said:

arty688 said:

Also people use the term free electricity usually around the iboost stuff , it's not free its what ever you export rate is say 5p. So if you are comparing against gas and savings you need to use that figure. Having said that it defiantly greener and is a good thing to do.

I don't think it necessarly is 'greener'. Exported electricity almost exclusively reduces gas generation at ~ 60% round trip efficiency. A gas boiler is typically 80% efficient so it's likely 'greener' to burn gas at home and export the electricity.

For those of us on deemed exports, however, it is use it or lose it. 

But apparently about 38% of UK electricity come from fossil fuels (27% gas)

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1032260/UK_Energy_in_Brief_2021.pdf

so the issue is whether your argument for 'almost exclusively reduces gas generation' is valid or not. I can see the logic for this but I'm not convinced it's necessarily the case. It could just as easily reduce imported nuclear leccy from France? 


The argument that each kWh of home useage is money saved does of course need more investigation. I'm sure most homes generating from solar change their habits, some more about convenience rather than pure money saving: putting on a tumble drier rather than hanging out washing on a sunny day, doing that extra load of laundry or dishwasher cycle because the electricity is free etc.. 

I think the fact that solar panels change one’s mentality is quite a positive thing. When we first got solar I became more conscious of which devices used more power and shuffled my usage around to optimise usage of my solar generation. 

You are right, though, in that such a mindset can also lead to power being used inefficiently. Take my IBoost. Most days my hot water cylinders are heated by solar power. I am very conscious though that heating water at 10am which is then stored until I have a shower at 8am the next day is not efficient. Perhaps a third of what I store disappears as heat from the cylinder. That is useful heat in winter but less so in summer although my wife enjoys a warm airing cupboard. 

Diverting solar into batteries, is also wasteful of electricity as up to 20% can be lost in the DC  to DC to AC cycle. (Perhaps we need to think about using DC batteries more). From a global perspective batteries consume energy and resources in their manufacture so if you are trying to be green adding a domestic battery is pretty much a no-no. It is far better to export surplus energy to the grid. 

If you could just time your battery usage to coincide with when the grid is at its dirtiest that would help but the reality is that many solar filled batteries are being discharged between 7pm and 7am. 

Understandably having invested a lot of money in solar panels which is a positive move I am not criticising anyone who wants to make the most of that investment putting their own household economics first. 

Solarchaser

Exporting pv to grid and helping reduce ff generation is very very much area specific.
Middle England you will almost definitely be doing just that, same with South Wales,  however North Scotland highly unlikely. 

Those who have batteries can play a good role in decarbonising the grid as most will be on a time of use tarriff, and so will be discharging the batteries to cover the house load when the grid is dirtiest and most expensive, and also refilling the batteries when the grid is greenest.
Win win really.

JKenH

What we really need is those with batteries to discharge to the grid between 4.30pm and 7pm. Outgoing Agile tariff encourages this but no other tariffs drive such behaviour. 

Petriix

Exiled_Tyke said:

Petriix said:

arty688 said:

Also people use the term free electricity usually around the iboost stuff , it's not free its what ever you export rate is say 5p. So if you are comparing against gas and savings you need to use that figure. Having said that it defiantly greener and is a good thing to do.

I don't think it necessarly is 'greener'. Exported electricity almost exclusively reduces gas generation at ~ 60% round trip efficiency. A gas boiler is typically 80% efficient so it's likely 'greener' to burn gas at home and export the electricity.

For those of us on deemed exports, however, it is use it or lose it. 

But apparently about 38% of UK electricity come from fossil fuels (27% gas)

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1032260/UK_Energy_in_Brief_2021.pdf

so the issue is whether your argument for 'almost exclusively reduces gas generation' is valid or not. I can see the logic for this but I'm not convinced it's necessarily the case. It could just as easily reduce imported nuclear leccy from France? 


The argument that each kWh of home useage is money saved does of course need more investigation. I'm sure most homes generating from solar change their habits, some more about convenience rather than pure money saving: putting on a tumble drier rather than hanging out washing on a sunny day, doing that extra load of laundry or dishwasher cycle because the electricity is free etc.. 

It doesn't matter how much of the overall generation comes from gas. The fact is that *immediate demand* is almost exclusively met by burning gas. They're the main type of power station able to ramp up fast enough so they effectively carry the load for all the small changes in usage across the grid.

It's obviously vastly more complicated than that. But, in practice, the more solar you export the less gas generation there is. Obviously there are some other technologies and, as grid scale storage increases, the picture is changing. However, I don't think it's reasonable to claim that the nuclear base load has any impact on the CO2 emissions caused by drawing an extra 3kW from the grid. 

mickyduck55

Today is the perfect example of the benefit of batteries... 

Yesterday the batteries were full from solar, so no grid import for daily use and 2.5 kWh imported to charge the batteries last night to cover a predicted very dull day.

Today the base load of house is just about being covered from the panels (see below) so I still have 80% of my battery .. admittedly not all useable but this will get me through the day and if the sun shines they may even get topped up a little.  I am baking later so they will get depleted but I don't expect to import any electricity until my TOU tariff kicks in.

Thanks to a member of this forum I have a Pi that decides how much power I need to import overnight based on the solar forecast, this is not perfect as we all know that forecasting the weather is not yet an exact science.

ATM the forecast for tomorrow is OK, not great but OK so maybe tonight I will import 2-3 kWh.

I totally agree that battery manufacture is quite destructive and household systems are not cost effective.. but as I have stated before I like the idea and it does work for me 

Magnitio

JKenH said:

What we really need is those with batteries to discharge to the grid between 4.30pm and 7pm. Outgoing Agile tariff encourages this but no other tariffs drive such behaviour. 

Although I am not actively exporting to the grid at peak (unless the battery is full and demand in the house is low), the use of the battery means I am not placing a demand on the grid during these hours when there is often a heavy load from cooking etc. From an environmental perspective, I did contemplate whether the purchase of a battery made sense. If I lived on my own, I probably wouldn't have done as I'm quite happy to change habits to maximise the use of solar. However, encouraging members of the family to also adopt new routines would be stressful and the battery provides lots of flexibility.

Going back to the original question about what percentage of PV energy is usable, my percentage is only 66% this month and will reduce as we go into Summer. I could obviously increase this by consuming more, but to me that would be wasteful and I would prefer to export the excess. More importantly is the amount I consume from the grid which is only 16% of our overall consumtion and 85% of that is during off-peak period.

Martyn1981

Petriix said:

Exiled_Tyke said:

Petriix said:

arty688 said:

Also people use the term free electricity usually around the iboost stuff , it's not free its what ever you export rate is say 5p. So if you are comparing against gas and savings you need to use that figure. Having said that it defiantly greener and is a good thing to do.

I don't think it necessarly is 'greener'. Exported electricity almost exclusively reduces gas generation at ~ 60% round trip efficiency. A gas boiler is typically 80% efficient so it's likely 'greener' to burn gas at home and export the electricity.

For those of us on deemed exports, however, it is use it or lose it. 

But apparently about 38% of UK electricity come from fossil fuels (27% gas)

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1032260/UK_Energy_in_Brief_2021.pdf

so the issue is whether your argument for 'almost exclusively reduces gas generation' is valid or not. I can see the logic for this but I'm not convinced it's necessarily the case. It could just as easily reduce imported nuclear leccy from France? 


The argument that each kWh of home useage is money saved does of course need more investigation. I'm sure most homes generating from solar change their habits, some more about convenience rather than pure money saving: putting on a tumble drier rather than hanging out washing on a sunny day, doing that extra load of laundry or dishwasher cycle because the electricity is free etc.. 

It doesn't matter how much of the overall generation comes from gas. The fact is that *immediate demand* is almost exclusively met by burning gas. They're the main type of power station able to ramp up fast enough so they effectively carry the load for all the small changes in usage across the grid.

It's obviously vastly more complicated than that. But, in practice, the more solar you export the less gas generation there is. Obviously there are some other technologies and, as grid scale storage increases, the picture is changing. However, I don't think it's reasonable to claim that the nuclear base load has any impact on the CO2 emissions caused by drawing an extra 3kW from the grid. 

Yep very complicated, and something that comes up every so often.

The 'marginal generation' argument for gas might now be starting to get questionable, especially whenever there is some RE curtailment. Also, if the PV is, for example heating water during non heating periods, then there's a chance the GCH efficiency will be as low as gas generation (around 50-55%) since the boiler won't be operating at best efficiency, and all heat losses in the pipework are losses, whereas in the heating months, it's still a benefit. Also, reduced use of the boiler, possibly 5-6 months per year might be beneficial, and the boiler pump probably consumes 80-100W.

Then there's the use of PV to operate heatpumps and heatpumps in general, which in the shoulder months and daytime may have a COP of ~4+, but if we just take the UK average of about 2.9, then 1kWh of gas via a GCH boiler might provide 0.9kWh(t), but to make up for that lost export, via a gas generation plant, and grid losses, the 0.45kWh to 0.5kWh of leccy generation would provide 1.3 to 1.45kWh(t) via a heatpump.

So we have a 'how long is a piece of string' situation, but I think it's all fun, and the direction of travel makes it easier (conscience wise), and additional PV, no matter how it's used* will always make a positive contribution v's not installing.

*LOL, that opens the door to some deep thinking, I bet some negatives can be thought off.


Back to the OP, my PV consumption was around 35%pa, but now with small heaptumps and BEV's it's about 55%.

JKenH

Petriix said:

Exiled_Tyke said:

Petriix said:

arty688 said:

Also people use the term free electricity usually around the iboost stuff , it's not free its what ever you export rate is say 5p. So if you are comparing against gas and savings you need to use that figure. Having said that it defiantly greener and is a good thing to do.

I don't think it necessarly is 'greener'. Exported electricity almost exclusively reduces gas generation at ~ 60% round trip efficiency. A gas boiler is typically 80% efficient so it's likely 'greener' to burn gas at home and export the electricity.

For those of us on deemed exports, however, it is use it or lose it. 

But apparently about 38% of UK electricity come from fossil fuels (27% gas)

https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/1032260/UK_Energy_in_Brief_2021.pdf

so the issue is whether your argument for 'almost exclusively reduces gas generation' is valid or not. I can see the logic for this but I'm not convinced it's necessarily the case. It could just as easily reduce imported nuclear leccy from France? 


The argument that each kWh of home useage is money saved does of course need more investigation. I'm sure most homes generating from solar change their habits, some more about convenience rather than pure money saving: putting on a tumble drier rather than hanging out washing on a sunny day, doing that extra load of laundry or dishwasher cycle because the electricity is free etc.. 

It doesn't matter how much of the overall generation comes from gas. The fact is that *immediate demand* is almost exclusively met by burning gas. They're the main type of power station able to ramp up fast enough so they effectively carry the load for all the small changes in usage across the grid.

It's obviously vastly more complicated than that. But, in practice, the more solar you export the less gas generation there is. Obviously there are some other technologies and, as grid scale storage increases, the picture is changing. However, I don't think it's reasonable to claim that the nuclear base load has any impact on the CO2 emissions caused by drawing an extra 3kW from the grid. 

This is the point I have made about charging one’s EV from domestically generated solar (which of course I do). Until I plug my car in that solar is going to the grid and helping to supply my neighbours but once I plug my car in a gas power station somewhere has to make good that shortfall. 

We have been burning massive amounts of gas over the last 8 days and everytime someone plugs something extra in a bit more gas has to be burned even during the night. It is cheaper to charge a battery overnight on a TOU tariff but switching that battery to charge does burn gas at the power station. The battery then wastes 10 or 20% of that gas generated electricity in going through its AC to DC and back to AC cycles.

We confuse what saves us money with what saves the planet. As @Exiled_Tyke pointed out adding solar can cause us to be more wasteful with the use of items such as tumble dryers because the electricity is free to us, (particularly if we are on deemed export). The same with batteries. Because we get our electricity free from solar and if we know there will be loads more of it the next day then the tendency is to use more of it than if we were paying for it. That PV that we are using from our batteries (perhaps needlessly just because it’s free) would have provided 10-20% more useful electricity had it gone straight to the grid. 

Having bought the panels, solar energy is free and I feel I am wasting it if I don’t use as much of it as possible. I think this is what often drives people to get home batteries even if they aren’t actually economic. You’ve paid for it so you don’t want to  waste it.