What percentage of PV energy is usable?

k6chris

Trying to get my head around what percentage of PV produced electricity is usable, in the real world.  For example, is a 4kWp system produces (say) 3,600 kWh througout the year, if I only used electricty when the sun was shining, and only needed 3,600 kWh all year, the answer would be 100%.  The problem is, I would be spending all evening / night and every dull day in the dark, with no appliances running!  Is there a rule of thumb?  I'm trying to get my head around a realistic payback period.

Thanks

Bluebell1000

As a rough guess I'd say 1/3, maybe up to 1/2 if you are home all day when sunny, though it'll vary through the year, and also depends what your consumption is. You can get it higher if you use some to charge an electric car and/or battery storage.

QrizB

Personal opinions vary but here are mine.

Without a battery, most people manage to use 30-40% of their generation. With a battery, perhaps 50-60%.

But the smaller the system, the greater % you use. The larger the system, the lesser. So without a battery you might manage to use almost all of the generation from a 1kWp system, but barely 20% from a 10kWp one.

And, broadly, the more electricity your household consumes, the more of your generation you'll use. A low-consumption household, using only 2000kWh/yr, can only use so much solar power. A household using 6000kWh/yr will be able to use much more.

Petriix

Before we had an EV, even with working from home, we used around 30%. With the EV it's over 50%. It was over 60% when electricity prices were lower and we were more liberally supplementing solar with grid imports. With a home battery (and some judicious dumping of the battery into the car) we'll get that up to 80%.

The real question, however, is how much can you reduce your peak imports by; otherwise you get sucked into the false economy of assuming all your usage is a 'saving'. I could (for example) use the tumble dryer on a sunny spring day, then count that as 4kWh of peak electricity saved - about £1 at my current peak rate. However, I could have just hung it out for zero cost, or even waited until the 5p Octopus Go rate kicked in so it's really only a 20p saving at best. 

Spies

Actual numbers from me with no battery.

Jan 60% self use
Feb 54% self use
March 37% self use so far

When I ran my numbers before I had it installed I had assumed 60% in winter months and 36% in summer months.

arty688

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.

Petriix

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. 

arty688

I try to forget about the FIT scheme as its something I wanted to do when I built the house but my dreams of solar died with the first week with clay soil and trees that made the footing £20k more than budget. 11 years on and I'm about to finally get solar. At least its cheaper per KWh now though.

Magnitio

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. 

That's an interesting argument. I've been using some of my excess solar generation to reduce gas usage e.g. immersion heater and electric heaters rather than using the gas boiler. So, I've been thinking about it purely from a personal CO2 saving rather than the potential savings at the power station. From a cost point of view, there's not much difference for me between gas per kWh and electric export per kWh.

ispookie666

My current system has been up since end of Jan 2022, its a 6.6KWp system which has panels in 3 different orientations.  Total generation since install has been 628KWh and export 235KWh making it 63% consumption. 
As we go into longer days the self consumption is going to go down unless I run the aircons throughout and get the batteries installed.  

JKenH

My stats for the last 12 months (minus October for which data was not available) show export of 1046kWh out of generation of 5078 kWh - 20.6% exported and 79.4% self consumed. I do have an electric car but no domestic  battery. I also have 2 wall mounted ASHPs which we use quite a lot in spring, autumn and winter. In December we exported just 0.9 kWh, 3.7kWh in November and 3.4kWh in January.