Solar panels - which electricity am I using?

[bitstreams]

I have just moved in to a house with solar panels on the roof of the garage. They are an older installation and therefore, I'm told, benefit from a reasonable feed in tariff which is currently being transferred to us.

What I don't understand is how we are using the electricity they generate. There are no batteries in the setup and they have their own meter that I can see incrementing.

When I switch on a light in the house, am I using purely mains supplied electricity or do I use any currently being generated from solar, topped up by mains? How does it actually work?

 

[Murdochcat]

At a very basic level , when the sun is out your home will use the electricity produced by the PV up to the point of generation, then any more is drawn in from the grid.

Eg

Our PV is currently producing about 2.3 kW

Our home needs about 0.3 kW back ground useage

The “excess” 2kW goes to our hot water tank via an automatic divertor

If I turn on the cooker the “load” would rise to about 3.6 kW , and in this case, 2.3 kW would come from the PV and the 1.3 kW would come from the grid. The automatic divertor to our hot tank only works when there is excess generation

On winter cloudy days the output could be almost zero for a whole day

Hope this helps

 

[ericmark]

What matters is the rate you get, at the moment I get nothing for export, but export rate can be from 6p to 26p per kWh, in the early days installers were offered more for export, than they paid for import, so there was no reason to try and store it.

I don't know if the deal is transferable, latter the export rates dropped, so it was a good idea to store energy in the DHW, I have an iboost+ which auto diverts power into my water once the export exceeds I seem to remember 200 watts, as we progressed further the export rate went much lower, and often you needed to have batteries to qualify for the export rate.

So a typical solar yesterday with all those spikes it is near impossible to run anything other than immersion heater with iBoost off solar alone, to use just solar it needs some method to smooth out the spikes, so with a modern solar system, the battery is an essential part, so my use v import etc is the purple line state of charge is the one I watch, as long as over 10% then unless I draw more than 5 kW then I will not use the grid power, but to do this needs an inverter designed to do it, my inverter holds back 10% for emergency, so with a power cut my freezers and central heating still works, and I also use off-peak as well as solar to charge the batteries, it is annoying to see 4.6 kWh exported for no payment, April my contract with British Gas ends and I will move to someone who will pay for export.

Yes, a solar panel payment for export deal, typically referring to payments under the Smart Export Guarantee (SEG), is generally transferable to a new owner if you sell your property.

The original agreements were for 25 years, and you were paid for having panels, they did not measure how much they exported. It was agreed what the average export would be, and you got paid that amount.

Then they fitted export meters, and you were paid only for how much exported, and now they insist on a smart meter, not as good with just solar panels, but with batteries as well, it at least has halved my electric bill even with no export payment. But each supplier has different rules, so with British Gas we get EV rate at night, Octopus you, it seems, need and EV and EV charger. Plus it changes area to area.

 

[bitstreams]

Thanks for lots of info. I have skimmed through the documentation left by the installers and previous payments from EDF. The agreement was indeed for 25 years, of which there are 13 left and the estimate on the install was around £2300 per year, which is line with what we were told when buying the place. We have signed the transfer of the FIT agreement and it's in progress.

The system is rated at 4 KW (or 4650 kwh) - I assume that's an annual figure? and the rate is now 21p / KWh and 3.1p for export - must confess I don't understand these numbers.

Heating water during the day sounds interesting as we have two immersion heaters, but generally water is heated on demand with an oil boiler.

 

[EFLImpudence]

4,650 @ 3.1p is only £144.15.

To get £2,300 @ 3.1p you would need to export 74,193kWh which @ 4kW is 18,548 hours which is 50.8 hours per day.

 

[Murdochcat]

A 4 kw system is the maximum output at any one time which is around 16A

So if it’s a sunny day run your white goods one after the other rather than at the same time to maximise your savings

 

[ericmark]

the rate is now 21p / KWh and 3.1p for export - must confess I don't understand these numbers.

Neither do I, my import rates are 8.95p and 31.31p per kWh and export rates when we finely get it will be between the two rates, 3.1p per kWh is really low.

Across the world, the export rate has steadily fallen, at one point areas of USA import and export were the same rate, so the grid was used as a huge battery, but today we are seeing some days in the UK where we are getting near to having too much solar power.

I have an iboost+ but will admit not so sure when I get paid for export, if it will help, as in the main if using the same supplier for both import and export the off-peak supply cost is less than the export rate, so a simple time clock will save me more money.

Here the smart meter helps this shows ratio between peak and off-peak, first the kWh, then money including standing charge, and final money without standing charge. It shows 60% of my electric cost was for off-peak power, and that's February, and no off-peak heating, and no EV charging, as summer arrives the percentage will increase, so the main gain I get is due to battery, not the solar panels.

 

[bitstreams]

4,650 @ 3.1p is only £144.15.

To get £2,300 @ 3.1p you would need to export 74,193kWh which @ 4kW is 18,548 hours which is 50.8 hours per day.

Yes, but I'm also getting a FIT payment on top of that I think.

 

[EFLImpudence]

I only worked out the numbers you gave us and said you did not understand.

 

[ericmark]

I must admit, living with solar has changed my lifestyle. However, not sure how one would use them without the battery? But the direction also matters. Today bright sunny morning, but my solar panels are pointing south-west, so morning sun only giving 170 watt, this is not a problem as the battery is 63% charged, and looking forward to a bright day, I can run dishwasher etc, knowing latter the sun will recharge the batteries.

At first, we did not have off-peak power overnight, and having no solar in the morning resulted in not being able to use it until midday, with the batteries also re-charging overnight, it means if likely a nice day we can run heavy users, washing machine, tumble drier and dishwasher in the morning so by midday we have loads of room in the batteries to accept solar power.

It seems in the USA, washing machines and dishwashers are hot fill, but here they are today cold fill, so even with a tank full of free hot water, we still have to use electric to heat the water in our machines.

Out of interest, how do you know what has been generated and when? I rely on the supplier of the inverters web page to tell me what is going on, the smart meter is in the main next to useless, it simply shows zero all day, its only function is it shows when battery has run out. It did not run out yesterday, so the in home display (IHD) would have only showed any power used midnight until 5 am.

It seems the inverters have been improving over the years, so the features we have, are likely missing on yours, ours will charge the batteries, and use the stored power, plus give us a UPS supply for central heating and freezers, but does not have the ability to use a generator and mains power, it is either/or for us. The server is in China, so we sometimes see software shows importing, and the battery has loads of power, this is it seems due to the delay over the internet, and is not really happening, just some bits shown direct and some via China.

I am intrigued how people without a battery use the solar power? As hard enough with a battery to anticipate how much sun we will get.

 

[baldykev]

I have PV ,I live in Spain . we have an app from the PV suppliers that show us what the PVs are making ,what we are using in the house and what we are sending back to the grid in KWs
Also our power suppliers Octopus has an app so we can see also how much we have banked during the days , they buy it of us that credit goes into my next bill

 

[Lower]

Yes, but I'm also getting a FIT payment on top of that I think.

the feed in tarriff gives you a payment for every KW you generate and use. If your FIT payment is 21p (which sounds more like your import rate rather than the feed in rate) per KW for every KW you generate use, you get paid that for generating and consuming your own energy plus the saving on the electricity you don't import as you've generated your own. If you don't use the electricity you generate and export then you only get paid 3.5p per KW. Its obviously much more benficial to generate and use your own electricity rather than export it.

 

[JohnD]

To maximise your gain, time your usage of high power devices to run during hours of daylight, and preferably one at a time. The only ones that use high power for long periods are tumbledrier, immersion heater, electric heater, and air conditioning. The habit of daytime usage will bring you benefits without fretting.

Ovens, dishwashers, washing machines, kettles and toasters are high power, but only during the heating cycle, which is measured in minutes.

You can get an electricity monitor to tell you what you are generating or using, and this will help you to be obsessive while you get used to it. After a while you will know the moment you look out of the window.

You can get a device to divert energy to an immersion heater, and I have considered using one to power a storage heater, but mine cost around £250, and if you have a gas boiler, the amount of gas you will save in summer (there is very little solar power in winter) will not repay the investment.

 

[ericmark]

You can get a device to divert energy to an immersion heater, and I have considered using one to power a storage heater, but mine cost around £250, and if you have a gas boiler, the amount of gas you will save in summer (there is very little solar power in winter) will not repay the investment.

I have the device, mine is called iboost+ and it feed pulsed DC into the immersion heater, so would the storage heater work with pulsed DC? As to being worth it, for me, it seems unlikely, at least once I start getting paid for export.

Gas or oil boilers heating DHW it depends on boiler size and pipe run. Winter no problem as boiler not cold, but summer, it is the cost of the match, how much does it cost to heat the boiler and pipework? I would fire my oil boiler up 4 times a week, it would never run full ½ hour, the hot return water would turn off the boiler after 20 minutes, boiler is 20 kW and not modulating type, so that's approx 25 kWh to keep water warm.

The immersion heater has a display showing how much saved, in other words how much solar used, this never exceeded 3 kWh, clearly the figures will vary, size of boiler and pipes etc, but clearly I was wrong saying oil was cheaper than electric, even if not using solar power.

The CT coil is wireless connected to the main device, and one can set how much is being exported before it cuts in, had to increase the amount on mine is interfering with other devices, now set to 200 watt I think, likely could return to 100 watt, as all the tails have been renewed when smart meter fitted, there is a minium gap between CT coils.

But yesterday quite good, but this

shows in the troughs only getting 535 watt, the peak was 5278 watt, but any equipment used during the sun shine without the battery will be using peak power. Here

on 22nd of February, my used did not match solar, the battery resulted in no grid power being used until 10 pm, but without it, would not have all gone to waste. Other days by chance it does match, but my tumble drier runs for around 2.5 hours at 600 watt, getting it to run when getting solar is not easy.

 

[JohnD]

The one I bought works by clipping the peaks of the AC waveform IIRC, not pulsed DC. So it worked a fan heater (but not really suitable if supply is intermittent).

DC erodes the contacts of switches, which I think would include immersion thermostats. The pulsing should break arcs though.

BTW my gas usage in summer (excluding standing charge) is about 50p a day. Not all days are sunny, so the device would not save me much.