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QrizB

You'll find many forum members (including me) will point to PVGIS as the best publicly-available model of solar generation for the UK:
https://re.jrc.ec.europa.eu/pvg_tools/en/

Deleted_User said:

We, without really trying, tend to settle at 5kw of electricity a day. We can reduce that, but 5kw is comfortable.
We have a 2kw solar array and in the summer that's more than enough.

I don't know exactly where you are, but in the centre of England (near Oxford) PVGIS suggests that an optimally-aligned 2kWp array will generate this much electricity:

If the owner of that array needed 5kWh/day, 150kWh/month, you'll see that for roughly half the year, on average, they wouldn't have enough and will need another source. (I seem to recall you posted that your solar panels generated an average of 1.5kWh/day last month. This seems quite low, considering that October 2022 was a month where pretty much everyone on this forum out-performed PVGIS by a substantial margin. PVGIS suggests the array in Oxford should generate over 4kWh/day.)

But - do be careful looking at monthly averages. Particularly in the winter, solar power comes in lumps. Here's a graph of the output from my solar array in winter 2021:

As you can see, there was a period of more than a fortnight where my array rarely managed 1kWh/day. PVGIS suggests an average December should see almost 3kWh/day from my array, but if I'd been relying on meeting that average during December 2021 I'd have been out of luck.

It's often been said on this forum that no-one ever complains of having too much solar panel. If you're going off-grid, that might be less of a truism but even so, it will be interesting to see how your first winter goes.

Reed_Richards

When you say you have a 2 kW array you mean an array with a peak output of 2 kW.  This is sometimes written as 2 kWp.

Power is measured in Watts but energy, which is what the electricity company charges us for, is (in this context) measured in kilowatt hours (kWh).  So if you have a fridge that runs on an average power of 100 W it will use 100 x 24 = 2.4 kWh in a day.  So when you write that you were using 1900 W per day on average in October I presume you mean 1.9 kWh per day (or an average power of about 79 W).

Because you are not always using the correct units and don't clearly distinguish between power and energy, I lose the thread of your argument as it progresses. 


      

Raxiel

We have a (new) 3.6kWp solar array, and no storage. We use on average 6kWh a day.

On the gloomy days we've had over the last few weeks, its been enough to generate a couple hundred watts over most of the daylight hours, enough to run my WFH setup (CAD spec laptop and two extra screens) as well as the other background usage like the router and fridge. If we had a smaller system it would probably still have to import.

Perhaps the inverter isn't being as efficient in these low light conditions, but I'm not sure why that would matter?

Sure, if I was running off a generator I had to fuel it would, but for solar that energy falls on the roof whether I capture it or not.

Alnat1

For many it's as simple as the excess generated in summer which goes back to the grid, pays for the shortfall in winter that you import from the grid.

Boffinboy24

Scarter, I think you are potentially confusing people for a couple of reasons, 1) as Reed mentions you are using two different types of unit interchangeably. kW and kWh. 1 kWh is equivalent to running an appliance pulling 1kW for 1 hour, or an appliance using 500W for 2 hours. You arguments become very hard to follow when you constantly refer to Watts, which is not what you mean in many cases. 2) I believe elsewhere you have said your solar kit is not grid tied. Most people on the forum will have or will purchase a grid tied setup. Your discussions re: inverter running overnight to power fridges etc and efficiency are therefore not relevant to most folks, their solar inverter won’t be running low load appliances. Perhaps I misunderstand your setup, apologies if so, just wanted to clarify why people may be confused by your posts. 

Reed_Richards

Deleted_User said:

I have already made this clear - I am not talking about kwh at all. I am ONLY talking about kw/watts.

No really you are not.  Referring to the number of Watts per day is no more meaningful than saying that a car does 50 mph per day.  Watts is to energy as speed is to distance.

If a car travels 50 miles in an hour then its average speed is 50 miles per hour.
If a freezer consumes 100 Wh per hour then its average power consumption is 100 W

The S.I. unit of energy is the Joule.  A Watt is a rate of energy consumption of 1 Joule per second.  It would be much less confusing if we were billed for our energy consumption in Joules (or Mega Joules since 1 kWh = 3.6 MJ).  But, sadly, we are not and the result is a lot of mistakes and confusion.   
    

ABrass

Deleted_User said:

If I run a low draw, long duration appliance from solar much of my generated solar will be used to power the inverter (i.e. wasted).
If I run a high draw, short duration appliance from solar almost all of my generated solar will be used to power my devices.

In the winter months where there's little sun optimizing efficiency will yield better results than even doubling the size of your solar array. So make sure all your collected solar is powering appliances rather than powering the inverter. 

This is literally the opposite of reality.

Your solar array is 2kW. In perfect conditions it can't run a kettle. And you do not get perfect conditions that often.

Also, the inverter losses you're describing would happen equally if you're using power or not. Keeping the inverter on uses power, if there's sunshine it powers itself, if there isn't then you draw from the grid to keep the pretty lights on. That doesn't change depending on your load.

You need to recheck your assumptions here.

Boffinboy24

Sorry scarter, now you misunderstand my point re: low loads. Of course low loads are being run, such as lightbulbs. But at night the solar inverter will not be operating because there is no sun. The low loads will run from the grid coming in as AC. Inverter efficiency is irrelevant. If you run a large load like a kettle that exceeds your incoming solar coming in (easily done with a small array and imperfect sun) then that will draw from the grid for the excess. These were the points I was making, that for most setups it doesn’t matter. Perhaps you are talking about an off grid system, or one that is running off battery storage or something. Sorry if I’ve missed that. The reason I mention all this is you may confuse others too. 

Reed has already explained the other point. When you say it generated 500 watts a day on average in December, what do you mean? You say “per day” which suggests a time based total which is why we are asking about watt hours, and may be the cause of confusion. I assume maybe you mean whilst the sun was shining your solar was producing an average 500 watts until the sun stopped shining (though a lot more in the middle of the day and a lot less on the tails)? Anyways, will leave this now. Was just trying to be helpful to other readers as I thought it was an interesting but potentially confusing thread with what sounds like an unusual setup. 

Magnitio

I suspect there is little point continuing this debate. Life's too short!

Martyn1981

Deleted_User said:

Boffinboy24 said:

Scarter, I think you are potentially confusing people for a couple of reasons, 1) as Reed mentions you are using two different types of unit interchangeably. kW and kWh. 1 kWh is equivalent to running an appliance pulling 1kW for 1 hour, or an appliance using 500W for 2 hours. You arguments become very hard to follow when you constantly refer to Watts, which is not what you mean in many cases. 2) I believe elsewhere you have said your solar kit is not grid tied. Most people on the forum will have or will purchase a grid tied setup. Your discussions re: inverter running overnight to power fridges etc and efficiency are therefore not relevant to most folks, their solar inverter won’t be running low load appliances. Perhaps I misunderstand your setup, apologies if so, just wanted to clarify why people may be confused by your posts. 

I have already made this clear - I am not talking about kwh at all. I am ONLY talking about kw/watts.

1 KW = 1000 watts. 

Sorry mate, but I think you are talking about energy (Wh's) not power (W's).

For instance you've repeatedly stated that you use on average 1,900W per day. Well that's power not energy. You might be using 45.6kWh per day (at an average power demand of 1.9kW). So I think you mean you use an average 1.9kWh per day. That would be an average power demand of 79W through the day.

With regard to diminishing returns, if you are grid-tied like the vast majority, then almost always better to go as big as possible, within reason. There are a lot of fixed costs with PV, such as scaffolding, AC side wiring etc etc, so going bigger with more PV panels, and extra roof racking gets proportionately cheaper. And adding more later is vastly more expensive, as costs need to be duplicated, such as scaffolding, and if the inverter is too small, an additional one will be needed.