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From: KevinJ93 <kevin_es@whitedigs.com>
Newsgroups: sci.electronics.design
Subject: Re: Grand Apagon - Electricity (not) in Spain
Date: Wed, 30 Apr 2025 11:18:40 -0700
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On 4/30/25 1:20 AM, Martin Brown wrote:
> On 30/04/2025 01:07, Don Y wrote:
>>>> Or, just solar "farms"?
>>>
>>> They can drop out entire blocks of switchgear to take a given region 
>>> or zone offline (as would happen if a fault condition trips a breaker).
>>>
>>> The big problem on a really sunny day is that an individual house 
>>> roof 4kW PV installation in late afternoon in the UK will be 
>>> potentially exporting all of it to the grid. That is about 20-30 
>>> houses worth of electricity for each solar roof.
>>
>> Huh?  A single residential PV is enough to *power* 20 homes?
>> A 4-5KW installation would barely cover the home on which
>> it was sited.
>>
>> E.g., our "average" (24/7) load is about 1KW.  Of course, that
>> neglects the peaks that we see OFTEN throughout the daylight
>> hours (night load is relatively small -- a few LED lights
>> plus my computers)
> 
> Different environment. The temporal average base electricity load of a 
> UK home in summer is somewhere around 200W mostly less unless the kettle 
> (2 mins) or immersion heater (1 hr) is on 3kW. By late afternoon the 
> immersion heater will not be in use as hot water will be fully hot.
> 
> Domestic aircon is virtually unknown here. Fridges and freezers come in 
> at about 4-500W but are very intermittent so a working average power 
> consumption for a home is somewhere between 200-300W.
> 
> As a calibration point my base load is 100W with no computers on and 
> twice that with my main box on and idle. I have a lot of electronic 
> gadgets running 24/7 too so most homes base load ought to be less!
> 
> It crosses my mind now you come to mention it that UK NESO might be 
> using US figures for domestic electricity consumption which would 
> explain why their load shedding sums went so horribly wrong.
> 
>>> They drop say 100MW of load or approx 500k houses @ 200W but with 2% 
>>> of them generating 4kW then they also drop off 40MW of local generation.
>>
>> The 200W figure is mystifying.
> 
> Why? We don't have any aircon and in the daytime so the only serious 
> loads are the fridge and the freezer for a few minutes per hour.
>>
>>> So the net load shedding is only 60MW which isn't enough to restore 
>>> the balance and then the cycle repeats until it hits the low 
>>> frequency total panic limiter. UK stopped it spreading by manual 
>>> override dropping more than the algorithm wanted but leaving a big 
>>> area without power.
>>
>> Hence my comment about dropping individual loads (cogenerators).
>>
>>> It didn't help that by the time they did that the low frequency had 
>>> put a lot of electric trains into a disabled state requiring a hard 
>>> reset by a qualified service engineer visit and at random positions 
>>> on the intercity train lines. The guys who could do that were in 
>>> short supply.
>>
>> "Unforeseen consequences".  A reason simpletons can't deal with
>> complex systems.
> 
> Key assumption was that there are never power cuts in first world 
> countries - but when there are all hell breaks loose. Why they decided 
> it needed an engineer to visit I really don't know.
> 

It is very unlikely that a house in th eUK with a 4kW system is 
exporting 4kW.

4kW of solar panels rarely produce 4kW of power, especially at the high 
latitude of the UK.

Typically PV systems are rated at by the panel output under a standard 
irradiance of 1000W/sqm with the panels at 25C. At noon with clear skies 
and the sunlight hitting the panel at 90deg the output could be slightly 
greater or more likely lower (especially in the UK with the longer path 
through the atmosphere compared to lower latitude locations).

The inverters fed from that panel are usually specified at about 80% of 
that panel rating or ie 3.2kW for a 4kW system. The inverters have 5-10% 
losses as well. It is expected that there may be some power limiting 
under some conditions but that energy loss is not usually worth the 
extra expense and idle losses of larger inverters).

At any other time than around solar noon (panel angle will affect 
precisely when) the panels will achieve maximum output) there will be 
less output.

The output will decline approximately according to the sine of the 
incidence angle although at lower angles the output drops off faster 
because of increasing path loss through the atmosphere and reflection 
losses at the panel.

In this example it is unlikely that there will be more than about 2kW 
export power once the local consumption is taken into account. Maybe 
8-10 houses if they are consuming 250W each.