Solar PV vs Grid

Talk Electrician Forum

Help Support Talk Electrician Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

coulter

Member
Joined
May 12, 2022
Messages
19
Reaction score
6
Hi Folks,
I am considering having solar PV panels installed with a grid tie invertor and have a basic question that I am struggling to find the answer to:
If my PV system is generating say 2kw at a given time and I turn on say a 2kw convector heater, will the 2kw come from the PV system and 0kw from the grid or will 1kw come from the PV and 1kw from the grid?
I would have thought that the load would be spread evenly across the two supplies and if that's not the case then what causes the total load to only come from the PV system?
Your thoughts and help would be appreciated.
Many Thanks.
 
your inverter will monitor grid voltage and output a few volts higher, so like water at a higher level, energy will flow from the solar first, grid second.
 
Hi Binky,
Thank you for your reply, and yes very good illustration and explanation. However, would that not mean that the majority of the supply would run from the higher voltage or higher pressure, but yet some would run from the lower voltage or lower pressure too?
Many Thanks.
 
If there's insufficient leccy from the panels, it tops up from the grid. If you have sufficient from the panels, then it pulls nothing from the grid. If you are generating more from the panels than you need, it backfeeds into the grid.
 
Hi again folks, I decided to go ahead get a quote for a PV system and was wondering about the best location & orientation. I've got a quote for 12 x 370 watt panels and 3.6kw invertor but I'm flexible with these numbers.

I have two options and would like to know what others think;
01. House roof at 150 degrees south east, pitched at 40 deg,
02. Large garage roof running east/west pitched at 25 deg.

My thoughts are the garage roof would be the easier install as its a metal clad roof and much larger than the house roof which is tiled and has various Velux windows. If I increased the number of panels on the east/west roof would I have a system with equal output but over a longer day than using the roof on the house?

Many thanks for your advice.
 
Hi again folks, I decided to go ahead get a quote for a PV system and was wondering about the best location & orientation. I've got a quote for 12 x 370 watt panels and 3.6kw invertor but I'm flexible with these numbers.

I have two options and would like to know what others think;
01. House roof at 150 degrees south east, pitched at 40 deg,
02. Large garage roof running east/west pitched at 25 deg.

My thoughts are the garage roof would be the easier install as its a metal clad roof and much larger than the house roof which is tiled and has various Velux windows. If I increased the number of panels on the east/west roof would I have a system with equal output but over a longer day than using the roof on the house?

Many thanks for your advice.
box section profile roofing is the easist and quickest to install PV on, plus East/West will give you a spread of power over more of the day, and if you can fit a bigger system, even better. Peeing about with velux windows, is no fun and looks messy, scaffolding will also be cheaper for the garage as it's a single lift. Just make sure your rafters are strong enough.
 
Thanks Binky,
Sounds like I'm heading in the right direction so, the roof is indeed box profile on steel purlins so no issues with strength. I've just rechecked the orientation and it seems to be about 100 East and 280 West with a 25 degree pitch.

Based on the 3.6kw invertor, what max panel loading do you think I could fit in that East/West configuration? I was thinking maybe 8 or 10 x 370 watt panels on each side!

Should I be thinking of a larger invertor? The garage has an existing 6sq mm SWA supply cable but rarely has any load other than lighting.

Many Thanks.
 
Most 3.6 inverters will take 4 kw of panels, you need to check the spec as it varies between manufacturers. Undersizing inverters to 80% used to be the norm when they were less efficient, as you have a split array that will probably work well for you. If you go bigger, you would need to run a comms cable back to the house. It may be worth talking to the DNO about how much they will allow you to backfeed the grid. They may allow you to have a bigger inverter without any export limitation, which would avoid the need for the comms cable.
 
Hi Coulter

have you decided what usage you will make of all the electricity?
Will you have batteries? Do you intend to generate cash flow from selling to the grid?
In the time of FiT, one was limited to what could be done. Theses day (also considering the batteries are much better), I would have had a different approach to than in 2016.
My view is to try to generate as much electricity as possible.
If budget allows it, I would put panels on the main roof and the garage and have batteries (that you would expend in a year or two once you have master its integration (usage point of view) into your needs.

In 2016, I got the standard 16 x 250W panels and no batteries. Now I am planning to add 9 panels (in different orientation) and had 10kWh batteries.
Since I joined this forum. I am also considering 3 panels on my garage roof facing North. It will surely produce only from April til September but if they generate 50 to 100kWh a year, it is better than nothing (I won t put panels on my main roof facing north because it won t be easy to clean regularly. (not that I have ever cleaned my 16 on my main roof facing south...)
 
(not that I have ever cleaned my 16 on my main roof facing south...)
within reason, panels are self cleaning. They have a coating that degrades in UV over time, a bit like the way your skin sheds dust. In theory this makes them non-sticky, and allows dirt to wash off with the rain. However, if you have a lot of birds or trees around, they can still build up algae against the lower frame edge, so I would double check them, I'm seeing more and more dirty panels in my area.
 
Thanks Binky,

Here are some photos from 2020
1658077388117.png1658077423443.png


and some from April 2022
1658077481066.png
you can see the big thick moss that built up in 2020 has gone

1658077534021.png
Some lichen are building up but I think this is still minimal.
I did get a quote for a professional cleaning but as a full scaffol is necessary and my conservatory is in the way I thought I wait a bit long.
 
You can clearly see the algae creeping over the cells, and the lichen looks very healthy. The yellow stuff is called 'bird poo lichen'. Stop feeding the birds 😃 Definitely in need of a clean , you can usually get a window cleaner with the long poles to do the job. Alternatively you may be able to get a van mounted cherry picker, which can be hired by the hour, cheaper than scaffolding. If you did have to scaffold, get a tower rather than full width set up.
 
So far I do not notice a significant drop in production. Granted, maybe for me to notice I would need a big drop maybe!!
I did get a quote about 500 - 600 pounds. It does need a full scaffold. When I asked they asked for photos. I did explain the set up and they first thought a tower will do.

1658091691943.png

Even a scaffold will be a pain because on one side there is only about 20cm between the wall of the conservatory and the neighbour fence.

I also asked local window cleaners and they all say no. Also some think it is like like cleaning a window....

Anyway, this is the As Is situation at the moment.

For info

WhenBest Day in kWh Month in kWh
June 202227.50531.10
June 202127.20539.20
June 201928.35460.06
June 201828.43632.66


If I do lose a bit because of the dirt, it is not yet significant enough in my opinion to spend over 500 pounds on cleaning.
I do produce less but this could also be due to the natural degradation of the panels and of course the variability of the weather.
If the cleaning was 50 or 100 why not.

Here is my humble opinion :)

PS: I think we have hijacked the original topic. Sorry.. I hope I will not be kicked off by an admin :(
 
Hi Bob, welcome to the thread. I haven't went any further since with my PV solar dreams. My main thoughts are to install an air to water heat pump and make use of the electricity generated in that way. My original PV quote did include a 5kw battery at a cost of £1600 so I was thinking that putting in a heat pump might be a better option to make use of the generated power.

My attention has been diverted in the short term to the heat pump design and pricing as here in Ireland we can now get a heat pump installation grant of €6500. But I certainly do intend to refocus on the PV system later on and probably go ahead with installing a system on the garage roof as Binky has pointed out the advantages earlier. Also as we are contractors ourselves then we should get best value for money in the terms of cost!
 
Hi Bob, welcome to the thread. I haven't went any further since with my PV solar dreams. My main thoughts are to install an air to water heat pump and make use of the electricity generated in that way. My original PV quote did include a 5kw battery at a cost of £1600 so I was thinking that putting in a heat pump might be a better option to make use of the generated power.
The problem with that approach is that there will be times when you have say 200w to 4000w of excess energy but you cant run the heatpump on that so that power is lost / fed into the grid. A battery could absorb that power to be used later.
 
The problem with that approach is that there will be times when you have say 200w to 4000w of excess energy but you cant run the heatpump on that so that power is lost / fed into the grid. A battery could absorb that power to be used later.
Until the batterie is full :) even if that may not happen many days in a year or that the battery has been oversized. what happens to that excess of electricity then if it cannot be exported to the grid?
 
Until the batterie is full :) even if that may not happen many days in a year or that the battery has been oversized. what happens to that excess of electricity then if it cannot be exported to the grid?
It goes nowhere, so that capacity is lost. You could of course sign up with an export tariff and then get paid a little for it. On my system the batteries charge on off peak power until 04:30, the batteries then power the house until Solar wakes up. Once there is a surplus of Solar it charges the battery, when that’s full it heats the water using 100% of the spare energy I.e. if there’s 100w or 3000w it will use it all. Once the hot water tank is up to temp and/or there is spare energy the cars are charged and only when all of that has happened do we then export to grid.
The above priorities are set in the system and are easily changed.
It sounds complex but it just does the job nicely and hasn’t needed changing at all since the installation.
 
Thanks John

What system do you have in order to decide the priorities.
I have been in discussion with an installer and he kept telling that the water will always come after the batteries. I do not really like this as from April til September the water has usually time to heat up from the solar by the time we get up and have our morning shower.
If the batteries are charged first it will mean we will no longer have hot water in the morning (unless using our boiler on timer, I don t want that of course).
At the moment I do not know if the water could be programmed to be heated from the batteries.

Cheers
 
Thanks John

What system do you have in order to decide the priorities.
I have been in discussion with an installer and he kept telling that the water will always come after the batteries. I do not really like this as from April til September the water has usually time to heat up from the solar by the time we get up and have our morning shower.
If the batteries are charged first it will mean we will no longer have hot water in the morning (unless using our boiler on timer, I don t want that of course).
At the moment I do not know if the water could be programmed to be heated from the batteries.

Cheers
Bob,
This is easy, heat the water on off peak energy (7.5p kWh) and charge the batteries as well, of heat the water from the batteries until your panels come online.
The system I have has a Growatt inverter and battery, in the inverter i can set the battery priority over the load etc. I have a Myenergi EDDI that does the hot water driving the element at whatever level is needed and a ZAPPI for charging the car. The Myenergi hub sorts out the priorities and times between car and hot water etc.
 

Latest posts

Top