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Discussion Starter #1 (Edited)
I´d be interested to hear about other´s solar systems, as well as thoughts on ours, for those that are into solar.

Changed my mind
I originally decided against solar thinking it would not be economical since a) we are renting, so may have to pay for the cost of moving the structure and b) we cannot sell to the grid. I bought carbon offsets instead.

But I changed my mind. Offsets don’t work on their own, we have to stop using fossil fuels as well. I also think our solar panels might inspire a neighbour or friend to do the same.

We also have regular power cuts so the system helps us with that.

I also have an EV now and I work from home, so I can charge the car whenever it is sunny, which makes the problem of not being able to sell to the grid relatively less of an issue.

Specs
In recent years we used (from the grid, before buying solar and an EV) 2.7 MWh/yr (family of 4). I anticipate +1.3MWh/yr for the car`s home charging, so 4 MWh/yr demand now or 11kWh/day.

The system is ten 325W panels (Risen) so 3250Wp. We have the system pointed due north (I am in Chile), so we can get about 5.7 MWh through the panels per year, perhaps 5 MWH after allowing for system inefficiencies. That is about 70% more than if the panels were in England, and about the same as Los Angeles and Casablanca which are at the same latitude as us.

The projection from the Chilean website Explorador Solar is we`ll get an average 10kWH per day in June/July and 17kWH/day in December/January/February.

I guesstimate about 1 MWh/yr we will have to use grid for (mainly due to cloudy days). That would mean we only use 3 MWH of the solar and could waste 2 MWH/yr if we don´t make changes.

Using the Extra 2MWh
Could take extra trips in the car.

Put in another cooling fan we don’t really need just to feel ice cold in summer.

Kettle for washing the plates and shaving instead of gas on a sunny day; add another kettle´s worth to a vacuum flask for the evening or the following cloudy day.

Using the kettle to put pre-boiled water into the pans on the gas hobs at lunch time on a sunny day. This may make more sense than an electric cooker which would just add to the expensive grid electricity (39% coal) in winter and in evenings.

We have thought of the idea of connecting the solar to the hot water for showers but this is a rented property so there is a limit to how much we want to do when we may have to move out before recovering the investment. Also, there is no hot water tank; water is heated up on demand which doesn´t work as well with solar. But I might have a think about a solution for showers/washing.

We can use the electric heater in some sunny mornings to save on using the gas/heating oil ones, but obviously heating need doesn´t match well to solar availability, so this is a pretty limited option.

I did wonder if we could sell electricity to others somehow, but I can´t think of a practical idea about how to do that. I don’t think any of the neighbours has an electric car.

Energy storage
We have 4 Utracell sealed lead acid UCG 100-12 batteries (4.8kWh) but the last 25% is not used to protect the life of the batteries, then there are losses, so may be nearer 3kWH. We´ve had days where the battery is full in the afternoon and a little energy is wasted, but still ended up spending 1-2kWH overnight. I asked the installer to come back and add 2 batteries more but he said it has to be 4 which isn´t worth the spend.

I did wonder if you can get a plug-in battery of a decent size (few hundred WH to a kWH): i.e. plugs in to house socket to charge and you can unplug the battery from the socket when full and then plug perhaps the TV and set top box and a charger into it later in the evening or on the following cloudy day but so far when I googled it I couldn´t find a simple plug-in solution with no installation. But might have another look later.

I also wonder if you can buy something like a plug-in device with no installation that you could store hot water in. Something like a kettle but larger (maybe 5-20 litres) and very well insulated and running at a lower wattage (again: few hundred watts to a kW). Didn´t find anything on the first google search.

To try and reduce grid spend a little overnight, I´ve actually been charging up my computer and phone in the day time only (I work from home), and charging up my kindle and spare computer only on sunny days. I even had the idea the other day to turn down the fridge and freezer temperature a degree or two on a sunny day, and turn it back up again in the evening!

Cost
About £4000. It was actually slightly higher because we put a separate structure in the garden that can be demounted more easily and because the owner was worried about the roof. However, had it been a roof installation, it would have been a little under £4000.

I estimate this will reduce our annual electricity bill from £560 to £140, for a saving of £420. We should also be able to use more electricity to reduce on use of gas for hot water and heating, guess £80 saving per year, total £500.

That gives us a best case payback period of 8 years (if no battery replacement or other coponents or costs in 8 years) however realistically it will probably be worse since at some point we will have to pay for moving the system to another house. There is also a chance we will have to sell the system second hand at some point for a low price, if we move to another country or somewhere we can´t install solar. So the payback is between 8 years and never.
 

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My 4kW max system has a Solic 200 diverter, so a current-clamp detects when I'm exporting to the grid, and tries to put as much power into a separate circuit off my Consumer Unit box that goes to my immersion heater & hot water tank. Here in UK we can get under-sink water heaters like this one
Kitchen Water Heater
and I would think it's not too hard to connect one of these to a Solic to act as a miniature immersion tank in effect. The Solic does a variable power output to whatever's connected, so anything from 0W to 3kW here in UK will be diverted. Depending on what other smart systems you have, looking at the exported electricity & recharging batteries, this particular diverter might not work well with that, as the two might fight for the exportable electricity. But others may be more configurable; I just accept that the Solic has priority; if I really want the car to charge I'll turn that on anyway, as the Solic will adapt itself and simply wait until later.
 

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Installed solar 2 and a bit years ago after buying the i3 ... next door neighbour now has solar + power walls + Model 3P :). 2more model 3's in our road now, 1 round the corner.
I've had a lot of dog walkers stop and ask me questions while I'm on the drive washing the car, hopefully some more will be persuaded to make the jump, good for you
 

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Discussion Starter #4
I like the idea of solar with a hot water tank, although not sure if it's the right solution for us at this time.

Anyone else out there want to share what solar system you have?

Also, if anyone has good knowledge of batteries, I might have a question or two?
 

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Is it 1/2 full or 1/2 empty
To me you seem to be in a good position using 3MWh PV out of a demand 4MWh. Here in the UK few people manage to achieve 50% self use brought on by the fact that the ratio of winter/summer output is like 10:1 . The EV has helped a little.
It is possible to use smart plugs to switch things on/off in line with PV and this could be useful in winter months.

You need to use electricity as that and not turn it into low grade heat. Have you the space to fit a Heat Pump (HP) / HW storage combined unit and plumb in to the house system or fit seperate plastic pipes.

In summer you are always going to have excess PV that is the price for having sensible amounts at other times of the yr.
 

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Discussion Starter #7
Is it 1/2 full or 1/2 empty
To me you seem to be in a good position using 3MWh PV out of a demand 4MWh. Here in the UK few people manage to achieve 50% self use brought on by the fact that the ratio of winter/summer output is like 10:1 . The EV has helped a little.
It is possible to use smart plugs to switch things on/off in line with PV and this could be useful in winter months.

You need to use electricity as that and not turn it into low grade heat. Have you the space to fit a Heat Pump (HP) / HW storage combined unit and plumb in to the house system or fit seperate plastic pipes.

In summer you are always going to have excess PV that is the price for having sensible amounts at other times of the yr.
The winter to summer ratio of available PV energy here should be about 1.6:1 or 1.7 to 1.

I'll look into (google) smart plugs.

I do like the idea of heat pump but if it's an investment in the thousands which I think it is we will defer until if and when we own as we're already close to the limit of capital investment that we want to do in relation to a rented house.

I have been using electricity to heat one room (my office, I work from home) and it works quite well on the minority of winter days that are fully sunny in the morning. I have two heaters one that maxes out about 500W and another with two options that correspond to about 850W and 1700W so I can at least use the spare amounts on those days.
 

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Discussion Starter #8 (Edited)
Very happy to. You may also find our battery blog useful: PowerBanx Archives - Tanjent Energy
Thanks for offering to help. I'll bold my main question and let me know if you have any thoughts on the rest of it.

For the 4 Utracell sealed lead acid UCG 100-12 batteries, you can select in the inverter the minimum voltage of which the lowest option is 44V (which appears to correspond to about 25% capacity when the battery is not charging and loads are low).

I assume that is sensible anyway to keep the batteries always at 25%+ to improve the lifetime. Although I had a couple of cases when the battery hit 44V a few minutes before sunrise and started charging wastefully from the grid even though the sun was about to rise into a clear sky. I assume it wouldn't be too bad to let the battery go below 44V occasionally in those cases. However, I can't see a way to use the battery below 44V apart from actually flicking the rocker switch to disconnect from the grid?

So once it falls to 44V overnight, this then triggers the inverter/controller to start charging the batteries from the grid.

So there is another menu option where you select a voltage where the battery, once it has achieved that voltage, will stop charging from the grid and return to using the battery. The lowest option is 48V. However on this option, the battery will just charge up from 25% to perhaps 30% or 35% capacity, and will be pretty soon back to 44V/25%. Is that bad for the life of the battery, to be constantly cycling between 25% and 30% - potentially many times between sunset and sunrise? Would it improve battery life to set it to charge up to 50% or so, so it will only go cycle down to 25% once or twice every 24 hours?
 

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However on this option, the battery will just charge up from 25% to perhaps 30% or 35% capacity, and will be pretty soon back to 44V/25%. Is that bad for the life of the battery, to be constantly cycling between 25% and 30% - potentially many times between sunset and sunrise? Would it improve battery life to set it to charge up to 50% or so, so it will only go cycle down to 25% once or twice every 24 hours?
That doesn't sound healthy. From a quick look at the cycle life (https://www.mobasolar.com/wp-content/uploads/2018/04/Battery-UCG100-12-min.pdf) you want to be keeping well above 25% or even 30%.
 
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That's quite frightening. It strikes me that more batteries would be a good investment.
 

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Discussion Starter #11
Perhaps I will usually set the minimum voltage at 45V (around 30%) rather than 44V (around 25%) then. I also noticed that when I chose 44V it actually starting beeping low battery on the inverter and this came up as a fault message, although nothing seems to goes wrong because around the time of the fault message it hits 44V and starts charging up anyway.

If it goes from near 100% in late afternoon down to 30% in the evening and then moves between 30% and 35% 5 times after dark (late evening, overnight) before going back to near 100% by 11am the next day is that 5 cycles used up in the above graph in terms of lifetime? Or is it more like 1 cycle?

I asked the installer to come back and add 2 batteries more but he said it has to be 4 and quoted about £720 for installing four more. I wonder if there is a cheaper solution because I don't really want to spend that amount and I'm not sure you'd ever see that money back. As I mentioned before, I did wonder if you can get a plug-in battery of a decent size (few hundred WH to a kWH): i.e. plugs in to house socket to charge and you can unplug the battery from the socket when full and then plug perhaps the TV and set top box and a charger into it later in the evening.
 

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If it goes from near 100% in late afternoon down to 30% in the evening and then moves between 30% and 35% 5 times after dark (late evening, overnight) before going back to near 100% by 11am the next day is that 5 cycles used up in the above graph in terms of lifetime? Or is it more like 1 cycle?
1 cycle is a full 100% discharge, so that's just 1 cycle (5 partial cycles).
 

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Discussion Starter #13
OK, so then my assumption is having it constantly moving between 30% and 35% for example (charging from grid on the way up and providing load on the way down) is not going to do major damage to battery lifetime. It's more how low it goes.

Can I think of 100%--->30% (and back) once as having a similar effect on lifetime of battery as going from 40%---->30% 7 times? Just very roughly? Or maybe it doesn't work that way?
 

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OK, so then my assumption is having it constantly moving between 30% and 35% for example (charging from grid on the way up and providing load on the way down) is not going to do major damage to battery lifetime. It's more how low it goes.
It's more how low it goes, but 30% is quite low.

Can I think of 100%--->30% (and back) once as having a similar effect on lifetime of battery as going from 40%---->30% 7 times? Just very roughly? Or maybe it doesn't work that way?
No, more degredation occurs going from 40% to 30% than say 80% to 70%. There are no simple calculation that you can do to model the damage of repeated partial cycles onto full cycles.
 

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Discussion Starter #15
ok thank you both for your help. probably makes sense to wait a while and see if any ideas come to me.

at the moment the batteries are going to the low of say 30% and being charged up every day. i have only had one day when there was zero grid used in a 24 hour period since the system was turned on 6 weeks ago.

i think i need to see how it changes after things return to normal after COVID. at the moment there are 4 of us here constantly. once the kids go back to school and their bed time is earlier the will be a little impact.

also will need to see how things change when we go into the autumn/fall as currently in mid winter; see whether the longer days and the battery in a higher ambient temp make any difference.
 

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I asked the installer to come back and add 2 batteries more but he said it has to be 4 which isn´t worth the spend.
I think that you might find you review this once you get to the Summer and see all of the wasted energy and also consider the desirability of reducing the depth of discharge on each cycle.
 

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Discussion Starter #17
Perhaps. My expectation for summer is that we will be spending very little on grid electricity, and that extra batteries will cause very little saving. If I`m wrong, we might change our mind about getting the extra batteries.

Solar can be a complicated business, there is a lot to consider to avoid getting it wrong. If anyone reading this is thinking of doing it I´d recommend to do your research properly, or be very careful about who you have install it for you, or both.
 

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Discussion Starter #18
That doesn't sound healthy. From a quick look at the cycle life (https://www.mobasolar.com/wp-content/uploads/2018/04/Battery-UCG100-12-min.pdf) you want to be keeping well above 25% or even 30%.
Having watched the system more closely since I last posted and taken readings and done back of the envelope calculations a part of the reason that the batteries are constantly going down to 30% at night is that they rarely get much above about 80% in the first place. Even on very sunny days when the whole day is sunny and the batteries are already up to over 75% in the morning or middle of the day, and then there was another several kWH of unused energy after that in the afternoon, the batteries seem to only take a fraction of a kWH of it and top out around 80% or so.

I found from google that charging efficiencies can crash to 50%-60% for a state of charge of around 80%. However once it gets to 75%-80%, the battery seems to be performing much worse even than that. It is almost like it can´t go beyond 80% at times.

Does the document above you shared have the explanation for this? Can I interpret the below (copy and paste from your link), of 10 hours in relation to a 100AH capacity as meaning it must take at least 10 hours to charge from 0% to 100% (no matter what the input wattage from the panels)? Is that what that means?

104.0 AH/5.20A(20hr, 1.80V/cell, 25°C / 77°F)
100.0 AH/10.0A(10hr, 1.80V/cell, 25°C / 77°F)
88.0 AH/17.6A(5hr, 1.75V/cell, 25°C / 77°F)
76.2 AH/25.4A(3hr, 1.75V/cell, 25°C / 77°F)
63.8 AH/63.8A(1hr, 1.60V/cell, 25°C / 77°F)


Can we make interim/intermediate calculations here as well - if it says 76.2AH is 3 hours and 88AH is 5 hours so can I read that as it will it take 2 hours to charge from 76.2AH to 88AH? Not sure if that fits with my experience though. It seems in my experience that this takes longer than 2 hours, if it happens at all.
 

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Having watched the system more closely since I last posted and taken readings and done back of the envelope calculations a part of the reason that the batteries are constantly going down to 30% at night is that they rarely get much above about 80% in the first place. Even on very sunny days when the whole day is sunny and the batteries are already up to over 75% in the morning or middle of the day, and then there was another several kWH of unused energy after that in the afternoon, the batteries seem to only take a fraction of a kWH of it and top out around 80% or so.

I found from google that charging efficiencies can crash to 50%-60% for a state of charge of around 80%. However once it gets to 75%-80%, the battery seems to be performing much worse even than that. It is almost like it can´t go beyond 80% at times.

Does the document above you shared have the explanation for this? Can I interpret the below (copy and paste from your link), of 10 hours in relation to a 100AH capacity as meaning it must take at least 10 hours to charge from 0% to 100% (no matter what the input wattage from the panels)? Is that what that means?

104.0 AH/5.20A(20hr, 1.80V/cell, 25°C / 77°F)
100.0 AH/10.0A(10hr, 1.80V/cell, 25°C / 77°F)
88.0 AH/17.6A(5hr, 1.75V/cell, 25°C / 77°F)
76.2 AH/25.4A(3hr, 1.75V/cell, 25°C / 77°F)
63.8 AH/63.8A(1hr, 1.60V/cell, 25°C / 77°F)


Can we make interim/intermediate calculations here as well - if it says 76.2AH is 3 hours and 88AH is 5 hours so can I read that as it will it take 2 hours to charge from 76.2AH to 88AH? Not sure if that fits with my experience though. It seems in my experience that this takes longer than 2 hours, if it happens at all.
Sorry to hear that this isn't working for you.

The ratings from the spec are the discharge capabilities for the battery charged to different voltages, for example the 104 AH battery rating is designed to release 5.2 Amps for 20 hours but at 1.8 volts per cell etc.

To charge it you need to provide a sufficiently high voltage and I wonder whether that is the issue - that your charging voltage is not sufficiently high to actually charge the batteries beyond 80%? So simplistically put not much beyond 1.70 volts/cell rather than the 2.25 volts/cell for float or even 2.4 volts to achieve that more quickly before reducing. If you look at the spec it shows how the charge should occur (sadly from only two points of discharge - 50% and 100%)

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