Solar / Battery Storage considerations

Lounge
863

I really don’t think you should feel at all uneasy. Just crack on and take advantage. Any mixed views I have about this possibility are because (a) have shared supply of electricity on the farm and in laws opposed to smart meter (b) even if I could get a suitable tariff whenever the electricity has been free in this way or v low priced, because we have wind and solar generation here we usually have a surplus anyway so would not be able to take advantage

On the plus side the wind and solar has meant that from July to September this year the house was run effectively free with the car charging exceeding total
Imports of electricity. Not going so well in October so far with full days and little wind here…

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864

This only seems to be available in the South East and East. I’m wondering when to switch from Flux to Cosy. I’d go on GO if it weren’t for restrictions on switching back to Flux. I’ve been building a surplus from exports, even given the crappy weather since June. 1 kWh exported at peak still buys 1.65 at off peak. So I charge the battery and export the spare. But the ASHP will be needed soon so the exports will peter out.

Phil

865

New record in generation today from solar and wind combined - currently at 80.5 kWh with five hours of wind to go - and currently showing 3.5-4kW generation coming in. Probably won’t quite hit 100 but should beat 90 and almost none of it exported. Have filled my car battery; currently working on wife’s car battery; air source heat pump been turned up most of the day:m; washing machine and dishwasher been on and house battery about 70% full

Previous record in a day was 80.2. Best ever solar only was 60 in June.

Edited to add - final total generation yesterday was 99.8kWh!

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866

Good going that, near to 100 units in a day. I take it that most is coming from wind?
My pv delivered nealy 40 units at the start of june. Down to 5 yesterday !

867

I think it will be a long time before it’s matched! Yes vast majority from wind I think - haven’t got detailed metering of wind separate from solar. The theoretical max from wind in 24 hours is around 80-90 (that’s our one third share from an 11kW turbine x 14 hours. I think it must peak above 12kW maybe even 13kW as no way of explaining overnight peak of a bit over 4kW) but max total we have measured is about 220 so our share would be 73 from that. It was very sunny in the middle of the day yesterday and peak generation combined was 8kW. With wind maxing at around 4 or just over that suggests maybe 4 kW from solar at the peak yesterday

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868

Going back to the original question… with a year’s worth of data I think I have an answer to how large a battery I want; 5.8kWh or 11.6kWh.

If anyone’s interested…

I’ve got a 6kWp set of panels ground mounted, not shaded by anything and pretty optimally aligned for spring/autumn (at least that’s what the installer said and watching the sun I think it’s the case; the rising and setting point seems to ‘swivel’ during the year which I’d not noticed before).

In ‘peak solar months’, say May-September I’ve set the battery to charge to 50% capacity between 00:30 and 04:30 (Octopus Go; 9p/unit off-peak) and I’ve set the discharge period from 04:30 to 00:30.

As the day’s shorten I increase the maximum charge to 70% then 90% and start to move the discharge period to start later in the day to make sure I’m not consuming ‘mains’ electricity during the evening peak time (to avoid contributing to demand for electricity from gas/coal), my goal is to run the battery to close to the minimum I’ve set (12% capacity) at 00:30.

So… the battery size. I went with the 5.8kWh option rather than the 11.6kWh option because of the £2.5k additional cost (total cost of my system ended up at £11k).

From about mid-march to mid October, the 5.8kWh battery has been enough to run the house so I barely use any peak time electricity (say less than £5 a month).

From mid October-mid March I’m increasingly using peak time electricity with the maximum of around £50 a month in December.

image

Given the recent changes from Octopus there’s now only 1p difference between my export payments (8p/unit) and the off-peak cost (9p/unit) so any saving through charging my battery with excess solar production rather than exporting it and later buying off-peak disappears ‘into the noise’.

In the longer sunny summer days the battery can go without any mains charging overnight if we don’t cook after dark.

So, for the larger battery to make sense it has to be from moving peak consumption to using off-peak electricity via the battery in the shorter days (and for ease of calculation I’d assume 100% efficiency which won’t be the case).

So… from Mid October to Mid-March I used about £180 in peak time electricity, if I could have bought this off-peak I’d have saved about £130 or so.

The battery is rated for 6000 cycles so assuming that’s one a day it’s about 16 years (which I think may be optimistic) so for £130 a year saving over 16 years it’s still less than the cost of the battery.

So that’s my conclusion; the smaller battery made more sense.

Glad I purchased the smaller battery option since it makes my ‘system’ more usable allowing me to ‘buffer’ usage during the day; e.g. if I’m charging my car with the 3kWh mains charger or using the oven/washing machine/dishwasher/tumble drier, then if a cloud blows across the sun it’ll use the battery to supplement the solar until the cloud’s gone, then it’ll return to using 100% solar and recharging the battery with the excess.

Were I buying again I’d get the same sort of system (the only difference is I’d get a miEnergy device for water heating rather than the iBoost device I have because of the data feed and potential to include a car charger!)

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869

Stupid question…. what is the benefit of these mienergy devices over just turning on the immersion manually or by using a timer?

870

Because they use spare solar energy to heat the water, During the summer my water is usually heated to max by lunchtime by solar when sunny.
So in effect costing me nowt and getting 65p a KWh for generating it.
Over the last 12 months we reckon our FIT payout has paid for all our energy gas and electric.

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871

As said above; it’s using excess solar to heat water before I export (thus saving me a whole penny a kWh! (since I heat water off peak at 9p/kWh vs exporting power at 8p/kWh)). I prevaricated about solar for too long to get the 65p/kWh for generating (that would have got me over £3000 over the last year… blimey!)

They also heat in increments so it can supply 500w of power rather than the 3.2kW if you just switch it on at the mains. Often a solar system’s only delivering a few hundred watts more than the house is using so this can be used to slowly warm the water.

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872

In my case my diverter can divert anything from 70Watts up to the 3KW Max, and it can really add up, especially in winter. So worth checking the small print of the various devices available.

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873

Got it👍

874

Another month rolls by and it’s our first calendar month with both solar and wind all properly integrated including the garage usage (car charging, a freezer and heating for the cinema room)

Total consumption 1700 kWh, total car charging 500 kWh, total imports about 700 (generation (solar and wind combined) was about 1,000 kWh) so home imported consumption we have to pay for was 200kWh. That’s air source heat pump, lights, hifi, tv, ludicrously thirsty electric range oven (about 400 kWh per month on its own) etc

A year ago one month’s house consumption was about 800kWh so with solar and the battery added we are saving 75% electricity consumption in the house compared to a year ago

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875

I’m still confused over how these work. We have our hot water on a timer, so only heating from 10am to 5pm all year round, when solar generation is high enough to cover the usage, and a morning top up from 6am to 7am on the low night power rate. Being a regular hot water cylinder, when the timer is on, it will draw power and heat the water as soon as you draw hot water from the cylinder irrespective of whether it’s sunny and you have enough solar power generation.

If you have one of these efficient devices, what’s difference? If you draw hot water, isn’t the tank going to heat up again anyway, either from the solar or by importing power. Presumably it doesn’t delay heating the water until there is solar production, as you run out of hot water on a poor solar day? Or does the device effectively heat the water to a higher temperature before exporting power, overriding the safety thermostat?

876

No. My normal way of heating water is gas which is set to come on late afternoon for a hour or two, if it’s needed.
If it’s sunny the immersion heater is using surplus solar power to heat the water albeit it at 0.5Kw or 2.5Kw. Depending on what’s being used elsewhere.
Since I installed it I’ve saved 4666 KWh. That’s cost me nowt
On a sunny day the water’s usually heated by lunchtime. The afternoon I then use solar to charge the car.
Hope that explains it a bit better

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877

Yes, that makes more sense. With an older electric hot water setup, I’d need to tailor that some way but I know our solar guys view was that it was not worth it for our situation. Ideally, I’d like a heat pump hot water system, but the costs don’t add up for that being worthwhile now that we have solar.

878

We are very happy with the solar diverter heating our water. We are out in the sticks so don’t have gas and rely on oil for hot water and heating. When we are generating surplus solar, rather than exporting it we use it to heat our water via an immersion heater in our tank.
Before we installed the solar we used the oil boiler for 40 minutes early in the morning every day and that kept the water hot enough for normal use. Very occasionaly we would put it on for half an hour if there were a lot of guests showering or bathing. We never used the immersion heater.
With the the solar diverter we hardly ever have to resort to oil. We have a modest 3.2 KW set up but we only need an hour of decent sunshine to top the water up. During October we used the boiler for heating water twice for a total of about 1 hour. Another useful feature is that we can turn the diverter off when we are away but turn it on remotely a day or so before we return as it is controlled via an app.
Basically we are saving a hell of a lot of oil in exchange for a small reduction in solar export income.

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879

That can happen, particularly if you get a couple of cloudy days together. In my case, the central heating will take the water up to 50degrees, but if you dont have gas, these solar diverters tend to have a boost option where it will send 3KW’s to the heater for an hour

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880

Ideally your hot water should be heated to +60c at least every 2 days to make sure Legionella bacteria cannot breed in your water. You do not want to get Legionella.

881

Absolutely, and I set my Immersion to just over 60degrees, to ensure maximum storage of heat from the sun, plus it means it helps you when you get a following cloudy day.

Saying that I have read that the +60c is far more important in stationary water, and not so much when it’s moving. So particularly watch out when you leave your house for a few days with heating turned off.

NB I do know someone who died in hospital with legionnaires when the nurses hadn’t been carrying out the flushing routine correctly, so please check all this out this yourselves.

882

Unless Legionella comes through the water supply I don’t see how it could start breeding. I do a periodic 60c immersion run as the ASHP only takes it to 48c as that is warm enough for two days of hot water.

Phil