Solar Panel: battery ratio

[roubaixtuesday]

We're getting solar via local authority scheme.

Recommendation is for 14 panels, 450W per panel so nominal 6.3kW, and 5kwhr battery.

10 or 15kwhr batteries are available at a price.

We have an immersion heater, so will top up hot water which is another energy store.

We don't currently have an EV but will likely get one when we next change our car (and will get a charge point fitted while we're at it).

Current usage is ~8kwhr per day average

5kWhr battery feels small to me? But maybe another 5 (£1200) isn't worth it.

Any rule of thumb/ experience to offer?

[rjb]

If you haven't done so look at the calculator on the energy saving trust web site.
https://energysavingtrust.org.uk/tool/s ... alculator/

[PDQ Mobile]

My highly lack of experience ( but not lack of interest)
take on it would be get a car that also allows use as domestic storage.
And until then go for the cheaper panels.
Use the savings towards the car?

Otherwise... well it's obvious.

[Jdsk]

I asked a similar question in another place, followed the expert advice and consequently doubled the battery to 10 kW hr. But you have more peak solar power than we do, so you might be able to take advantage of even more.

I did a lot of detailed calculations in advance, but after several months' experience I think that the key points are:

1 Install enough domestic battery capacity to get you from the time that you stop generating to the time that the cheap tariff starts.

2 The battery capacity of the EV will be very high compared to your generating power or your domestic battery capacity. The big gain is here is charging on the cheap tariff. Definitely get the charger fitted as part of the initial work.

3 The next time that you buy the big consuming devices such as a washing machine or dishwasher make sure that they have built-in timers.

We were recommended not to run electric immersion heaters as energy stores, so check that calculation and ask the suppliers explicitly about this.

Jonathan

[Carlton green]

5kWhr battery feels small to me? But maybe another 5 (£1200) isn't worth it.

Any rule of thumb/ experience to offer?

I’ve no direct experience of solar but do have a few rules of thumb:
# know what you need and then (try to) buy something a bit better (to grow into and incase you’ve underestimated what you need).
# upgrading later always costs a lot and is difficult to do.
# stuff degrades, to prepare for that inevitable event build in a buffer (whenever you are able to do so).
# he who buys (too) cheap buys twice; so save up your pennies, spend a bit more on what’ll last and work, and only buy once - doing so almost always gives you a smaller total bill.
# there’s rarely a perfect answer to anything, so embrace the imperfect but still good.
Edit. # there’s sometimes a case for buying cheap to get a job done for now (well, one job or maybe a few more), but when you do that then you do so understanding that you’re putting a limited life solution in place.
^^ Those insights have been learnt the hard way …

Edit. TLDR. If you can afford then buy the bigger battery.

[roubaixtuesday]

Thanks all.

We were recommended not to run electric immersion heaters as energy stores, so check that calculation and ask the suppliers explicitly about this.

Using immersion seems like a no brainer to me, unless gas is cheaper than export tariff, which I don't think it is? But tariffs are fiendishly complex!

The quote gives typical payback 4 years for the immersion extra (vs 6 years for whole system)

get a car that also allows use as domestic storage.

V2G cars and chargers are both unfortunately like hens teeth, so it's much simpler to install standalone (and a small % of a car battery)

[Cowsham]

We have had solar for about the last 10 or so years but at that time the limit you could instal for micro generation was 4kW. The limit has now been increased to 7kW so I'll instal some more panels. I might go for more if I can get the room ( I need to strengthen the roof of a big shed first )

We recently been to the new Nissan leaf presentation show. It has V2G ( vehicle to grid ) and V2L ( vehicle to load ) At present V2G isn't available here in NI but will be soon we've been told so when it is available the car will do it.

I specifically asked if Vehicle to Load would still work ie be allowed if it's not through the solar inverter and just feeding the house and he said it would ( ie where you can run up to 3.5kW into the house eg if there was a power cut where it could replace a small generator so keeping the fridge, freezer, lights and central heating going ( we have a lot of oil fired boilers here in NI still ) . I told him about our change over switch and because we live out in the sticks there can be occasion where we are out for a few days when there's an outage and have to rely on the wee Jenny ( about 3kW )

The reason I asked is because I'm aware that there are dafties who will run a generator and plug it directly into a mains socket of the house with no changeover or pulling the big fuse which can electrocute some boy working to fix the outage and that solar inverters are designed to shut down preventing pushing generated mains back into the grid wiring and by the same token the car may be prevented from supplying the house when the grid is down.

It works something like this
-- the solar inverter is plugged into the mains to get it in phase with the grid and as a way of detecting that the grid is up and OK so feeding back to the grid is allowed.

-- I'd expect the car is very similar in that it must be connected via it's home charging point which will have the grid phase detection system incorporated and only allow feeding back to grid when the grid is up. This is why I asked the question " can I use it like a generator with V2L " to which he replied " yes "

Also feeding back to the grid when we're not using the car for a few days and charging on economy 7 would reduce our bills a bit ( when it gets introduced ).

Incidentally
The new 75kW car does around 380 +/- 5 miles on one charge and the 52 kW car will do around 275 miles to a charge. The new micra will retail from 20 grand ( range is about 270 miles iirc ) which is on a power with many supermini ic car prices now.

[Cugel]

As with everything else, its best to understand your motives for buying and using such stuff first. There's the cost motive but perhaps also the being-green motive. There can also be the motive of wanting to have resilience if the grid goes down, for weather or commercial-failure reasons.

Batteries are useful not just for storing solar but for storing cheap overnight lecky to use during the day. There are many days in Britain on which solar production is very low to zero so enough battery to store enough overnight lecky at 6p per Kwh rather than 28p per Kwh, say, is a good strategy.

If the house uses on average 8.5 Kwh a day its likely to use more than that per day on the sunless winter days (depending on your e-devices, with e-heating being the biggest factor). One easy short cut to deciding battery storage capacity, then, is to look at your biggest-usage day and assume you want enough battery storage to download that much overnight, in case the following day sees no sun on your solar panels.

A larger storage capacity than always needed is useful too for any periods when the grid goes phut. If you can ensure an UPS (Uninterruptable Power Supply) capacity in the system, a larger battery storage capacity could perhaps get you through two or even three days without the grid or solar after the winter storm downs the power poles or floods the local sub-station, needing 2-3 days for a fix. (We've suffered both in the last 20 years, although not where we currently live and so without electricity for those periods). If you become parsimonious with your battery usage during such periods, you'll be able to reduce your normal daily usage by, say, 30 - 50% but still have essential facilities.

An e-car with a larger-than-needed (for just transport purposes) battery, if used as a house battery too, is much less expensive than buying house batteries. In fact, you could regard the car battery as free house-battery storage since you pay for them anyway when you buy an e-car, whether the house also uses them or not. The rat chewing the wire (no flies or ointment involved) is the cost and availability of the inverter that can do V2H et al. Various suppliers have been saying "any moment now" about such availability, for about five years now! All the signs are that if they do ever appear, they'll cost A LOT.

But we have an e-car and the intention to use it to increase our battery storage for home use - if the inverter ever appears. If you want to do the same, make sure the car does V2H as many don't, even today.

*************
We're an ell electric house, including the e-car. We have 11 Kw solar generation, 30 Kwh house battery storage and two inverters with a UPS. We generate around 10 Mw per year and use 9Mw per year. But on the longer warmer days we export a lot of excess to the grid, some for no return, whilst on the colder shorter days we download varying amounts overnight from the grid for use next day. Because of an old preserved FiT agreement we end up:

* generating more electricity per year than we use;

* having an annual FiT payment made to us for some of what we export larger than the annual cost of downloading overnight cheap lecky when we need it.

We estimate that the capital paid to install all of the solar, batteries, inverters and UPS will be recovered in the form of free electricity and the income from the FiT exceeding the bill for the overnight lecky in around 10 - 12 years. We will need to buy new inverters after about 10 -12 years but all the other stuff will last much longer than that.

[roubaixtuesday]

As with everything else, its best to understand your motives for buying and using such stuff first. There's the cost motive but perhaps also the being-green motive. There can also be the motive of wanting to have resilience if the grid goes down, for weather or commercial-failure reasons.

Batteries are useful not just for storing solar but for storing cheap overnight lecky to use during the day. There are many days in Britain on which solar production is very low to zero so enough battery to store enough overnight lecky at 6p per Kwh rather than 28p per Kwh, say, is a good strategy.

If the house uses on average 8.5 Kwh a day its likely to use more than that per day on the sunless winter days (depending on your e-devices, with e-heating being the biggest factor). One easy short cut to deciding battery storage capacity, then, is to look at your biggest-usage day and assume you want enough battery storage to download that much overnight, in case the following day sees no sun on your solar panels.

A larger storage capacity than always needed is useful too for any periods when the grid goes phut. If you can ensure an UPS (Uninterruptable Power Supply) capacity in the system, a larger battery storage capacity could perhaps get you through two or even three days without the grid or solar after the winter storm downs the power poles or floods the local sub-station, needing 2-3 days for a fix. (We've suffered both in the last 20 years, although not where we currently live and so without electricity for those periods). If you become parsimonious with your battery usage during such periods, you'll be able to reduce your normal daily usage by, say, 30 - 50% but still have essential facilities.

An e-car with a larger-than-needed (for just transport purposes) battery, if used as a house battery too, is much less expensive than buying house batteries. In fact, you could regard the car battery as free house-battery storage since you pay for them anyway when you buy an e-car, whether the house also uses them or not. The rat chewing the wire (no flies or ointment involved) is the cost and availability of the inverter that can do V2H et al. Various suppliers have been saying "any moment now" about such availability, for about five years now! All the signs are that if they do ever appear, they'll cost A LOT.

But we have an e-car and the intention to use it to increase our battery storage for home use - if the inverter ever appears. If you want to do the same, make sure the car does V2H as many don't, even today.

*************
We're an ell electric house, including the e-car. We have 11 Kw solar generation, 30 Kwh house battery storage and two inverters with a UPS. We generate around 10 Mw per year and use 0.9Mw per year. But on the longer warmer days we export a lot of excess to the grid, some for no return, whilst on the colder shorter days we download varying amounts overnight from the grid for use next day. Because of an old preserved FiT agreement we end up:

* generating more electricity per year than we use;

* having an annual FiT payment made to us for some of what we export larger than the annual cost of downloading overnight cheap lecky when we need it.

We estimate that the capital paid to install all of the solar, batteries, inverters and UPS will be recovered in the form of free electricity and the income from the FiT exceeding the bill for the overnight lecky in around 10 - 12 years. We will need to buy new inverters after about 10 -12 years but all the other stuff will last much longer than that.

re motivation, green:economic 60:40*

So will install max panels (14) rather than any notional optimum number.

No need for UPS.

I want a simple one off installation - can't be doing faffing around with this sort of thing. V2G is a great idea, it's just not an option right now. So separate battery it is. just need to decide 5kWhr or 10. I think we'll go 10.

I'm not actually sure with the installation if you can set it to import cheap overnight eg in winter. Presumably you can. I spoke to a colleague who got an installation last year, who reckoned it's hardly worth it once two way losses from the battery are allowed for.

*Though in my experience, "expensive" and "green" are inversely correlated, at least within a technology. The less economic a solar scheme is, the less green it's also likely to be. Cost is a good indicator of the embodied carbon that you're putting into capital items, if the payback time is long.

[Jdsk]

Definitely at least 10 kW hr, and from what you’ve told us I’d probably go for 15. Check your evening consumption, as above.

I strongly recommend deciding on the tariff before finalising the spec. Have a look at Octopus with the cheap rate from 2330 to 0530. You can tweak that when the EV arrives and if your charger is supported.

Jonathan

[Carlton green]

As with everything else, its best to understand your motives for buying and using such stuff first. There's the cost motive but perhaps also the being-green motive. There can also be the motive of wanting to have resilience if the grid goes down, for weather or commercial-failure reasons.

Batteries are useful not just for storing solar but for storing cheap overnight lecky to use during the day. There are many days in Britain on which solar production is very low to zero so enough battery to store enough overnight lecky at 6p per Kwh rather than 28p per Kwh, say, is a good strategy.

If the house uses on average 8.5 Kwh a day its likely to use more than that per day on the sunless winter days (depending on your e-devices, with e-heating being the biggest factor). One easy short cut to deciding battery storage capacity, then, is to look at your biggest-usage day and assume you want enough battery storage to download that much overnight, in case the following day sees no sun on your solar panels.

A larger storage capacity than always needed is useful too for any periods when the grid goes phut. If you can ensure an UPS (Uninterruptable Power Supply) capacity in the system, a larger battery storage capacity could perhaps get you through two or even three days without the grid or solar after the winter storm downs the power poles or floods the local sub-station, needing 2-3 days for a fix. (We've suffered both in the last 20 years, although not where we currently live and so without electricity for those periods). If you become parsimonious with your battery usage during such periods, you'll be able to reduce your normal daily usage by, say, 30 - 50% but still have essential facilities.

An e-car with a larger-than-needed (for just transport purposes) battery, if used as a house battery too, is much less expensive than buying house batteries. In fact, you could regard the car battery as free house-battery storage since you pay for them anyway when you buy an e-car, whether the house also uses them or not. The rat chewing the wire (no flies or ointment involved) is the cost and availability of the inverter that can do V2H et al. Various suppliers have been saying "any moment now" about such availability, for about five years now! All the signs are that if they do ever appear, they'll cost A LOT.

But we have an e-car and the intention to use it to increase our battery storage for home use - if the inverter ever appears. If you want to do the same, make sure the car does V2H as many don't, even today.

*************
We're an ell electric house, including the e-car. We have 11 Kw solar generation, 30 Kwh house battery storage and two inverters with a UPS. We generate around 10 Mw per year and use 0.9Mw per year. But on the longer warmer days we export a lot of excess to the grid, some for no return, whilst on the colder shorter days we download varying amounts overnight from the grid for use next day. Because of an old preserved FiT agreement we end up:

* generating more electricity per year than we use;

* having an annual FiT payment made to us for some of what we export larger than the annual cost of downloading overnight cheap lecky when we need it.

We estimate that the capital paid to install all of the solar, batteries, inverters and UPS will be recovered in the form of free electricity and the income from the FiT exceeding the bill for the overnight lecky in around 10 - 12 years. We will need to buy new inverters after about 10 -12 years but all the other stuff will last much longer than that.

re motivation, green:economic 60:40*

So will install max panels (14) rather than any notional optimum number.

No need for UPS.

I want a simple one off installation - can't be doing faffing around with this sort of thing. V2G is a great idea, it's just not an option right now. So separate battery it is. just need to decide 5kWhr or 10. I think we'll go 10.

I'm not actually sure with the installation if you can set it to import cheap overnight eg in winter. Presumably you can. I spoke to a colleague who got an installation last year, who reckoned it's hardly worth it once two way losses from the battery are allowed for.

*Though in my experience, "expensive" and "green" are inversely correlated, at least within a technology. The less economic a solar scheme is, the less green it's also likely to be. Cost is a good indicator of the embodied carbon that you're putting into capital items, if the payback time is long.

My emboldening above.

The overall efficiency (Round Trip Efficiency or RTE) between supply to and from the battery inverter terminals (on battery energy storage and later use) would be very helpful to know. It likely varies between competing products so maybe it’s specified somewhere in sales literature (some of which might even be truthful).

“ It is beginning to look like an overall efficiency of 70-80% is to be expected for tariff shifting.”
https://diysolarforum.com/threads/total ... ins.74684/
I’d have thought that 70% RTE was to be expected, which makes using off peak supplied power still cheap but clearly dearer than might (incorrectly) be assumed to be the case.

Other suppliers are available.
https://givenergy.co.uk/what-is-round-t ... y-storage/


As an aside I thought Cugel’s answer was particularly useful to anyone contemplating a similar solar installation.

[roubaixtuesday]

Definitely at least 10 kW hr, and from what you’ve told us I’d probably go for 15. Check your evening consumption, as above.

I strongly recommend deciding on the tariff before finalising the spec. Have a look at Octopus with the cheap rate from 2330 to 0530. You can tweak that when the EV arrives and if your charger is supported.

Jonathan

Thanks.

Surely we can expect tariffs to change over time though, so I'd say current tariffs are only an exemplar?

[rjb]

Surely we can expect tariffs to change over time though, so I'd say current tariffs are only an exemplar?

dont rely on current tariffs. even OVO as recommended above are in dire straights and may be heading into administrstion according to several press briefings.

[Jdsk]

My point about thinking about tariffs now was that some broad features affect the specification of the hardware... the stage which you're now at. And battery capacity is an excellent example of this... the relevant broad feature being that grid electricity is much cheaper at certain times. So one of the crunch points is getting through the evening when you're not generating and are relying on the battery. That is likely to remain true even when the times and unit costs change.

And the reason that I've repeated this is that I hadn't understood it despite all of the projections and background reading. And of course once all of the gadgets are in place you have much better information on what's flowing where when than you did just before specifying and paying for the system. But of course we can share experiences and the learning curve. ; - )

Shirley

[rjb]

EV charging is likely to change the electricity demand curve as more vehicles come online and owners plug them in to charge overnight. I've seen this before in the 1970's when i started my apprenticeship with the electric industry. The National Grid were sending instructions to power stations to increase generation to meet the 10pm switch on of storage heaters where sales had unexpectedly exceeded expectations. It may only be time before overnight electric becomes more expensive then daytime. Watch this space.