Solar install, batteries, exporting and ev tariffs

[Dabz]

The inverter is the bit I understood least and now wish I’d pushed for a 5kw. On our 3.5 we frequently end up, even in winter on a sunny day, exporting to the grid where the batteries only charge at 2.5 max and the house isn’t using the rest.

I’d have thought that you (@Nickj) would be able to export enough in the summer to bank some money towards winter fuel bills. We typically get about £200 a year in export which pays the winter use.

Overnight last night I put 60kw in the car at 7p, while the batteries still had 40% in them this morning from yesterday’s sunshine. We’ve got all sorts running today and still charging the batteries, so by lunchtime we’ll likely be exporting unless I can find some washing to do!

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[f1eng]

Our 3.5kW installation with no batteries cost £10k many years ago, things are much less expensive now, despite money having continued to lose value.
I am paid for the electricity by FIT whatever that is, but with my system we can’t use the OVO charge anytime and being retired and at home all day plus a wife who wants to do whatever she wants whenever she wants having low price in the early hours of the morning wouldn’t work for us.
I just plug my Taycan and our 2 hybrids in whenever they need a bit of leccy.

I do try to do it when it is sunny since FIT pays whether we use it or not and it seems like a small win to be using the electricity ourselves.

 

[Nickj]

The inverter is the bit I understood least and now wish I’d pushed for a 5kw. On our 3.5 we frequently end up, even in winter on a sunny day, exporting to the grid where the batteries only charge at 2.5 max and the house isn’t using the rest.

I’d have thought that you (@Nickj) would be able to export enough in the summer to bank some money towards winter fuel bills. We typically get about £200 a year in export which pays the winter use.

Overnight last night I put 60kw in the car at 7p, while the batteries still had 40% in them this morning from yesterday’s sunshine. We’ve got all sorts running today and still charging the batteries, so by lunchtime we’ll likely be exporting unless I can find some washing to do!

just on the inverter bit, googled what they do eg dc/ ac conversion. Why do you say you’d wished you’d pushed for a bigger unit. Was this the dno application restriction also?

Does you current inverter restrict how quickly you can charge the batteries then? Does the same apply to exporting that you can export more with a bigger inverter and this is why the DNO applications apply restrictions?

 

[Dabz]

just on the inverter bit, googled what they do eg dc/ ac conversion. Why do you say you’d wished you’d pushed for a bigger unit. Was this the dno application restriction also?

Does you current inverter restrict how quickly you can charge the batteries then? Does the same apply to exporting that you can export more with a bigger inverter and this is why the DNO applications apply restrictions?

I didn’t do enough research, so I let the solar company guide me. They only ever mentioned a 3.5kw inverter and I didn’t really think anything of it. What it means is that the system is hamstrung at peak generation and has to dump any extra on the grid - it’s a first world problem indeed but it’s frustrating to see the batteries charging at 2.5kw when we’re generating 5kw and are away from home so can’t use the excess for anything useful. Similarly only drawing 2.5 from the batteries often means we’re importing from the grid despite having battery storage. I’m not sure why a 3.5kw inverter seems to limit charging and discharging of the batteries to 2.5kw, but my understanding is that a larger inverter would allow for more throughput

If I was having a new system then (finances allowing) I would opt for the bigger 5kw inverter, as many panels as I could fit on the roof and as much battery capacity as poss. Battery capacity being the priority - we originally had just 5.2kw and have since added an extra 2.4..even then I wish we’d gone for more but at the time I was being frugal

Panels are quite cheap now and we could fit a further 4-6 panels, but the cost of installation would be high due to scaffolding and the inverter would waste the extra capacity for 8 out of 12 months. If it was a new system I’d cover the roof which maximises winter generation.

 

[Dabz]

Here we are right now - dumping in to the grid despite the batteries not being fully charged, due to the inverter maxing out at 2.49kw in to the batteries. Frustrating!

 

[Nickj]

So the hybrid inverter suggested at 7.5kw looks ok on my quote?

 

[Nickj]

Here we are right now - dumping in to the grid despite the batteries not being fully charged, due to the inverter maxing out at 2.49kw in to the batteries. Frustrating!

when you say ‘dumping into grid’ are you getting paid by an export tariff?

 

[bn8959]

Again, brilliant info, much appreciated. I’ve had the DNO applied for. Why did they limit the inverter, and what does the smaller inverter mean to your system?

The Powerwall can export at 5kW - the DNO refused to support more than 8.6kW export (local capacity limitations). So I had a choice - 5kW inverter and cap the PowerWall to 3.6kW, or have a 3.6kW Inverter and allow the Powerwall to run at full 5kW.

If I had capped the PowerWall, then it would have limited its ability to supply the house too, plus its charge rate would also be capped. So the only real choice was to cap the Inverter.

This means my output gets clipped once the panels hit 3.6kW+, so on a sunny day I lose the peak output (my nice bell-curve outputs on a clear sunny day have a flat top at 3.6kw). It doesn't actually equate to that much lost PV energy in reality. I could add a SolarEdge DC-coupled battery to the system; this would allow that clipped lost energy to be soaked up by that battery, but in reality it wouldn't pay back, given the cost of the battery.

The PowerWall gives so many more benefits in my opinion.

Also, your below comment,

“I’m on Octopus Intelligent. 7.5p import cheap rate and 15p export”

I shouldn’t have any problem getting this set up should I? Previously I was told they wouldn’t let you have this as basically your charging cheaper and exporting higher? Do they manage/ check/ validate this?

This is allowed on this tariff. Very handy. Technically I can charge the battery at 7.5p and dump it back in the day at 15p.

Before I had an EV, the most appropriate tariff was ECO7. But this only gave a 4p/kwh export rate. So it was best to try and use everything, as there was little benefit to export - hence using the Eddi to heat the hot water.

Now with OI tariff, its better to charge the battery overnight and export the excess solar, instead of letting the battery discharge overnight and charge by solar. The Eddi is also pointless currently, as its cheaper to heat the water with gas and again export the excess solar. Similar to charging the car - better to charge overnight and not from solar!

But things change! If Gas goes up, or export rates go down...

The PowerWall is brilliant - you tell it your tariff and it optimises everything.

At one point when I was on ECO7, I had a play with being on Octopus Intelligent Export tariff (which you CANT have with Intelligent import tariff). I had to manually edit the tariff settings in the PowerWall each night, but when there were periods of 30+p/kwh export rates coming up, the Powerwall would fully charge overnight in preparation and dump out energy during that window automatically, but maintain just enough charge to see us through to when the ECO7 cheap rate started the next night. Was impressive. It just needed automatic integration to the dynamic tariffs.

Hope that all makes sense

 

[bn8959]

@Nickj There is differences between systems.

You can have both DC coupled and AC coupled batteries.

With a DC coupled battery, the battery and inverter are connected in parallel to the PV array. The battery can therefore charge directly from PV, independently of your Inverter. Usually the Inverter can also work 'backwards' to charge the battery from Grid (important feature - check you system supports it). The battery will have a peak charge and discharge rate, but ultimately you wont be able to get any more energy out of the system than your Inverter. But lets say I had PV capable of 8kW and a 5kW inverter with a DC battery, if the PV was outputting absolute peak, the Inverter would be outputting 5kw, leaving a potential 3kw that can go in to the battery. If your PV is generating 2kW and your house is pulling 4kW, then the extra 2kW can come from battery - but if your house is pulling 6kW, you'll get the max 5kW from your inverter and 1kW from Grid.

An AC coupled battery has its own separate inverter. There it can charge and discharge within its own separate limit. I have a PowerWall which can output 5KW and I have a 3.6kW inverter, so technically if my house was drawing 8.6kW it could all come from solar and battery without using grid.

DC coupled batteries tend to be more efficient, as there is one less DC->AC->DC conversion step. Plus they can soak up excess solar when output exceeds your inverter. However AC coupled have other benefits.

 

[Dabz]

when you say ‘dumping into grid’ are you getting paid by an export tariff?

we are yeh, I think we get 11p per kw for export so it’s not wasted, and in actual fact it’s better to charge the car at 7p from the grid and export the excess solar, but in an ideal world we’d be able to store more in batteries

 

[Nickj]

The Powerwall can export at 5kW - the DNO refused to support more than 8.6kW export (local capacity limitations). So I had a choice - 5kW inverter and cap the PowerWall to 3.6kW, or have a 3.6kW Inverter and allow the Powerwall to run at full 5kW.

If I had capped the PowerWall, then it would have limited its ability to supply the house too, plus its charge rate would also be capped. So the only real choice was to cap the Inverter.

This means my output gets clipped once the panels hit 3.6kW+, so on a sunny day I lose the peak output (my nice bell-curve outputs on a clear sunny day have a flat top at 3.6kw). It doesn't actually equate to that much lost PV energy in reality. I could add a SolarEdge DC-coupled battery to the system; this would allow that clipped lost energy to be soaked up by that battery, but in reality it wouldn't pay back, given the cost of the battery.

The PowerWall gives so many more benefits in my opinion.



This is allowed on this tariff. Very handy. Technically I can charge the battery at 7.5p and dump it back in the day at 15p.

Before I had an EV, the most appropriate tariff was ECO7. But this only gave a 4p/kwh export rate. So it was best to try and use everything, as there was little benefit to export - hence using the Eddi to heat the hot water.

Now with OI tariff, its better to charge the battery overnight and export the excess solar, instead of letting the battery discharge overnight and charge by solar. The Eddi is also pointless currently, as its cheaper to heat the water with gas and again export the excess solar. Similar to charging the car - better to charge overnight and not from solar!

But things change! If Gas goes up, or export rates go down...

The PowerWall is brilliant - you tell it your tariff and it optimises everything.

At one point when I was on ECO7, I had a play with being on Octopus Intelligent Export tariff (which you CANT have with Intelligent import tariff). I had to manually edit the tariff settings in the PowerWall each night, but when there were periods of 30+p/kwh export rates coming up, the Powerwall would fully charge overnight in preparation and dump out energy during that window automatically, but maintain just enough charge to see us through to when the ECO7 cheap rate started the next night. Was impressive. It just needed automatic integration to the dynamic tariffs.

Hope that all makes sense

That’s brill, thanks and mostly makes sense! The 8.6kwh export is that per day? Is there any common themes on why capacity is limited? Whilst we live in a new ish build, it’s on mainly old protected land with fairly old looking overhead power cables?! The installer did say they can refuse export completely in some cases when doing the site visit but I didn’t press on this point to understand more at the time.

 

[bn8959]

That’s brill, thanks and mostly makes sense! The 8.6kwh export is that per day? Is there any common themes on why capacity is limited? Whilst we live in a new ish build, it’s on mainly old protected land with fairly old looking overhead power cables?! The installer did say they can refuse export completely in some cases when doing the site visit but I didn’t press on this point to understand more at the time.

DNO’s can be a nightmare! They impose arbitrary export limitations and have no incentive to upgrade local equipment to support export (since they make money selling power TO you!).

The limits will be on your peak power (which will be in kilowatts-kW). They wouldn’t limit on total kWh exported per day.

As you say, export could be denied completely, but I’ve never heard of anyone with less than 3.6kW allowed.

I have seen some operators charging £1,000+ for a survey, which is ridiculous!

I wanted two Powerwall batteries and a 5kW inverter - which would have needed a 15kW export! No chance of that sadly!

its all about maintaining the correct voltage and frequency. Too much export on a local supply could drive it out of tolerance for other houses. It needs modern active power control gear in local substations to govern it all. Most places simply don’t have this.

 

[Nickj]

Thanks, so if limited to 3.6kw this is what I can export per hour?

Just been looking at the Tesla PWs, and noticed they work in power cuts. Do normal inverters/ batteries work in power cuts so no loss of power as we tend to suffer from power cuts!

 

[bn8959]

Yep 1kW for 1 hour is 1kWh
kW is a measure of power (ie how fast)
kWh is a measure of energy (ie how much).

Running ‘off grid’ requires a device to completely isolate you from the grid (great big automated switch).
The operator has to guarantee that when they turn off the power (eg for maintenance), there isn’t going to be a pesky house trying to feed back in to the grid and electrocute the engineer down the road - so it needs an isolation switch.

The Powerwall includes the ‘PowerWall gateway’ which does just that.

Other inverter manufacturers have it as an optional extra - eg SolarEdge.

I have a SolarEdge inverter but the Powerwall provides the isolation for me. This would allow me to continue to use solar power in the event of a power cut. Otherwise they have to legally shut down as soon as you lose grid power.

 

[Nickj]

Yep 1kW for 1 hour is 1kWh
kW is a measure of power (ie how fast)
kWh is a measure of energy (ie how much).

Running ‘off grid’ requires a device to completely isolate you from the grid (great big automated switch).
The operator has to guarantee that when they turn off the power (eg for maintenance), there isn’t going to be a pesky house trying to feed back in to the grid and electrocute the engineer down the road - so it needs an isolation switch.

The Powerwall includes the ‘PowerWall gateway’ which does just that.

Other inverter manufacturers have it as an optional extra - eg SolarEdge.

I have a SolarEdge inverter but the Powerwall provides the isolation for me. This would allow me to continue to use solar power in the event of a power cut. Otherwise they have to legally shut down as soon as you lose grid power.

Thanks again for the info. Did contact the solar provider and as you say for power to be maintained in event of power cut additional cabling and changeover switch required and cost of£600-£800 additional. I’m presuming this would be dependant on you having power in the batteries and during the winter months, when power cuts are likely, low solar input I’d imagine the batteries could likely be low. Not sure if £6-800 is money well spent?

Said 3.6kwh is normal minimum but would hopefully be unrestricted regarding export. Close to pulling the trigger.

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