PV & Battery install next week BUT which tariff please for my set up?

ddi

I’ll be having 12,5 kwp and 2x PW3 (se facing etc.) I’m having a charger fitted but no electric car as yet. Ppl on my other thread have said my location should generate 12000 kWh a year.

Usage is VERY high esp in summer as we have newly installed pool. Pre pool it was 14000 kWh and I’m estimating it’ll be more like 19000 kWh 

So, I’m thinking I’ll be purchasing 7000 kWh a year and won’t be generating enough to export. If I’m calculating this correctly which is the best tariff please? 

Huge thanks! 

PS our heating & water is gas. I’m at home so can run appliances whenever albeit everyone home weekends and from 6pm onwards. But I can change cooking times if needed. 

QrizB

I'd look at Intelligent Octopus Flux. The maths are complicated but it's likely to be your cheapest option, at least until late autumn when the skies go dark.

Once the seasons turn enough that you find yourself regularly having to import power, look at a tariff with a cheaper overnight rate. That might even mean leaving Octopus for the winter (the cheapest open-to-everyone overnight tariff is Utility Warehouse who have E7 with a 5p/kWh night rate, but the daytime tarrrif is high).

ddi

@QrizB thank you for this. I’ve started another thread because I’m now concerned that my ‘generation’ is limited to 8 (8 what I don’t understand) and I thought I’d be generating circa 12000 kWh with a 12.5 kwp but if it’s limited to 8 does that mean I’ll only be getting 8000 kWh per year. I’m so very very confused! 

Netexporter

ddi said:

@QrizB thank you for this. I’ve started another thread because I’m now concerned that my ‘generation’ is limited to 8 (8 what I don’t understand) and I thought I’d be generating circa 12000 kWh with a 12.5 kwp but if it’s limited to 8 does that mean I’ll only be getting 8000 kWh per year. I’m so very very confused! 

You are confusing energy and power. The inverter is the limit to how much power you can export and in most cases the inverter power is less than the peak output power of the array. You'll get some clipping on "perfect" days but that tends to be countered by the inverter starting at a lower voltage on dull days. You should still get your 12MWh of generated energy over the year, assuming the installer's calculations are correct.

ddi

@Netexporter thank you. But if the inverter is limited to 8; doesn’t that mean it can only covert 8 kW DC to AC for me to use at home? Or does the inverter limitation not include home use? Is that ‘8’ what I can export? And does that ‘8’ mean 8000 kWh over the year or something else entirely please? I really need a physics degree to get my head around this. 

Netexporter

Panels reaching peak power occurs less often than you may think. Basically the middle three hours of the day in the middle three months of the year, and only if it isn't cloudy! Having a larger array just gives you, overall, more power, for longer, in any given light conditions.

In theory, you can have an asymmetric output from the inverter, so exports are limited to what the DNO allows, but self-consumption can be at the maximum output of the inverter. You'd need to read the technical specs to find out.

ed110220

If your battery and inverter are big enough to basically cover all your peak time consumption then you're probably best with one of the tariffs designed for EVs and/or batteries which offer very cheap electricity overnight and then more expensive electricity the rest of the time (which you can avoid using).

I'm on Octopus Go, got a 15 kWh battery so almost all our imports are at 8.55p/kWh, they average under 10p/kWh (we use a little peak rate electricity when we draw more power than our inverter can take from the battery, say if I have oven and hob on at the same time, you might have this issue with your setup and in any case it doesn't add much to the cost). Octopus never asked if I had an EV (I don't). EON offer even better rates (7p overnight, 16.5p export I think) and specifically say it's for users with an EV and/or battery. Can't speak for their service but the tariff is very good.

Martyn1981

ddi said:

@Netexporter thank you. But if the inverter is limited to 8; doesn’t that mean it can only covert 8 kW DC to AC for me to use at home? Or does the inverter limitation not include home use? Is that ‘8’ what I can export? And does that ‘8’ mean 8000 kWh over the year or something else entirely please? I really need a physics degree to get my head around this. 

Hi ddi. Can I possibly confuse things even more for you, sorry.

I'm no expert on the Powerwall 3 specs, but a quick look seems to suggest that they can cope with up to 20kW of PV side panels. Now it's not as simple as that, as the PV will have to be spread appropriately across the MPPT's, and that 20kW may be for more than one Powerwall, I'm really not sure.

But, here's the point, that's a DC side figure, not the 8kW AC discharge/export figure. So as PV panels produce DC, that can flow into the batts to charge them at whatever the max DC side is (possibly 20kW, but I may be wrong on that). The 8kW is the output of the inverter from DC to AC.

So for example, if the PV is producing 10kW DC, and the battery isn't full, then there will be no clipping as the battery charges at 10kW, or say charges at 2kW whilst outputting 8kW AC. If the batts are full, then the 8kW AC output would be the limiting factor both for home use, and PV generation. Though as Netexporter explains, that won't happen as much as you might think. My sister has a 10.3kWp system (no battery) that runs through an 8kW inverter, and she's getting around 11,000kWh's of generation per year, slightly above what we expected.

But regarding your point about not being able to supply the house more than 8kW, then yes that's my understanding.

Before believing what I'm saying, hopefully someone else can check/confirm my understanding of the PW3 specs.

[Also, looking at the specs, I'm wondering if you need 2 PW3's to achieve a continous 7.6kW of AC output, or I'm misunderstanding the chart.]

Qyburn

Martyn1981 said:.

Before believing what I'm saying, hopefully someone else can check/confirm my understanding of the PW3 specs.

[Also, looking at the specs, I'm wondering if you need 2 PW3's to achieve a continous 7.6kW of AC output, or I'm misunderstanding the chart.]

There are a few things not stated in that spec sheet, for example it gives maximum battery charge rate of 5kW (relevant to your clipping scenario) but no figure for maximum battery discharge.

There's no reference to export limit separate from the total AC power limit.  This could well be an omission from that spec sheet, for example one of our quotes included a PW2 configured for zero export so clearly the PW2 has some form of export limit.  It seems likely that the PW3 does as well.

Finally I note it specifies 60Hz so presumably this is the North American variant.