Adding battery to existing solar PV system - any thoughts/advice please.

QrizB

EricMears said:

Fortunately we've had very little snow this week but from past experience when there's an inch or more snow on the panels generation will be zero and there'll be no melting effect.

I've occasionally wondered whether, when that happens, I could feed power into the panels to warm them and melt off the snow. A bit like what happens if a panel is shaded.

So far I've not attempted it!

JKenH

I have today switched tariff to Octopus Go which offers five hours off peak at 8.5p and the remainder of the day at 29.37p with a standing charge 9p lower. I had always been planning on doing this once our Christmas visitors ended their 3 week stay (when it would not have been economic) but a couple of things prompted me to get on with it. Firstly my NetZero app subscription was up for renewal on 9 January and at £6.99/month I decided this wasn’t good value for me. Most days I don’t import anywhere near the 13.5 kWh capacity of my battery so it was costing me around 3p/kWh of battery charge. Agile cheap rates canbe quite erratic and do need careful managing and NetZero was useful but not really a money saver. I decided the Go tariff would be much easier to manage so I wouldn’t need Net Zero.

Secondly, snow! The Powerwall anticipates the weather and sets the charge schedule accordingly. Today was forecast to be quite sunny - it was correct - and fortunately what snow we had on the roof burned off by 11pm but had it not, then there would have been no solar generation and to get through the Agile peak period of 4-7pm would have required grid imports which during the day was relatively expensive. Hopefully on Go, as cheap rate import is much lower than export rates, the PW3 will just charge from the grid overnight.

I don’t have an EV so I rang Octopus and asked if it was OK to sign up for Go. After some deliberation and referral the answer was yes, so I completed the switch online. At 12.14 I received an automatic email asking me to accept the terms and conditions and at 12.16 I was on Go. The rates start from 00.01 on the day of the switch so I got the benefit of the 8.5p rate on what charging and water heating had taken place during the night on Agile. I did have about a small amount of grid imports on the 29p rate but less than a kWh. 

I know some people complain about Octopus customer service but for me everything went very smoothly today.

I should add that I did consider Good Energy’s EV tariff (no EV required) which has a similar peak rate but only 6.6p/kWh for 5hours overnight but it also has an exit penalty of £50 and as I intend to switch to Intelligent Flux for the summer I wanted to avoid that.

NedS

@JKenH That's interesting.

For us, Cosy is the only tariff for winter as we need to recharge the Powerwall multiple times per day. There is no way we'd manage on a single charge + solar, so Cosy really is the only choice. And in summer, we can generate 4 times what we use, so IOF is again the obvious choice.

But that does leave the cross over months of March and October where we generate roughly what we use. Staying on Cosy is the safe bet as import (14.8p) is roughly the same price as solar export (15p), so we are cost neutral, but no you have me considering Octopus Go for march, where we can manage on a single charge per day and export any excess solar before recharging again each night. The alternative is the take a punt on Agile in the hope of some price plunges, but Go seems a more certain proposition and like you say is far easier to manage. In March we'd probably be buying just over half our electricity at 8.5p and the rest at 15p from solar, so may average out around 11p or so.

JKenH

NedS said:

@JKenH That's interesting.

For us, Cosy is the only tariff for winter as we need to recharge the Powerwall multiple times per day. There is no way we'd manage on a single charge + solar, so Cosy really is the only choice. And in summer, we can generate 4 times what we use, so IOF is again the obvious choice.

But that does leave the cross over months of March and October where we generate roughly what we use. Staying on Cosy is the safe bet as import (14.8p) is roughly the same price as solar export (15p), so we are cost neutral, but no you have me considering Octopus Go for march, where we can manage on a single charge per day and export any excess solar before recharging again each night. The alternative is the take a punt on Agile in the hope of some price plunges, but Go seems a more certain proposition and like you say is far easier to manage. In March we'd probably be buying just over half our electricity at 8.5p and the rest at 15p from solar, so may average out around 11p or so.

I don’t know what regime you operate your heat pump on but presumably you could take advantage of the cheaper Go off peak rate to boost your heating overnight, (maybe for longer). My son is on IOG and although not having a battery or heat pump he uses the cheaper rate for his electric underfloor heating overnight. I use the off peak for DHW as one of my hot water tanks is immersion only. (My other DHW tank is heated by oil which is currently about 6p/kWh plus losses in pipework to the tank).

NedS

JKenH said:

NedS said:

@JKenH That's interesting.

For us, Cosy is the only tariff for winter as we need to recharge the Powerwall multiple times per day. There is no way we'd manage on a single charge + solar, so Cosy really is the only choice. And in summer, we can generate 4 times what we use, so IOF is again the obvious choice.

But that does leave the cross over months of March and October where we generate roughly what we use. Staying on Cosy is the safe bet as import (14.8p) is roughly the same price as solar export (15p), so we are cost neutral, but no you have me considering Octopus Go for march, where we can manage on a single charge per day and export any excess solar before recharging again each night. The alternative is the take a punt on Agile in the hope of some price plunges, but Go seems a more certain proposition and like you say is far easier to manage. In March we'd probably be buying just over half our electricity at 8.5p and the rest at 15p from solar, so may average out around 11p or so.

I don’t know what regime you operate your heat pump on but presumably you could take advantage of the cheaper Go off peak rate to boost your heating overnight, (maybe for longer). My son is on IOG and although not having a battery or heat pump he uses the cheaper rate for his electric underfloor heating overnight. I use the off peak for DHW as one of my hot water tanks is immersion only. (My other DHW tank is heated by oil which is currently about 6p/kWh plus losses in pipework to the tank).

Generally we are wanting our house cooler overnight for sleeping, not hotter. Cosy works well as we can set back or turn off from midnight and then turn back on at 4am to start warming the house ready for when we get up. Running the heating full blast from 00:30 to 05:30 would be counter-intuitive, and pretty much the exact opposite of what we require for comfort.

Yesterday we used 43.5kWh total for the house and heating, and only generated 3.8kWh from solar, so lets assume we need to import 40kWh per day in winter, or around 1.66kW per hour.

If we consume 5 x 1.66kWh = 8.3kWh in the 5 cheap hours overnight, and charge the battery with another 12kWh, we may consume half our daily 40kWh usage at the cheap overnight rate of 8.5p and half at the day rate of 31.42p, so at a ~50:50 split, our average import price would be ~20p versus the 15p we achieve on Cosy now as we can charge the battery 3 times per day at cheap rate and directly consume 8 hours of cheap rate electricity at times of the day when it is more practical rather than just during the middle of the night. To achieve that on Go, we'd need 32kWh of battery capacity, so three Powerwalls instead of one. Hence for us Go is not an option in winter, and Cosy is the only choice unless we want to add 32kWh of storage (and we have no suitable inside locations for battery storage meaning it must go outside and be IP weather rated with good thermal management so the battery is still capable at charging at a high rate when it's -2C outside)

Everyone's usage profile is different, so understanding your usage profile and coming up with an appropriate strategy is essential. Where Go becomes viable is when our usage drops and our solar generation improves, and we can manage on a single charge per day plus any solar, which will be March for us. Then we are charging our battery at 8.5p instead of 15p so a definite saving.