Battery and extra panels?

Shortsy

@1961Nick. Thanks for the advice. I have been confused by the advice on the Which guide to solar panel battery storage which says that "DC batteries can't be charged from the grid, according to the energy saving trust" - I'd understood that to mean that you couldn't charge a DC battery overnight on a cheap rate tariff.
(the forum doesn't allow me to post the link)

Coastalwatch

Shortsy said:

Reading this thread with interest as I'm planning to get 10 380W panels but can't decide what to do about the storage (we have no hot water tank). I would have done the battery after a year or two, but then VAT is liable. I've been quoted for a 8.5 battery, but a smaller one might be enough? It only seems worthwhile if both capturing spare PV and overnight tariff - but my understanding is that you can't store grid electricity on the DC battery, only the AC ones (which then compromise on the efficiency of going between DC and AC). I'd be interested to hear if anyone has solved this problem.

Hi Our son has recently had PV and batteries installed alongside that of a single 3.5kW Air2Air hp. The batteries total 7.2kWh's and are frequently exhausted part way through the evening. He's getting storage increased as a Christmas present.

Shortsy

@Coastalwatch, thanks for that insight, that's interesting to know. What PV size does your son have and what is his electric consumption? I would be putting in a 3.8kWp system and my annual electric consumption is only 2200, ie less than 7 per day... although if I install a A2A heat pump that's going to increase.

Wolfkeeper

Shortsy said:

@1961Nick. Thanks for the advice. I have been confused by the advice on the Which guide to solar panel battery storage which says that "DC batteries can't be charged from the grid, according to the energy saving trust" - I'd understood that to mean that you couldn't charge a DC battery overnight on a cheap rate tariff.
(the forum doesn't allow me to post the link)

I googled it, and having read it, I think that's a misunderstanding by Which of a third party source, which also seems to be somewhat incorrect/out of date. Some of the early DC-coupled systems were intended for off-grid use and they kept everything DC, and did DC to DC charging of the battery from the solar panels and then had an inverter to power the house which probably used a diode or relay or something to select the battery when the solar panel voltage dropped at night. They CAN'T turn AC back to DC again for storing in the battery (and why would you want to, when there's no grid).

Modern DC-coupled systems use what are often called 'hybrid inverters' which are able to move the electricity in any direction (except back to the solar panels obviously) and allow you to charge the batteries whenever there's solar or grid electricity available and have a charging and discharging schedule to allow you to operate with cheap tariffs and can still run your house off grid when there's a power cut (up to some power limit).

AC-coupled systems are mostly not quite as good, they're more expensive and less efficient, but are more flexible because you can add them to any system without changing any existing hardware, which potentially might lose you your feed in tariff and costs more to perform. They do however add inverter capacity, so that can prevent you from importing electricity from the grid quite as often.

Coastalwatch

Shortsy said:

@Coastalwatch, thanks for that insight, that's interesting to know. What PV size does your son have and what is his electric consumption? I would be putting in a 3.8kWp system and my annual electric consumption is only 2200, ie less than 7 per day... although if I install a A2A heat pump that's going to increase.

@Shortsy, PV array circa 5.2kWp and consumption prior to A2Ahp circa 3000 kWh's. Depending upon the size and consumption of your heat pump then in Winter with little sun to assist generation, the house may need to run entirely upon the battery contents each day. In cold weather with heat pump running, say a minimum of ten hours, then at 500 to 600W's/h adds another 5 to 6kWh's.

Many will advise that storage batts are not worth the investment, with diminishing returns as storage size increases, but it's very much a personal choice so you pays your money and act accordingly.

Being reliant on electricity for all our energy needs we have 30kWh's worth with absolutely no regrets while also applying for a V2H trial which will add the Leaf battery into the mix!

Please bear in mind that we have taken this route as our effort to become as independent as we can upon energy generation while travelling towards a net zero lifestyle. Absolutely appreciating that not everyone will have the appetite or capital to follow a similar course!

Shortsy

Thanks again for the advice, very useful indeed. The online solar battery calculators I've found aren't very good, they only seem to focus on energy used vs energy exported on a flat rate and don't seem to take into account time shifting by buying cheap overnight electricity as well. It looks like I might have to build my own calculator!

1961Nick

One thing to be careful of when using an ashp with storage is the COP when the temperature is very low. During last week with an overnight of -8C & daytime -3C the COP of my ashp was somewhere in the low twos. That made it more expensive to run than gas whenever it used peak rate electricity or was the cause of the batteries running out before the off peak period started.

Wolfkeeper

Shortsy said:

Thanks again for the advice, very useful indeed. The online solar battery calculators I've found aren't very good, they only seem to focus on energy used vs energy exported on a flat rate and don't seem to take into account time shifting by buying cheap overnight electricity as well. It looks like I might have to build my own calculator!

I've been using the SMA Sunny Design website. It's very detailed and free up to quite a high level and allows you to set time-scheduled tariffs. They only let you model SMA equipment, but that works roughly the same as other equipment.

1961Nick said:

One thing to be careful of when using an ashp with storage is the COP when the temperature is very low. During last week with an overnight of -8C & daytime -3C the COP of my ashp was somewhere in the low twos. That made it more expensive to run than gas whenever it used peak rate electricity or was the cause of the batteries running out before the off peak period started.

Yes. That's a common issue. However, these very cold snaps are relatively rare, so sizing your batteries and for that matter heat pumps for worst case conditions isn't necessarily the best move. ASHPS are widely used in Norway, and they usually size them for something like 85% of the worst case, and then top up with resistive heating. Similar things are at play with batteries.