Solar phase II

I am doing exactly this, I can get 1.5kw on the garage roof and will be buying the battery at the same time to get it at 5%. Think about it, you get a 1.5kw system for around 2k, the battery is around 4k, with 5%, with 20% it is nearer 5k, so almost getting the 1.5 system for half price. Saying that I won't be going for a 14kw system as I have never used 14kw in a day so don't see the reason to have such a large battery.

warrenb wrote: »

Saying that I won't be going for a 14kw system as I have never used 14kw in a day so don't see the reason to have such a large battery.

Hiya. That's how I've done most of my battery pondering, suggesting 4kWh useable (around 5kWh Li-ion) would do the job.

But, I'm having second thoughts now, for a number of reasons, though none are entirely compelling:

1. The bigger the batt, the more you can avoid full charging, and discharging, which will extend the life of the battery.

2. Bigger batt, allows for longer viability as it loses capacity, so if 20%+ overcapacity at the start, then if down 20% after 10yrs, it's still going to meet needs.

3. When I'll need it the most, shoulder months, with plenty of export, but still high import, Feb, Mch, Apr & Sept, Oct coincide with when I'll be using the small ASHP. A battery would allow for more ASHP use without import. EG 2kWh per day, could mean 4hrs of heating, outside of generation, and 6-10kWh of heat.

4. The larger batt would allow cross day storage in the 6 best months, allowing for 100% PV supply on days which are extremely poor. Not really a concern normally, as these days are so rare, and going larger just isn't economical on this basis alone, but as part of this list, perhaps.

5. Future EV use. On average in the summer I have 20kWh of generation, reducing import from 7.5kWh to 2.5kWh per day. After allowing for domestic use of the battery I'd still have 10kWh+ of spare charge, or around 30-40miles of motoring per day (which we don't do). So a larger batt may help to future proof for a PV, though the savings here would be smaller as presumably the alternative would be a cheaper night rate for charging.

6. Bigger batt, especially the PWII is cheaper per kWh.

As I say, none of these arguments are convincing, and I'm not sure all of them together are, but worth a ponder I think, if the decision on battery size wasn't clear cut.

[I'll cross post this on the domestic battery thread as it might be useful, or not! M.]