Extra solar battery

russ_t5

I'm new to this site and I'm after some advice please.

Our current situation is this.
We have 10 375w solar panels on a south facing roof, coupled with a 3.2 kwh greenlinx battery.

We are on the Octopus flux tariff,  and currently we run the washing machine and dishwasher in the cheap period through the night, and charge the battery up.
Our average daily use is 8 kwh.

The question is whether it would be worth adding a second battery?

JKenH

I have previously suggested that Octopus Flux negates the argument for a battery but others on Flux believe a battery enables you to make better use of the Flux tariff by arbitrage (importing at cheap rate and forced export at peak rate). That will earn you around 20p/kWh. Let’s say you buy a 5kWh battery you could earn £1/day. In practice though there are cycling losses in charging and discharging the battery so to earn £1/day you would perhaps need a 6kWh battery. Assuming Flux rates remain unchanged you would earn £365/year or over 10 years £3650. If the installed cost of a 6kWh battery is less than that then it may be worthwhile. 

That, though, is a somewhat simple view ignoring the opportunity cost of that £3650 which if you offset against your mortgage might be saving you £180 per year at current rates. Take this into account and it is 20 years before the cost of the battery is paid off (that’s assuming of course you don’t move house).

Buying an additional battery just to store surplus solar and use in the house will not be worthwhile. You have already picked the low hanging fruit. You haven’t said whether your 8kWh daily usage (is that consumption or import?) is spread evenly through out the year but for most people summer consumption is less than winter so it might be 4kWh  in summer and 12kWh in winter. In the summer when you have excess solar there is no household demand to use it up and in winter when you might be able to use it there won’t be much surplus to put in the batteries. 

Exiled_Tyke

I looked into this pre-Flux and the marginal benefits were too small to make it worthwhile.  Although I would have still liked to have done it as I would have been able to buy more cheap electricity when the PV doesn't produce enough.  The other benefit would have been to double my maxim rate of draw. Unless the figures work by buying and selling using Flux I wouldn't bother

ed110220

Do you find you've got enough battery capacity to avoid significant imports during peak times? That's probably where I'd start.

I have 6 kWh of batts and it's enough to avoid almost all import 16:00 - 19:00 except when we're drawing more than the inverter's 3 kW max from the batts (for some reason the amount it can supply from batteries is less than what it can supply from the PV). 

[Deleted User]

The question is whether it would be worth adding a second battery?

What is your site export limit? 

EricMears

Dolor said:

The question is whether it would be worth adding a second battery?

What is your site export limit? 

Surely a 'site export limit' would be in kW and unless a larger or extra inverter is added at the same time as the proposed extra battery there wouldn't be any increase in the max possible export rate (just the possibility of sustaining it for longer).

[Deleted User]

EricMears said:

Dolor said:

The question is whether it would be worth adding a second battery?

What is your site export limit? 

Surely a 'site export limit' would be in kW and unless a larger or extra inverter is added at the same time as the proposed extra battery there wouldn't be any increase in the max possible export rate (just the possibility of sustaining it for longer).

Actually, my site export limit is in kVA. I have a PW2 which has a built in inverter which can export at 5kW. If I buy a second PW2 it can also export at 5kW: hence my question about a site export limit. Even if the intention is to store rather than export, it still might require DNO approval unless the second battery was DC.

EricMears

[Deleted User] said:

EricMears said:

Dolor said:

The question is whether it would be worth adding a second battery?

What is your site export limit? 

Surely a 'site export limit' would be in kW and unless a larger or extra inverter is added at the same time as the proposed extra battery there wouldn't be any increase in the max possible export rate (just the possibility of sustaining it for longer).

Actually, my site export limit is in kVA. I have a PW2 which has a built in inverter which can export at 5kW. If I buy a second PW2 it can also export at 5kW: hence my question about a site export limit. Even if the intention is to store rather than export, it still might require DNO approval unless the second battery was DC.

[Deleted User] said:

EricMears said:

Dolor said:

The question is whether it would be worth adding a second battery?

What is your site export limit? 

Surely a 'site export limit' would be in kW and unless a larger or extra inverter is added at the same time as the proposed extra battery there wouldn't be any increase in the max possible export rate (just the possibility of sustaining it for longer).

Actually, my site export limit is in kVA. I have a PW2 which has a built in inverter which can export at 5kW. If I buy a second PW2 it can also export at 5kW: hence my question about a site export limit. Even if the intention is to store rather than export, it still might require DNO approval unless the second battery was DC.

Original question referred to 'extra battery'.  A PW2 with its own built-in inverter is a rather different beast.