How can extra Solar Battery save £

ninjaef

So, I have a PV system with a 4kwh battery. 
The inverter is set to charge the battery during cheap rate tariff time between 0400 and 0700.
The cheap rate is 10p per kwh.
The normal rate is 25p per khw
So my battery charges to 100% by 0700
Assuming my battery is used for load demand , from 0700 onward, does this mean the battery can deliver 4kwH , so if my load is 1kw then the battery will discharge to empty after 4 hrs, which means i have saved 60p, that being the difference between cheap rate and normal rate , i.e. 15p per kwh


?

I ask because I was considering adding a slave 4kw battery, but they cost around £1200 so if it saves me 60p a day then that's a lot of years to get the investment back....
60p x365 = 219
1200/219 = 5ish years

Netexporter

Well, it will be slightly less than that as there are charging and discharging losses, but you should have saved over 50p. Also, is 4kWh the useable capacity of the battery, or the gross capacity? If the latter then the saving would be slightly less.

ninjaef

@Netexporter ... thank you.

4kw is usable 

I was just looking for estimates, but critically, confirmation that my assertion that another 4kw battery would take approx 5 years to pay for itself based on these rough order of magnitude numbers

?

Oscarmax

ninjaef said:

So, I have a PV system with a 4kwh battery. 
The inverter is set to charge the battery during cheap rate tariff time between 0400 and 0700.
The cheap rate is 10p per kwh.
The normal rate is 25p per khw
So my battery charges to 100% by 0700
Assuming my battery is used for load demand , from 0700 onward, does this mean the battery can deliver 4kwH , so if my load is 1kw then the battery will discharge to empty after 4 hrs, which means i have saved 60p, that being the difference between cheap rate and normal rate , i.e. 15p per kwh


?

I ask because I was considering adding a slave 4kw battery, but they cost around £1200 so if it saves me 60p a day then that's a lot of years to get the investment back....
60p x365 = 219
1200/219 = 5ish years

I have kept a detail Excel spreadsheet over the past few years, I have a 4.4kWh system and a Solax 5.8kWH, we import approximately 3000kWh charging our PHEV requires 1900kWh leaving 1100kWh.

We have been on Octopus Go, Flux when it was introduced and Agile from November this year, I have considered adding an additional Solax triple 5.8 slave battery but like yourself cannot justify the cost at £1850 it would take over 20 years to payback the investment.

ninjaef

@Oscarmax ... EXACTLY.

 I moved into this property that has octopus cosy tariff  with PV solar, battery and ASHP. 

The slave battery as you say is £1200-£1890 depending on spec

and yes, exactly, i cant justify buying a slave battery at these numbers. i thought maybe i was miscalculating, but from this thread and insightful replies.. i am not

THANKS 

Oscarmax

You have based you figures on battery fully charged from the grid 365 days a year in reality is a lot different in April to the start of October the majority of your energy will be off your panels, March and October about 50/50 it is only in January,  February, November and December were you are heavily reliant on the grid, in my humble opinion the solar battery at £4,500 has not been as cost effective as I expected, I am now looking at at least 8 to 10 years.

Reed_Richards

What you need is enough battery capacity to see you through to the time when you have enough sun to see you through to when your battery starts to recharge from your panels.  And later, enough capacity to see you through from the time when your solar power drops away to the next cheap rate charging period.  So if you save 15p per kWh at present with 4 kWh usable battery capacity you should save 15p x 4 x 2 = 120p per decently sunny day.

ninjaef

Oscarmax said:

You have based you figures on battery fully charged from the grid 365 days a year in reality is a lot different in April to the start of October the majority of your energy will be off your panels, March and October about 50/50 it is only in January,  February, November and December were you are heavily reliant on the grid, in my humble opinion the solar battery at £4,500 has not been as cost effective as I expected, I am now looking at at least 8 to 10 years.

indeed, i was being liberal with my estimates.

yes, 8 to 10yrs payback for another 4kw slave unit is just not viable.or.tempting.

EricMears

ninjaef said:

yes, 8 to 10yrs payback for another 4kw slave unit is just not viable.or.tempting.

My solar panels took seven years to achieve 'payback' .  After another six I've received 200% of initial outlay and I've still got twelve years of accelerating returns to go.  After that,  my earnings will drop somewhat but I'm still expecting lower import costs for many more years.

Heedtheadvice

....and financial aspects are not the only considerations for generation or storage!

Magnitio

Reed_Richards said:

What you need is enough battery capacity to see you through to the time when you have enough sun to see you through to when your battery starts to recharge from your panels.  And later, enough capacity to see you through from the time when your solar power drops away to the next cheap rate charging period.  So if you save 15p per kWh at present with 4 kWh usable battery capacity you should save 15p x 4 x 2 = 120p per decently sunny day.

That assumes no charge/discharge losses and that you don't get paid for exporting excess solar.