Storage

[Marvo]

No probs, it can stay here away from the riff raff .

 

[The Solar King]

I would endorse Worcester's comments. We are all feeling our way in. The public side thread is also important to help visitors to the forum find credible information and advice.

 

[whinmoor]

using economy 7 is in no way economic when you take into account the lifetime cost of storage + the economy 7 cost of leccy.

There really is far too much pie in the sky stuff being spouted on this at the moment.

The SMA guy said it in front of maybe 150 people. He stressed that it would be "affordable". I was very surprised. Without knowing the cost of the unit, it's impossible to do accurate calcs but it seems pretty fanciful.

 

[Gavin A]

The SMA guy said it in front of maybe 150 people. He stressed that it would be "affordable". I was very surprised. Without knowing the cost of the unit, it's impossible to do accurate calcs but it seems pretty fanciful.

6.5p/kWh economy 7 tariff, plus 6p/kWh cheapest lifetime energy storage cost I've found yet* (not including inverter costs) means it'd be 12.5p/kWh vs 11.9p/kWh for the daytime rate for the same economy 7 tariff. Not including battery efficiency losses.

The SMA guy was probably just another sales idiot who doesn't really understand what he's doing, must be to have stood up and said that in front of 150 people.

Those economics should change when energy prices rise further, but at the moment they seem to have stopped doing that for a bit, so it's a bit of a hard sell unless you rig the figures (as many companies appear to be doing).

*big lead acid batteries, cheapest Lithium battery I've seen is around 8p/kWh lifetime storage costs.

 

[Worcester]

Simple Math:
Take British Gas's economy 7 rates:
Unit rate: 16.870p per kWh
Night rate: 5.950p per kWh
Use say LG's 6.4kWh with a daily DoD of 90%
and 15% losses charge / discharge
(I'm being generous all round..)

=(16.87-5.95)*6.4*0.9*0.85 = 53.464p saving / day = £195.145 per year.

 

[Gavin A]

you're missing the lifetime cost of storage of the battery.

cost of battery / capacity x lifetime number of discharge cycles.

on the LG I think that works out at around 8p / kWh.

 

[Worcester]

There were no costs in my example (inverters , charge controllers , batteries , installation , ... ) it was just to illustrate the electricity 'saving' , for example (from memory - haven't done the math for a while) unless you use in the region of 30% of your leccy in the night time economy 7 is more expensive than flat rate.

 

[Gavin A]

but it's that sort of calc that's confusing people into thinking it could be viable. The simplest way to show it isn't is to include that lifetime storage cost in the calc.

I wish BRE had included that in their guide.

 

[The Solar King]

you're missing the lifetime cost of storage of the battery.

cost of battery / capacity x lifetime number of discharge cycles.

on the LG I think that works out at around 8p / kWh.

Gavin

Could you expand on your calculation please?
In the example given is this just the cost of the battery or all equipment in the storage system?
Is your capacity the usable capacity or total capacity?
Where are you taking your discharge cycle figure from?

A worked example would be really helpful.

Thanks

 

[The Solar King]

One other point of note. In a conversation with the guys at Wattstor, I raised the issue of VAT. They have been advised by HMRC that installation as part of a PV system is at 5%, but retrofitting to an existing PV system is 20%.

This may all become academic if the rate of VAT is raised to 20% in July for all new PV installs. We (the STA) are fighting hard against this. Consultation closed yesterday. Trust you all responded.

 

[Worcester]

I might have this wrong, and happy for someone else to come up with a better formula, using their published figures (believe them or not..)

Trade List Prices, ignoring VAT and Installation costs

Aquion:
Price £875.00
Capacity 2 kWh
DoD 90%
Cycles 6000
Total Lifetime Capacity = Ca*DoD*Cy = 10800 kWh
Cost / kWh £8.10 p/kWh

LG Chem
Price £2,780.00
Capacity 6.4 kWh
DoD 90%
Cycles 6000
Total Lifetime Capacity = Ca*DoD*Cy = 34560 kWh
Cost / kWh £8.04 p/kWh

 

[TedM]

The attached ssheet is how I would work it out. This is for adding a battery storage system to existing PV.

Note, apart from the input values, the assumptions that are built-in to that model - that the batteries are charged/discharged to their max every day. How many households use 5kWh a day, every day, when their PV system is not generating or are capable of changing lifestyle to do that?

Also no account is taken for the PV generation being taken to charge the batteries being unavailable for any other on-site use during the day, i.e. the assumption is that this is all currently exported.

 

[solarfred]

Thanks Ted. Can you add an energy inflation feature for the Grid supply?

 

[solarfred]

I might have this wrong, and happy for someone else to come up with a better formula, using their published figures (believe them or not..)

Trade List Prices, ignoring VAT and Installation costs

Aquion:
Price £875.00
Capacity 2 kWh
DoD 90%
Cycles 6000
Total Lifetime Capacity = Ca*DoD*Cy = 10800 kWh
Cost / kWh £8.10 p/kWh

LG Chem
Price £2,780.00
Capacity 6.4 kWh
DoD 90%
Cycles 6000
Total Lifetime Capacity = Ca*DoD*Cy = 34560 kWh
Cost / kWh £8.04 p/kWh

If you take the mid range discharge capacity of Aquion (c10) then 2kwH is correct but if you use the c20 rate the capacity is nearer 2.5kWh. It is hard to compare different technologies with different parameters ie. the voltage range of the LG is smaller than the Aquion but the Aquion has higher resistance so will charge slower but I calculated that over an E7 time period that was sufficient to fully recharge a single stack within its limits. If the LG goes outside the voltage range it will trip off sooner than the Aquion. If a large load is applied to the LG will it drag the voltage down enough to trip a half charged battery? I don't have this data yet unless anyone has seen it. Aquion can take a big hit and not trip if the inverter is set lower and then the voltage bounces right back. Still working on how deep it can go.
Looking at the warranty they are both on a sliding scale but LG is logged inside the BMS so if the temperature drops below 0c (garage??) then the warranty is invalidated. Aquion warranty requires a load profile for one day to check the site is within range. It can operate down to -5c.
If you have LG you need some rather scary safety gear! Aquion is non toxic and virtually inert in comparison. It does not need a BMS.
I would not tell anyone "it will last x years..." as no one knows - yet. I can refer the the manufacturers data.

 

[whinmoor]

I've slept since but I'm 99% sure the SMA guy said that while the Sunny Boy Storage would be compatible with most of the leading battery brands, their preferred partner was....Tesla.

It doesn't exactly sound like a budget option!

Even if Worcester's (very generous IMO) £200 a year savings on E7 are accurate, there's no way this can make sense.