Battery storage & PV questions

[Ben Weston]

Hi all,

 

We're currently doing a full renovation and, as part of that, our single phase supply is being upgraded to three phase to support EV charging, air source heat pump, hot tub (in the future), etc. Our planning permission didn't include PV so we've been advised to wait until it's signed off and add the panels under PD rights straight after. We'll therefore run the cabling for a solar array but won't install it to begin with.

 

However, I'm very keen to get some home battery storage installed from the start to, ideally, charge on the cheap EV tariff between 2am and 6am and use throughout the day (as much as possible). This will also be supplemented by the aforementioned PV down the road. We don't have the funds for expensive Sonnen or Tesla batteries so were investigating various Chinese options to couple with a Victron Multiplus inverter/charger, or similar.

 

I'm not sure how we'll 'split' the three phases yet (or if we even would?). I'd considered one phase for garden office & garage EV charging, second phase for induction hob, ASHP and hot tub and third phase for the rest of the house. This might be the wrong way to approach it?

 

Basic questions time.

 

If I bought 3x 12 kWh 200Ah 48V LiFePO4 batteries and used them through an inverter, would this roughly translate to 36 kWh of AC capacity (minus inverter losses)? We use around 30-40 kWh a day (I know) so I'm keen to understand if the battery storage translates to AC the other end?

 

And is 48V the right choice? I've seen 12V and 24V systems but I don't see how they'd provide sufficient current for a house?

 

Lastly, would I only be able to integrate this on one of the phases mentioned above (probably the house phase as there's no single item with a huge draw) or is there a better way to split the phases and battery/PV (i.e. one battery per phase with three inverters instead)?

 

I'm not interested in exporting back to the grid at this stage as I gather that gets more complicated with the DNO, etc.

 

Thanks in advance for any help!

Edited March 25, 2022 by Ben Weston

[J1mbo]

The short answer is: batteries tend not to cost-in for rate shifting.

 

e.g. 36kW of battery capacity, cycled 80% daily for ten years. Total energy provided by them would be 36 * 0.8 * 3650 = 105MWh. Night rate saving about 8p as of 1st April typically, giving a benefit of about £8k allowing for charging losses.

 

Obviously charging them from PV is a different equation.

[Ben Weston]

52 minutes ago, J1mbo said:

The short answer is: batteries tend not to cost-in for rate shifting.

 

e.g. 36kW of battery capacity, cycled 80% daily for ten years. Total energy provided by them would be 36 * 0.8 * 3650 = 105MWh. Night rate saving about 8p as of 1st April typically, giving a benefit of about £8k allowing for charging losses.

 

Obviously charging them from PV is a different equation.

 

Thanks, yes this was my understanding. It is definitely our intention to add PV as soon as we can but, with rates as they are at the moment, it seemed prudent to add the batteries straight away as they don't require extra planning permission.

 

I'm keen to understand if my thoughts of using a 48V 200Ah LiFePO4 battery is along the right lines? Obviously it's cheaper to down-rate to 150Ah or 100Ah (and also to 24V) but that seems possibly like a false economy for our needs?

[J1mbo]

I would suggest sizing the batteries to deal with the solar and use them for rate-shifting "for free" as a benefit. 36kWh seems very large.

[JohnMo]

I would look to buy everything before completion, then you can claim Vat back.  Get Electrician to run DC cables, DC isolation switch, DC cables to inverter.  AC cable to PV meter, AC isolation switch and then into consumer box.  All you need do then is install panels, install inverter and connect up.

[billt]

2 hours ago, Ben Weston said:

If I bought 3x 12 kWh 200Ah 48V LiFePO4 batteries and used them through an inverter, would this roughly translate to 36 kWh of AC capacity (minus inverter losses)? We use around 30-40 kWh a day (I know) so I'm keen to understand if the battery storage translates to AC the other end?

 

And is 48V the right choice? I've seen 12V and 24V systems but I don't see how they'd provide sufficient current for a house?

 

Lastly, would I only be able to integrate this on one of the phases mentioned above (probably the house phase as there's no single item with a huge draw) or is there a better way to split the phases and battery/PV (i.e. one battery per phase with three inverters instead)?

 

Yes to the first question less any losses, which could amount to 15-20%.

 

Yes to the second question. The peak power available depends on the peak discharge current capability of the batteries, but increasing the voltage reduces the current demand. My system uses 400Ah LiFePO4 batteries which can be discharged at 3C or 1200A, theoretically that's 57kW, but the inverter is limited to 6kW continuous, 8kW for 30 minutes or 9kW for 5 minutes. However the inverters (SMA Sunny Island 8.0H) can be paralleled to increase output or 3 of them can be used in a 3 phase system.

[Ben Weston]

31 minutes ago, J1mbo said:

I would suggest sizing the batteries to deal with the solar and use them for rate-shifting "for free" as a benefit. 36kWh seems very large.

 

Our general daily use is around 30 - 40 kWh, admittedly a little less than this in the summer. But yes, a fair whack and ££££ even now, let alone with next week's increase. Ideally, I'd like to run as much of the house on solar and reduced-rate stored grid electric as I can.

 

16 minutes ago, JohnMo said:

I would look to buy everything before completion, then you can claim Vat back.  Get Electrician to run DC cables, DC isolation switch, DC cables to inverter.  AC cable to PV meter, AC isolation switch and then into consumer box.  All you need do then is install panels, install inverter and connect up.

 

That's the plan. Will purchase it all and get everything except the panels themselves in place. When we're signed off circa October, we'll add the panels.

 

1 minute ago, billt said:

 

Yes to the first question less any losses, which could amount to 15-20%.

 

Yes to the second question. The peak power available depends on the peak discharge current capability of the batteries, but increasing the voltage reduces the current demand. My system uses 400Ah LiFePO4 batteries which can be discharged at 3C or 1200A, theoretically that's 57kW, but the inverter is limited to 6kW continuous, 8kW for 30 minutes or 9kW for 5 minutes. However the inverters (SMA Sunny Island 8.0H) can be paralleled to increase output or 3 of them can be used in a 3 phase system.

 

This is really helpful information, thank you. I can't foresee a situation where we'd even be close to 6 kW continuous and I presume, in the event we breached those sorts of numbers, the inverter/BMS would begin to pull more from the grid?

 

Any idea on how best to plan the 3 phase supply with this? Keep it on the 'main' phase or bridge all three? 

[J1mbo]

Solar is now zero rated for VAT anyway 

[RobLe]

The three phases are I think independent, from your perspective.  That is, you could pull in 3kW to charge your car, but if the batts and PV are on a different phase you will be importing and exporting simultaneously on the different phases,  likely the import rate is lower than the export rate and you will lose out.  I'm not aware of consumer phase balancing equipment, which could fix the issue.

 

I suggest you split the battery amongst the phases, and also split your loads.  If you will find one of the phases needs more than the others, it might be helpful if you could move a batt over in future.  It's a lot simpler in a straight single phase system!  Maybe you only need to use 2 of the 3 phases?

 

Also - if you do get a hot tub, get massive amounts of insulation for it!  I bet they are an energy hog.  Can it be solar thermal heated ?  Or a cheap pool ashp will give a cop of 4 in summer.

[Ben Weston]

17 minutes ago, RobLe said:

The three phases are I think independent, from your perspective.  That is, you could pull in 3kW to charge your car, but if the batts and PV are on a different phase you will be importing and exporting simultaneously on the different phases,  likely the import rate is lower than the export rate and you will lose out.  I'm not aware of consumer phase balancing equipment, which could fix the issue.

 

I suggest you split the battery amongst the phases, and also split your loads.  If you will find one of the phases needs more than the others, it might be helpful if you could move a batt over in future.  It's a lot simpler in a straight single phase system!  Maybe you only need to use 2 of the 3 phases?

 

Also - if you do get a hot tub, get massive amounts of insulation for it!  I bet they are an energy hog.  Can it be solar thermal heated ?  Or a cheap pool ashp will give a cop of 4 in summer.

 

That was my understanding too. We'll actually need all three phases and will utilise them fairly equally (hence our huge daily demand at the moment). I'll have to give it some thought how we implement this.

 

On the hot tub — yes, this is what we're going back and forth on at the moment. Hot tubs are a huge energy hog and we're not too keen on the environmental impact. If we could run the majority of it off solar or cheap electric from the grid (usually mostly renewables at that time of day), our conscience would be a little clearer!

[J1mbo]

Surely a heat pump is ideal for a hot tub

[SteamyTea]

5 hours ago, billt said:

can be discharged at 3C or 1200A

C is usually used for coulomb, not sure that 3 of them is 1200A.

Or have I misunderstood something.

3C.s^-1 = 3A