Considering more solar and a battery

NedS

I'm looking for some help/advice to help clarify my thinking - we are considering a battery and adding more solar.

What we already have

3.645kWp south facing solar array on a non-hybrid 3.0kW inverter

Air Source Heat Pump for heating / hot water

Current Usage

The current system was installed last June (so not yet a full year), 

Our winter use is typically 25-35kWh per day and may peak at 40kWh on the coldest days. The recent period of settled 2-5C days has seen usage of 30-35kWh. It's generally mild where we live and we do not regularly see temps below freezing (2C is far more common low temp for us). We are on the Octopus Cosy tariff for heat pump owners.

Summer usage is 10-15kWh per day with import averaging around 6.5kWh per day due to solar usage.

What we are considering

I'd like to add a battery.

Installation may be a bit of an issue - we do not have any good indoor locations which rules out the cheaper fogstar type systems, so we are looking at something that can be installed outside where we have a couple suitable locations. We do have an outbuilding, but cable runs are far from ideal.

Sizing a battery

As we are on Cosy, we have 3 cheap slots (2x3h and 1x2h) where we could grid charge at cheap rate. There are 6h periods between cheap slots. something like a 5kWh system may get us 70-80% of the way there, especially if we turn down the heat pump whilst on battery, but a 10-13.5kWh system should easily see us through to the next cheap slot. Due to the outside location requirement, we are considering a Tesla PW3 (13.5kWh, with inbuilt inverter and 3 MPPTs)

At present, our average cost of import on Cosy is ~20p. Assuming we can shift all usage to cheap rate with a 13.5kWh battery, that drops to 12.86p giving a leverage of 7p per kWh. I calculate that will save us in the region of £400-500 per year (estimated as I don't yet know what a full year's usage looks like). Payback for a Tesla PW3 is looking like close to 20 years. In summer, 13.5kWh is going to roughly cover our daily usage so we'd be running totally on solar (which I price at 15p SEG rate) or cheap import - not sure if we could switch tariffs in summer to import and charge at an even cheaper rate overnight and then export all our solar during the day at a higher 15p SEG rate?

Although the payback on a Tesla PW3 is not great, there is also a consideration of increased easy of use and comfort, not having to turn off heating and not cook at peak rates but clearly that comes at a cost.

More Solar

We currently have 3.645kWp south facing on a 3.0kW FoxESS inverter.

I have scope to add another 8 panels on a second south facing roof. 2 Panels would experience shading in the late afternoon. 8 x 445W panels would give a second array of 3.56kWp

As I'm considering the Tesla PW3, an inverter is already included so these could be connected directly into the PW3's inverter.

I'd like to dump the existing FoxESS inverter and also wire the existing solar array into the Tesla PW3 as a second string. Is that advisable, switching it from AC coupled to DC coupled, or would I be better off leaving the first array on the existing FoxESS (I see some clipping in the summer)?

I'm trying to understand how the system would work in the summer, and whether I'd be able to leverage cheaper imports against a higher SEG export rate.

Adding a second south facing array would give me just over 7kWp and would roughly equate to our annual usage.

Obviously the additional solar array would depend on what the DNO grants us and not worth doing if they do not grant 7kW?

Costs

I'm initially spoken with a local installer who gave me an initial indication of ~8K for a Tesla PW3 installed. Not sure what an additional 8 panels would cost (scaffolding to house required but fairly simple), but I'm guessing the payback on a south facing array is going to be pretty good. They are contacting me back this week to visit / survey with a view to quoting.

I'd appreciate any thoughts / discussion to help me work this through and see where we end up.

Screwdriva

Your instinct to replace Fox with Tesla is a wise one. No other system performs as well outdoors. 

The existing array can connect to one of the three PV inputs. DC coupled works really well with the PW3 so I wouldn't hesitate with that option. 

 8 Sharp 435W Bifacial panels + Tesla PW3 should cost ~£10.5K all inclusive. Just the PW3 is £7.5K. You should be able to sell the existing inverter on. 

badmemory

My batteries are in the porch but there was a slight suggestion of putting them in the loft.  My meters are also in the porch so it seemed a sensible place.  I would not have had it installed (last year) without a battery due to my usage pattern.  This has been a massive learning curve for me as I am pushing 80, so I am looking forward to reading what people have to say.

NedS

@Screwdriva thanks, I was hoping you may reply.

Given our annual usage of around 7000kWh, is doubling the solar array to 7.2kWp reasonable? I'm assuming it's pretty much a no brainer to fill our available south facing roof space? Are we likely to get DNO approval?

Assuming we do get DNO approval, do we need to update our supplier (Octopus) given we are already registered for export with them, albeit at 3.68kW or can we just export more up to our new DNO limit?

Are there any issues with having a battery with an integral inverter? What happens if/when the inverter fails outside of the warranty period, as one would hope the battery may be good for well past it's 10 year warranty life.

Any advice for siting a Tesla PW3. Is south facing in direct sunlight an issue (for overheating in summer?) or would it be better placed in a shaded spot.

Finally, can you/anyone recommend any national installers as I fear my local installer is going to come in above ~10.5K for a PW3 + 8 panel installation. At least I may be able to get other quotes to try to haggle them down with (I think I'd prefer a local installer just in case of any issues).

Martyn1981

I was pondering this part:

Obviously the additional solar array would depend on what the DNO grants us and not worth doing if they do not grant 7kW?

Can the PW3 output be limited/capped? If so, then an export limit of 4kW or so, would be doable, hopefully with no clipping, as the battery is DC side.

For example, even on a good day when the system is generating say 6.5kW, then as long as the battery has been emptied ready, that's over 5hrs of battery charging at 2.5kW (with 4kW use/export) before it will be full.**

If the PW3 can do this, then any DNO export limit of 4kW+ should be OK for you. Although there may be times in the winter when you want more than 4kW output*, but just saying, in case that helps.

*Way out of my depth now, and no idea if the PW3 can do this, but it may even be possible to set a max discharge rate higher than the max export rate (these aren't exactly the same thing). For instance 4kW export, but 6kW discharge, so if you needed 6kW for the ASHP, there would actually be zero export, so in compliance with the DNO.

Apologies if I'm misleading you.

**Edit - Just to say, I have 6.72kWp of PV, running through a 3.68kW capped inverter with no clipping, so long as there is space in the battery, as it can handle (upto) 5kW DC + (upto) 3.68kW AC.

badmemory

I thought the export limit when applied only referred to the amount they would pay for.

QrizB

badmemory said:

I thought the export limit when applied only referred to the amount they would pay for.

No, it's the amount your local mains connection can handle.

Screwdriva

NedS said:

@Screwdriva thanks, I was hoping you may reply.

It's a privellege to be able to help other MSE forum members. 

NedS said:

Given our annual usage of around 7000kWh, is doubling the solar array to 7.2kWp reasonable? I'm assuming it's pretty much a no brainer to fill our available south facing roof space? Are we likely to get DNO approval?

I would and have expanded my system to the maximum with no regrets. Hard to say for certain but the installers I use have had great success with PW3 DNO approvals. 

NedS said:

Assuming we do get DNO approval, do we need to update our supplier (Octopus) given we are already registered for export with them, albeit at 3.68kW or can we just export more up to our new DNO limit?

Are there any issues with having a battery with an integral inverter? What happens if/when the inverter fails outside of the warranty period, as one would hope the battery may be good for well past it's 10 year warranty life.

Any advice for siting a Tesla PW3. Is south facing in direct sunlight an issue (for overheating in summer?) or would it be better placed in a shaded spot.

Ideally yes but I would wait till your installation is complete and MCS Certificate is in hand before letting them know.

The PW3 has a product replacement warranty - if there is any issue, they don't repair, they replace the whole thing, no questions asked. This is an unbeatable level of after sales support in the industry and fills me with confidence on the product's longevity. I am genuinely slightly envious of every PW3 I see installed

I always recommend a South facing location or a location right besides the ASHP's air intake, so that the ASHP can cannibalize the PW3's exhaust in the winter. 

Finally, can you/anyone recommend any national installers as I fear my local installer is going to come in above ~10.5K for a PW3 + 8 panel installation. At least I may be able to get other quotes to try to haggle them down with (I think I'd prefer a local installer just in case of any issues).

Feel free to DM me and I'll make sure atleast one installer honours the price point I shared. That said, I don't recommend local installers over national installers unless they've been operating for more than 7 years or offer really good brands of components. (Too many fly-by-night types in the industry who will be there for you today and gone tomorrow)

Qyburn

Martyn1981 said:

I was pondering this part: can the PW3 output be limited/capped? If so, then an export limit of 4kW or so, would be doable, hopefully with no clipping, as the battery is DC side.

For example, even on a good day when the system is generating say 6.5kW, then as long as the battery has been emptied ready, that's over 5hrs of battery charging at 2.5kW (with 4kW use/export) before it will be full.**

Also requires pretty fancy battery management as many systems prioritise battery charge over export, so left to their own devices the battery gets filled up pretty quickly. I have 8.5kWp panels with 6kW inverter and in the Summer the battery can be filled by 09:00.

Qyburn

NedS said:

Installation may be a bit of an issue - we do not have any good indoor locations which rules out the cheaper fogstar type systems, so we are looking at something that can be installed outside where we have a couple suitable locations. We do have an outbuilding, but cable runs are far from ideal.

The battery, or its inverter will almost certainly need a signal cable to the incoming grid connection.

NedS

Qyburn said:

NedS said:

Installation may be a bit of an issue - we do not have any good indoor locations which rules out the cheaper fogstar type systems, so we are looking at something that can be installed outside where we have a couple suitable locations. We do have an outbuilding, but cable runs are far from ideal.

The battery, or its inverter will almost certainly need a signal cable to the incoming grid connection.

We do have a few outdoor locations which may be suitable, although none I'm particularly over the moon about, all of which have reasonably sensible cable runs back to the Consumer Unit. If Tesla, the gateway can go in the house next to the meter/CU.

We don't really have any suitable indoor locations - one or two possibilities, but not ideal. I have a couple local installers visiting this week to take a look who may be able to at least offer suggestions or guidance.