New solar install. Go for battery or not?

[HABEL]

Hi all.

I have been looking to get solar for some time and have decided to finally bite the bullet after having numerous quotes, I have found a supplier that is over decent quality panels (n-type) and a good price. I just can't decide if a battery is going to be worth the investment at the moment, and what size to go for. I have two options:

14 panel Jinko 415w (5.7 KWP) with solar edge optimisers and inverter: £8140

17 panel Jinko (7.1 KWP) with solar edge optimisers and inverter: £9138

The battery they are recommend for my size and usage is the solaredge 10kwh battery (9.5 usable) at £6999.

I am also looking at getting a zappi charger installed for our Nissan leaf for £1395

So some information about our usage. We currently use on average 16kwh a day in the winter. Our estimated annual usage is 5878kWh. We have a Nissan leaf that my wife uses 3 days a week for work mon-wed and is generally not used much on the other days apart for going locally to schools clubs or shopping. So I plan to switch to the octopus ev scheme, so that we can get cheap electricity over night to charge on a Mon Tuesday night, and the car can hopefully be charged on a Thursday onwards from solar energy. The at that size would almost be enough to run our house during the summer months independent from the grid, then in the winter we could take advantage of the cheaper rate over night to charge our battery storage for use in the day / evening. We currently have a combi boiler, but the long term plan is to get a heat pump installed as we have wet underfloor heating throughout downstairs so this could take advantage of the setup even more.

I am sold on the solar setup for 8 or 9k but unsure if the battery investment will pay off. 7k is quite an investment on something that may only last 10 years?

Here or some pics of the solar panel location and our roof. It is almost south facing. One thing to note is that we have a dormer which can provide some shading down either side during early morning and late evening. This is one of the reasons I opted for solaredge, along with the software. The 3 extra panels in the 17 panel install will be most likely to be affected by shaded so was unsure if they were worth the extra cost?

Any advice or input from people who have had these systems installed would be appreciated!

 

[binky]

I'm a little confused, their shading charts says none (SF1 means none) but photo shows some hard shading from your dormer bedroom.

You can also use off peak tariff to charge up batteries, as well as the car. If you have a read of the various battery psots on here, installing batteries for this purpose is very popular, as well as charging EVs. Battery life is by charge cycles, usually 6000, which if charged and discharged fully once a day equates to around 16 years life. So another way to look at the battery is a rough calculation of 10kw at say £0.40 per kwh 6000 times = £24,000 potential saving over its life, and of course that's not adding any inflation to the cost of leccy.

 

[HABEL]

Thanks that makes a bit more sense to me now. Yes I did think the equation did not take into any consideration the shading. I'm not sure how well the solar edge can compensate for this. I looked up the warranty on the battery and it offers 70% guarantee after 10 years, unlimited charge cycles, I couldn't find a number on how many it can do but that seems like a good warranty.

 

[johnb2713]

You dont mention the capacity of the inverter?

Definately go for the battery, if you install at the same time as the solar PV you get the battery VAT free.

 

[Bob Smith]

Hello

The batteries will last you more than 10 years. Like the panels, there is degradation over time which can be limited by using the batteries within their optimal limits (depth of discharge, temperature)... Like for any batteries. So after 10 your batteries will still be working except their storage capacity will be reduced compared to when they were new.

Depending on the type of shade and frequency, you may consider panels with microinverter in those areas and have the other panels on string inverter but if you are talking about a couple of panels, then probably best to have an optimizer to ensure they are bypassed when they are in the shade so not to reduce the amps too much on the string.



Regarding the batteries, the kWh is of course an important factor but also their nominal/standard or max discharge.
There is no need to have a 20kWh battery full, when you can only discharge at 2kW while you need 5kW (because you have the oven, the washer etc.. on).
If the batteries cannot give you the instant power needed by the house, then the house will take the remaining from the grid.

eg (at night).
You need 5kW for 1h. This is a total of 5kWh.
Your batteries discharge at 2kW. This means that you will need an extra 3kW from the grid. This results of 2kWh from the batteries and 3kWh from the grid.
You still use 5kWh in total but because of the discharge capacity of the battery, you can't take it all from the battery.

Of course if this occurs in the day and your panels are generating 3kW, then you are OK.

It is important to understand the instant power consumption and the energy consumption and see what piece of the equipment will be the bottleneck (number of panels, inverter, batteries). You will also need to find the sweet spot as trying to fulfill all the requirements may end up being very expensive. (for instance, your house has a peak power demand of 20kW a few times a year. Will your size your system to deliver 20kW only a few times a year?)

 

[Bob Smith]

This has been written on a different social media by somebody else. I thought it could help with your reflexion


When solar generates it goes to the inverter as DC which converts it to AC and sends it to the house consumer unit to be used by the house. If the house doesn't use it then it's excess and goes out to the grid. If you have a battery then it's inverter monitors the house grid connection (using a CT clamp) looking for any excess being exported and if there is excess it charges the batteries. When there's no solar and the house has demand, that same CT clamp determines that there's electricity coming from the grid and it tells the batteries to discharge.

In terms of loads that exceed the capacity of your solar and/or battery, the demand is first satisfied by available solar, then by battery, then by grid. Each adding as much as it can to meet the demand. I say as much as it can as parts of the system have limits which restrict the flow. Think of the inverter and batteries as each having a tap that lets current flow. Each battery and inverter has its own discharge rate and depending on which is the least it becomes a bottleneck.

So here's a couple of examples to try to explain:

Scenario 1 Solar only - a 5kwp solar array on a 4kw inverter with the sun shining brightly giving the full 5kw of generation. When a 10kw load (say an electric shower) comes on the inverter becomes the limiting factor. So even though 5kw is being generated by the solar array, only 4kw is given to the house with 1kw lost through clipping. In this case 6kw would need to come from the grid. On the basis the shower only takes 15 minutes it would consume 2.5kwh, the solar would supply around 1kwh and the grid would supply 1.5kwh.

Scenario 2 - Battery only - a 12kwh battery with a 2kw max discharge rate, behind a 5 kw inverter. Using the same 10kw shower example, the battery max discharge rate becomes the limiting factor. So only 2kw will be supplied by the battery inverter combo an 8kw would need to come from the grid. On the basis the shower only takes 15 minutes, the batteries would drain by around 0.5kwh and the grid consumption would be 2kwh.

Scenario 3 - Solar and AC coupled Battery - as above but with a separate 5kwp solar array on its own 4kw inverter. Again the sun was shining brightly giving the full 5kw of generation. So in the 10kw shower example, 4kw will come from the solar (the solar inverter limiting output from solar to 4kw with 1kw lost through to clipping), and it will be topped up with 2kw from the batteries (limited by the battery discharge rate), leaving 4kw to be supplied from the grid. The 15 minute shower will consume 1kwh from the solar, 0.5kwh from the battery and 1kwh from the grid.

Scenario 4 - as per scenario 3 but it’s really cloudy and the solar is only generating 1kw. Now that 10kw shower will get 1kw from the solar, and 2kw from the batteries (the batteries are limiting the draw due to discharge rate), leaving 7kw needing to be drawn from the grid. The 15 minute shower will now be 0.25kwh from solar, 0.5kwh from the battery and 1.75kwh from the grid.

Scenario 5 - Solar and battery on Hybrid inverter - now imagine the battery is not AC coupled but plugged into the same inverter as the solar - a 4kw hybrid inverter with that 5kwp solar array and the 12kwh battery connected it. Once again the sun is shining brightly giving the full 5kw of generation from solar. The inverter is still the limiting factor and only 4kw of the solar is given to the house with 1kw going to charge the batteries, or if they're full it will be lost through clipping. So in the 10kw shower example, 4kw comes from the solar and 6kw from the grid. The 15 minute shower will be 1kwh from the solar and 1.5kwh from the grid.

 

[HABEL]

Thanks for all the insightful info. I forgot to mention the inverter.

It is the

SolarEdge 5000W Single Phase Inverter with HD-Wave Technology (SetApp Configuration)​

This is for the 14 panel config. It doesn't specify the wattage on the 17 panel quote but the same solar edge inverter, so not sure if this would need the 6000w one?

The battery has a charge and discharge rate of 5KW

 

[Mrjmegson]

I'd say batteries are an absolute no brainer, especially as solar energy has a really bad habit of coming while you're not at home to use it.

And as mentioned, if you get on a time of use energy tariff, you can really benefit in the darker months.