PV Solar and/or Battery Review Please

Toki

Good morning,

I have decided to get PV solar panels for climate change and also Return On Investment reasons.  I’d appreciate some of the experts here reviewing my calculations and assumptions and also correct me on my lack of general knowledge on these systems and any errors I’ve made please.

Apologies for the long post.

I currently use around 2200 kWh per year so quite a light user.  I have a ESE (120 degrees) facing roof in Aberdeen, Scotland and should be able to get 13-14 panels so around the 4.5 to 5kWp depending on panel.  PVGIS and EST sites estimates this would produce 3600 kWh.  Using the Energy Saving Trust calcs and not being home all day every day, I could use approx. 750 kWh of this.

My reason for getting isn’t purely financial as I said but be good to have a reasonable payback period.  Based on the EST calc and amending to an estimated 25p/kWh from the grid once prices go up in April, and 5p/kWh for SEG payments from April onwards, this would take around 15-16 years payback based on £5200 to install and £330 savings each year.

I’ve also been looking into batteries.  Originally to complement the solar.  Reading up on here, a few have Lux / Pylontech.  If I got 3x 2.4kWh, giving 7.2 kWh of storage and not getting PV solar panels at all (if this is even possible?), this would do me most days and not have to take much from the grid based on my average daily use of 6-7 kWh, depending on load, odd tumble dryer use etc would be more.  I made an assumption of 80% would come from the battery, is this reasonable assumption?

If I got Octopus Go at say 7.5p per kWh and a battery, this could save me 1760 kWh (80% usage a year if assumption is correct above) say at 25p and only cost 7.5p saving £350 a year.  Based on the Solar People price of £4250, this could pay for itself in 16 years or so. 

My other choice is to get both PV solar panels and the battery.  I am not sure on what estimate to put in here for energy I could save using solar and a battery.  Would 70% be reasonable based on winter and cloudy days or should this be lower?  750 kWh as above for the solar then an additional 1015 kWh (70% of total usage 2200 – solar usage 750) from the battery.  This would mean based on the import of 435 kWh and my SEG export I’d only be paying £17 per year.  Payback period 17-18 years.

How does this all sound please?  I’ve attached a copy of my calcs to the post.  Many thanks in advance.

ccbrowning

Personally I’m just not sure it’s wise to use the cheaper Octopus Go rates in any ROI calculation, beyond the initial fixed term. Is there any guarantee those will stick around? 

mickyduck55

Dont know if this helps:

My real life data.. 3 weeks short of two years since battery install, retired so no empty house Monday to Friday

Solar  yield 7813 kWh

Household Usage 8388 kWh

Export to Grid 1642 kWh

Imported from grid.. Average cost circa 7p kWh 2435 kWh (£170) ... "Octopus Go Faster"

These are figure from my Lux Software.. Sola PV yield is slightly higher using the direct Sunny Boy inverter

If you half this you are getting close to your yield.. but everything else depends on your own circumstances.. 

jimjames

You can see the actual data for solar panels by month here. It's SE England so will be higher than Scotland but gives an idea of the pattern across the year.

http://solar-panels-review.321web.co.uk/index.php/yearly-comparison-of-solar-pv

So although the panels generate over 3000kWh across the year they wouldn't cover usage of 3000kWh that is evenly spread across months as winter is much lower generation than summer and usage tends to be higher. Battery will help spread some of that load but it will have very little impact in the winter as it won't store the generation from summer to winter. If the panels only generate 1kWh for the day then a battery isn't going to have much effect, 30kWh generated over the summer will but I suspect the battery will be full and not really discharging/recharging for much of that time. It should mean that you have zero import from the grid over the spring to autumn though. 

QrizB

I think the maths for the solar-only system stacks up fairly well. You're only assuming self-consumption of 25%, not the 70-80% that some people are promised by solar installers (and which they never achieve).  25p/kWh as a 15-year average for import might prove to be a little high, but at the same time 5p/kWh for export is likely to be a bit low so they'll balance out.

The battery maths are a bit less certain; you're relying on a specific tariff remaining available - and do you have an EV? If instead you had to switch to something like E7 with an off-peak of 17p/kWh it changes the outcome appreciably.

Toki

Thanks everyone, much appreciate for your input.  Good points about the Octopus Go tariff and not assuming this will be available for life.  Who knows with the current turmoil in the energy markets what will happen in the future.  It certainly rules out battery only system as not worth taking the chance.

Appreciate the real world numbers also, gives an idea and can tailor to our own usage. 

QrizB, we don't have an EV currently but thinking about it in the next couple of years most likely. 

How does the solar & battery numbers look?  Does 70% seem reasonable guess as to amount we can use, 30% import based on the battery capacity being equal to a full days usage.  Obviously some days will get next to nothing and winter also considerably less.  I haven't considered Octopus Go tariff on this setup but could be worth it to charge overnight on bad solar days, even with the more expensive day rate of 31p.

Thanks again everyone.

mickyduck55

From my figures above I use 79% of mine... If I have done my calculation correctly ?
This year I should use more "IF" we get some hot sunny weather as I have installed some AC

Magnitio

You mention climate change and return on investment. If you are considering a reduction in your co2 emmissions, then it is worth looking for solar panels from a company that miminises their environmental impact during manufacture. Here is an example:

https://www.recgroup.com/sites/default/files/documents/wp_-_recs_class-leading_carbon_footprint.pdf

You might also consider panels that achieve higher output from your available roof space e.g. 14 * 400w panels providing 5.6kWp.

I haven't looked at the co2 emmissions for production of batteries, but I doubt they currently make much sense purely from a climate-change perspective. The manufacture, transport (usually from China) and installation result in quite a significant co2 impact. They do help with the reduction in peak demand on the grid if used correctly and can also encourage other ways of using the energy from the solar panels e.g. immersion heaters and other forms of heating instead of gas/oil.

arty688

Sorry if I have missed this on another answer but you also need An EV for Octopus go and the SEG on it is 3p.

1961Nick

Other factors that you shouldn't overlook are the usable battery capacity & the round trip loss (particularly when grid charging).

7.2kWh of pylontechs have a usable capacity of 90% = 6.5kWh. You'll also suffer discharge round trip losses of 17.7% so the available discharge will be around 5.35kWh.

There is a further loss on the charge side of 4.2% but this doesn't affect the actual discharge.

If you're using an off peak rate in your spreadsheet, add 20% to the tariff to cover the losses.

(I've fitted a meter to monitor the input/output of my inverter & those percentages have been calculated over a 34 month period. The inverter is showing a 10.7% loss but that's only about half the truth - it's probably reporting the battery loss & ignoring the AC/DC conversion losses x 2.)

orbit500

1961Nick said:

You'll also suffer discharge round trip losses of 17.7% so the available discharge will be around 5.35kWh.

Many thanks for that, amazingly hard to find empirical data on this. I had built my projections for my own pylontech install at 15% using my best estimates so great to see a real figure not too far off.