Noob Solar / battery questions and confusion about potential cost savings?

marchesini46

Like everyone, we are concerned with the rising cost of fuel prices so are looking into battery or solar + battery options in order to reduce our electricity bill.  Our average electricity usage is approx 500-550 kWh as I work from home.  Our monthly bill (combined) has increased from £185 to £385 pcm and with further increases looking likely in October, we are looking at all options to reduce our monthly bills. If there are any additional routes to explore, as to how we can also reduce our gas bills, then that would also be useful.   We use a standard boiler + Megaflow style hot water tank system.  

However, based on some online calculators, it only estimates that we will save approx £575 per year - which seems to be very low?  By my very basic maths, if I am currently spending £200 pcm (on electric) which may double again in October, I am potentially spending between £2400-£4800 per year which would mean it would only take 2-4 years to break even based on an £8000 system and best and worst case fuel price scenarios. Is my maths correct or have I overlooked anything or should I take into account other random factors to explain the price difference - maybe down to the battery storage? The quotes are also dramatically different as well - hence the confusion??

For context, our property is a 5 bed detached house in SE London with a large roof that can accommodate quite a few panels. We have had a quote (see below) on a system 

I last looked into solar about 15 years ago but was put off by the overall price and that batteries were not a cost effective option. Fast forward to today and a neighbour told me about solar / battery options and was pleasantly surprised at the cost.

So onto my questions:

• is this a good / reasonable quote?
• An electrician friend suggested to go for a single battery first and then add another one at a later date as we may not need 2 batteries and also battery prices are likely to drop as the technology gets better. Is this a good idea or false economy due to install costs etc?
• The thinking is to go onto the dual tariff (from Octopus) so that the batteries are charged overnight on the cheap rate tariff and then use this stored power for the evenings / non-sunny times. Is this a good energy saving strategy?
• Can the batteries also be charged by the solar panels during the day - assuming that the rest of the house is being powered by the panels and there is excess capacity to also be able charge the batteries at the same time?
• How long will a fully charged battery last for based on my typical monthly usage?

Here are my existing quotes so far:

——————————————————————————————————————————

QUOTE 01 - LIGHT RENEWABLES

Size of system 4kh. With battery storage

Equipment;

10 x 400w Hyundai Solar black panels, 25 year warranty to 86.2% efficiency

  1 x Growatt 4000w  Hybrid inverter and AC/DC Battery storage management system 10 year product warranty

  1 x Growatt 6.5kh of battery storage 10 year warranty giving you 10kh of battery storage

  1 x Emergency power back up supply

  1 x Grid trading option

  1 x All cable run’s isolators and fitting’s

  1 x Micro generation meter

  1 x System Wi Fi monitor  

  1 x System registration

  1 x Access Equipment

  1 x MCS and G98 Certificates

 All service and maintenance under the terms of the warranty.

Fully fitted price of £7,559.00 This price includes 0% Vat. No extra’s

 Income and savings based on a 4kh system. Example only. [YOU CAN SAVE MORE BY GRID TRADING ONLY WITH A BATTERY]

The expected power this system will produce is 4,000 kWh per year. This is an estimated income and savings.

Income;

Output; This system will generate;

4,000 kWh If you export 10% = 400 kWh of the power, the Export tariff is 0.075p  you get paid  = £30.00p

 

Total income of £30.00p per year

 

Saving’s

Output; This system will generate;

4,000 kWh per year if you use 90% = 3,600 kWh of the power per year and you are paying .296p per kWh you will save £1,065.60p per year on your Electric bill

Extra saving’s by charging your battery at night in the winter at 0.07p per kWh. 0.07p per kWh x 150 hours x 6.5kh winter days saving .216p per kWh = £210.60p

 

Total income and saving of £1,306.20p per year.

——————————————————————————————————————————

QUOTE 02 - ECO RENEWABLES (£7495 inc)

Solar†PV†´†Battery

12†x†375w†Solar†Panels

1†x†SoFar†Inverter

1†x†4Æ8kW†Pylontech†Battery

1†x†Mounting†Kit

1†x†Electricals

1†x†MCS†Sign†off

1†x†HIES†Warranty


——————————————————————————————————————————

Apologies for all the questions so any advice would be welcome so any advice is welcome before I go ahead and place an order.
 

Thanks

 

Pulpdiction

Similar system on this thread here:

https://forums.moneysavingexpert.com/discussion/6350098/new-quote-help-growatt

Pricing doesn't look too bad, but mixed reviews on Growatt batteries from what I have seen.  Also given your high usage you might want to consider a larger than 4kw system if you have the room and can get permission.  Also depends on the orientation of the roof and shading, but presumably all of that is taken into account.  A 4kw system might cover your use in May - September but it'll get no where near for most of the rest of the year if you are using 550kw/h, at approx 18kw/h per day then you'd need a bigger system and battery for the winter, some days you'll generate nothing. 

But I don't think you are too far away given the optimal cost vs saving.  Maybe consider a slightly larger battery, or system or look at a solar hot water diverter as well depending on your hot water set up.  

QrizB

There's a lot to unpick in your post - more than I can do in the next 5 mins - but those payback calculations are unlikely to be achieved.

Light Renewables are assuming 90% self-consumption; even with a battery and your high electricity use, 60-70% would be more reasonable.

They've also assumed you get onto Octopus Go (for the cheap overnight electricity) and onto Octopus Outgoing (for the 7.5p export) but you can't have both, it's one or another.

• Self-consumption: 2800kWh @ 28p/kWh - saves £784/yr
• Export: 1200 @ 7.5p/kWh - earns £90/yr
• Total ROI - £874/yr
• Payback - 8.65 years.

If you prefer Octopus Go, with the cheap overnight electricity, you'll need to own an EV.

Your consumption of 500-550kWh/month is around 18kWh/day so your fully-charged Growatt 6.5kWh battery will last roughly 8 hours.

arty688

Also when doing the maths remember its no good taking your annual figure dividing it by 12. Have you got your last years bills ? If so I would do a monthly break down then compare it with PVgis . For example I used 2300Kwh in Jan 21 and 1100Kwh in August . I would say without GO or similar battery might struggle to justify itself ROI wise.

marchesini46

arty688 said:

Also when doing the maths remember its no good taking your annual figure dividing it by 12. Have you got your last years bills ? If so I would do a monthly break down then compare it with PVgis . For example I used 2300Kwh in Jan 21 and 1100Kwh in August . I would say without GO or similar battery might struggle to justify itself ROI wise.

The breakdown of my electricity usage seems to be consistent month by month but not sure what you mean by "compare it with PVgis".

My electricity monthly breakdown:

Mar 22: 492 kWh

Feb 22: 548 kWh

Jan 22: 590 kWh

Dec 21: 582 kWh

Nov 21: 587 kWh

Oct 21: 560 kWh

Sep 21: 550 kWh

Aug 21: 540 kWh

Jul 21: 552 kWh

Jun 21: 490 kWh

May 21: 502 kWh

Apr 21: 640 kWh

Mar 21: 660 kWh

Still confused and sooo many questions??

marchesini46

If you prefer Octopus Go, with the cheap overnight electricity, you'll need to own an EV.

Your consumption of 500-550kWh/month is around 18kWh/day so your fully-charged Growatt 6.5kWh battery will last roughly 8 hours.

I wasn't aware that you had to own an EV.  Do you have to show proof? I understand they do another tariff (can't recall the name) that is approx 21p at night and 31p during the day so still a saving but nothing near the 7.5p Go tariff.

QrizB said:

There's a lot to unpick in your post - more than I can do in the next 5 mins - but those payback calculations are unlikely to be achieved.

Light Renewables are assuming 90% self-consumption; even with a battery and your high electricity use, 60-70% would be more reasonable.

They've also assumed you get onto Octopus Go (for the cheap overnight electricity) and onto Octopus Outgoing (for the 7.5p export) but you can't have both, it's one or another.

• Self-consumption: 2800kWh @ 28p/kWh - saves £784/yr
• Export: 1200 @ 7.5p/kWh - earns £90/yr
• Total ROI - £874/yr
• Payback - 8.65 years.

Again, I am trying to get my head around the terminology and measurements etc but as I understand it, the proposed system won't generate enough power to be fully powered by the panels so ideally I need more panels?  The quote estimates that the system will generate 4000 kWh whereas my household uses at least 6500 kWh per year.  

Also, in the winter, the panels may not generate any electricity at all so I will have to assume that I will have to pay the going rates from the supplier and possibly rely on part of my daytime usage from the cheaper rate electric from batteries?

Have I understood this correctly?

marchesini46

Pulpdiction said:

Pricing doesn't look too bad, but mixed reviews on Growatt batteries from what I have seen.  Also given your high usage you might want to consider a larger than 4kw system if you have the room and can get permission.  Also depends on the orientation of the roof and shading, but presumably all of that is taken into account.  A 4kw system might cover your use in May - September but it'll get no where near for most of the rest of the year if you are using 550kw/h, at approx 18kw/h per day then you'd need a bigger system and battery for the winter, some days you'll generate nothing. 

Yes, the more I look into it, it appears the proposed system seems to be not powerful enough for my needs.  Is there a chart that shows how much electricity is generated on cloudy v sunny days inc number of panels etc?

But I don't think you are too far away given the optimal cost vs saving.  Maybe consider a slightly larger battery, or system or look at a solar hot water diverter as well depending on your hot water set up.  

What batter size should I be looking for?  Also, what is a solar hot water diverter?

gefnew

What batter size should I be looking for?  Also, what is a solar hot water diverter?

Two large portions of haddock and chips will suffice.

QrizB

marchesini46 said:

If you prefer Octopus Go, with the cheap overnight electricity, you'll need to own an EV.

Your consumption of 500-550kWh/month is around 18kWh/day so your fully-charged Growatt 6.5kWh battery will last roughly 8 hours.

I wasn't aware that you had to own an EV.  Do you have to show proof? I understand they do another tariff (can't recall the name) that is approx 21p at night and 31p during the day so still a saving but nothing near the 7.5p Go tariff.

Owning an EV is a condition of the tariff. I don't know what checks they currently carry out.

The other tariff sounds like Economy 7, and that's open to everyone with no qualifying requirements.

marchesini46 said:

Have I understood this correctly?

Now you've shared your electricity breakdown, we can compare it to PVGIS. Dropping a pin in Bromley and assuming an optimised 4kWp array, I get the following output:

Plugging the numbers in (these are only typical, not guaranteed):

• Mar 22: 492 kWh - generate 353kWh
  
• Feb 22: 548 kWh - generate 209kWh
  
• Jan 22: 590 kWh - generate 159kWh
  
• Dec 21: 582 kWh - generate 154kWh
  
• Nov 21: 587 kWh - generate 193kWh
  
• Oct 21: 560 kWh - generate 277kWh
  
• Sep 21: 550 kWh - generate 392kWh
  
• Aug 21: 540 kWh - generate 451kWh
  
• Jul 21: 552 kWh - generate 500kWh
  
• Jun 21: 490 kWh - generate 488kWh
  
• May 21: 502 kWh - generate 486kWh
  
• Apr 21: 640 kWh - generate 460kWh

Let's assume for now that you fit a big enough battery that you can smooth out variations (we'l talk about that later).

marchesini46 said:

Again, I am trying to get my head around the terminology and measurements etc but as I understand it, the proposed system won't generate enough power to be fully powered by the panels so ideally I need more panels?  The quote estimates that the system will generate 4000 kWh whereas my household uses at least 6500 kWh per year. 

You'll see that in December you used 582kWh but a 4kWp system will only generate, on average, 154kWh in that month. You'd need a much bigger system, 15kWp, to generate as much electricity in an average December as you use.

That's around £15k worth of solar panels.

But not every December day is equally sunny. December generation consists of the occasional sunny day separated by lots of dull ones. To be fully self-powered you'd need a big enough battery to bridge, say, a week's power use. In your case that's 140kWh of battery. A rough rule of thumb is £400 per kWh so you'd need £56k-worth of battery.

So now you're looking at a total system cost of £71k.

This is why no-one would ever choose to be self-powered unless they really had to, and (if they really had to) they'd be looking at a daily energy budget of maybe 3-4kWh, not the 15-20kWh you are using.

Fundamentally:

• Without a battery, you can meet your daylight-hours needs for 6-9 months of the year. Any excess wiull be exported to the grid. A 4kWp system without a battery will be around £4k. You could go larger, to maybe 6kWp for £6k, but you will need permission from your electricity network operator (likely to be a formality but you won't know until you apply).
• Adding a battery lets you use your solar electricity in the dark. For most average households, using 3000kWh/yr (8-10kWh/day) a battery of 6-8kWh is the "best" size at current prices. In your case you're such a large user that a larger battery might be worth considering but a larger battery will only benefit you on those days of the year where your panels are generating that much excess power. The time taken to pay back the cost will extend accordingly.
  

marchesini46

QrizB said:

marchesini46 said:

If you prefer Octopus Go, with the cheap overnight electricity, you'll need to own an EV.

Your consumption of 500-550kWh/month is around 18kWh/day so your fully-charged Growatt 6.5kWh battery will last roughly 8 hours.

I wasn't aware that you had to own an EV.  Do you have to show proof? I understand they do another tariff (can't recall the name) that is approx 21p at night and 31p during the day so still a saving but nothing near the 7.5p Go tariff.

Owning an EV is a condition of the tariff. I don't know what checks they currently carry out.

The other tariff sounds like Economy 7, and that's open to everyone with no qualifying requirements.

I can't quite find the tariff but I don't think its an Economy 7 tariff but it gives you a night time rate of approx 2/3 the daytime rate.  Octopus Go rate is a non-starter as you have to show you own an EV or have an order for one

Now you've shared your electricity breakdown, we can compare it to PVGIS. Dropping a pin in Bromley and assuming an optimised 4kWp array, I get the following output:

Plugging the numbers in (these are only typical, not guaranteed):

• Mar 22: 492 kWh - generate 353kWh
  
• Feb 22: 548 kWh - generate 209kWh
  
• Jan 22: 590 kWh - generate 159kWh
  
• Dec 21: 582 kWh - generate 154kWh
  
• Nov 21: 587 kWh - generate 193kWh
  
• Oct 21: 560 kWh - generate 277kWh
  
• Sep 21: 550 kWh - generate 392kWh
  
• Aug 21: 540 kWh - generate 451kWh
  
• Jul 21: 552 kWh - generate 500kWh
  
• Jun 21: 490 kWh - generate 488kWh
  
• May 21: 502 kWh - generate 486kWh
  
• Apr 21: 640 kWh - generate 460kWh

Let's assume for now that you fit a big enough battery that you can smooth out variations (we'l talk about that later).

marchesini46 said:

Again, I am trying to get my head around the terminology and measurements etc but as I understand it, the proposed system won't generate enough power to be fully powered by the panels so ideally I need more panels?  The quote estimates that the system will generate 4000 kWh whereas my household uses at least 6500 kWh per year.  

You'll see that in December you used 582kWh but a 4kWp system will only generate, on average, 154kWh in that month. You'd need a much bigger system, 15kWp, to generate as much electricity in an average December as you use.

That's around £15k worth of solar panels.

But not every December day is equally sunny. December generation consists of the occasional sunny day separated by lots of dull ones. To be fully self-powered you'd need a big enough battery to bridge, say, a week's power use. In your case that's 140kWh of battery. A rough rule of thumb is £400 per kWh so you'd need £56k-worth of battery.

So now you're looking at a total system cost of £71k.

This is why no-one would ever choose to be self-powered unless they really had to, and (if they really had to) they'd be looking at a daily energy budget of maybe 3-4kWh, not the 15-20kWh you are using.

Fundamentally:

• Without a battery, you can meet your daylight-hours needs for 6-9 months of the year. Any excess wiull be exported to the grid. A 4kWp system without a battery will be around £4k. You could go larger, to maybe 6kWp for £6k, but you will need permission from your electricity network operator (likely to be a formality but you won't know until you apply).
• Adding a battery lets you use your solar electricity in the dark. For most average households, using 3000kWh/yr (8-10kWh/day) a battery of 6-8kWh is the "best" size at current prices. In your case you're such a large user that a larger battery might be worth considering but a larger battery will only benefit you on those days of the year where your panels are generating that much excess power. The time taken to pay back the cost will extend accordingly.
  

Once I'd read through the calculations a few times, it all makes much more sense to me. 

A larger panel system does seem to make more sense as it covers our daytime usage as well as generating enough to keep the batteries charged up.

I also just have to accept that in the winter months, that our electric bills will increase as the solar panels are not likely to be efficient to cover our needs.  I could possibly use the cheaper night rate tariff to charge the batteries to enable us to make use of the cheaper rate in the daytime?  Does that thinking make sense?

Many thanks for all of the responses.  It's really helped me understand the realities of seasonal variations etc and being totally self-powered.  

marchesini46

gefnew said:

What batter size should I be looking for?  Also, what is a solar hot water diverter?

Two large portions of haddock and chips will suffice.

No mushy peas with that?  I though they might help from a green perspective?