Not sure if this is worthwhile

[up_north]

Thought I'd start a new thread as my last one was more about quotes.

This is hurting my head. I know I really want solar but just not sure if I can make it work financially.

We use about 9kw/h per day. Picking some arbitrary time blocks for a typical day it looks like this:
1.30am-9.00am - 1.6kw/h
9.00am-1.00pm - 2.1kw/h
1.00pm-5.30pm - 1.6kw/h
5.30pm-8.30pm - 1.4kw/h
8.30pm-1.30am next day - 2.6kw/h

This excludes car charging.

We're on Octopus Go Faster and have cheaper electricity between 8.30pm and 1.30am. Although we're still on the old rates I need to do the calculations based on the new rates - 38p peak and 8p off peak.

We can physically only have 10 panels on the roof so realistically it's a 3.6kw system. I have a quote for £8600 for such a system with a 5kw/h battery with 90% depth of discharge. The figures from multiple installers all suggest an annual output of about 3600kw/h.

So it seems to me that in the height of summer there will easily be enough solar to power the house during the day and top the battery up to use when the sun goes down early evening and before it comes up again and possibly also during that cheap 8.30pm-1.00am slot - but then the current draw becomes the restricting factor - dishwasher, washing machine etc. It's quite possible I'll have too much solar and unfortunately my car charger doesn't have solar integration to capture the excess energy (yes I should have got a Zappi but the Andersen looks nicer ) .

But then in winter on a short gloomy day, I have no idea how much solar will be generated, in which case I guess charging the battery in the cheap overnight period becomes the obvious thing to do.

So I'm trying to work out the payback time and would really like to hear how other people worked it out and especially keen to know on how bad it really does get in winter.

Sorry this is a bit rambling guys!

Cheers.

 

[johnb2713]

Have a look here:-

https://www.in2gr8tedsolutions.co.uk/info/solar-generation-calculator.php
that will give you a breakdown month on month.

My take is a decent size inverter (mine is 6kW but is being upgraded), a battery is essential, and as much solar PV as you can accomodate / afford.

My system charges the batteries on Octopus GO tariff off peak, that feeds the house from 04:30 until the solar wakes up. The solar then provides the house power, recharges the battery and if anything is spare, heats the hot water tank. When the panels stop producing the battery provides the power until 00:30 when off peak starts again. This works really well over the last 3 months, I know we wont have enough storage for winter, I'm dealing with that.

Definitely go for an inverter 6kW ish and 7kWh battery, that should keep you covered. Smaller inverters, typically 3.6 kW will be calling for grid power when you run a couple of appliances eg oven & washing machine, kettle & toaster.

 

[up_north]

Hi John. Thanks for your reply. Unfortunately 10 panels is the most we can have. So at say 390w per panel we’d only ever be able to generate not much more than 3.6kw anyway. I can understand that a bigger inverter might still be better for battery draw but my (possibly wrong) understanding is that the inverter size should be as close as possible to the array size, or ideally slightly under? Unfortunately we have a combi boiler so can’t use solar for heating.

 

[johnb2713]

There are loads of solar experts around that will tell you to match or undersize the inverter. My experience has shown quite the opposite when a battery is installed. With a battery there you can handle peak demands if you have enough inverter power i.e. Kettle & Toaster on = 5kW demand, the battery and a 6kW inverter will meet the demand, a lower power one wont and will import from the grid.
I'm just about to increase my battery capacity be 16kWh and an additional 5kW inverter for these reasons.

 

[Murdoch]

This is the production off our 16A system for last year and we’re in Surrey.

I’m not convinced it’s worth us adding batteries ATM as most of our spare power goes to our immersion

As for calculating pay back I think this could be really tricky, if not impossible. In 2020 April was the sunniest month

 

[johnb2713]

Do you use any daytime peak rate units? If the answer is yes then a battery is worthwhile, charge up off peak / solar and stay off the expensive jWh.

 

[up_north]

Thanks Murdoch - that’s an interesting graph. And it highlights my problem. In total it will generate what we roughly consume. But not when we want it. Looking at those summer months, we would be producing energy that we have no use for. - can’t use it to heat water due to having a combi and car charger isn’t solar friendly although I guess I could charge it manually.

 

[Murdoch]

Yup.

When our daughter is at home from uni, she and my wife can empty our hot tank with ease, so the divertor to our immersion comes in very handy, as it does the rest of the year when we’re out, and the sun comes out

 

[johnb2713]

An energy diverter such as MyEnergi EDDI is a good addition. Whatever surplus solar energy you have be it 200w or 3kW will get dumped into the hot water tank very efficiently until it reached max temperature set on your immersion heater. It's a good way of storing energy.

 

[Andy1733]

I'm pretty new to solar and I'm learning quickly that unless your pockets are deep in the UK you will always need the grid.
My way of thinking is. Ignore November - February.
If you bought the equipment to cover the ROI. It would take a lifetime.
Get that fine Ballance from march- October. I have a 3.6kw inverter with 4kw panels. And 4.8kw battery's.
EG yesterday when the sun was shining my battery's were on 13% at 8am. And 100% at 12 noon. The amount I sent back to the grid was enough energy to fill another 7kw of battery's.
At 6pm After cooking a chicken dinner (electric over, hob, and kettle) the battery's were down to 32%. This was a mix of battery and solar.

The previous day it rained most of the day and the highest my battery's got were 32%. By 6pm they were spent. I don't charge over night so basically the battery's are to gain 4 maybe 5 extra kw per day.

Come winter I will swap to a low over night tariff to get max uses from the battery's

 

[up_north]

Thanks Andy - that’s useful and a similar setup to what I have a quote for, although we already have the low overnight tariff.

 

[Andy1733]

I would check your consumption as we're out at work all day and our typical usage is over 20kwh per day.

 

[up_north]

Hi Andy - Our consumption figures are right. Smallish house and only two of us. 9kw/h on a normal day, 12kw/h on a big wash day! . (Excludes car charging). We also use smart plugs to make sure TVs are not in standby. I worked out that 18 hours standby can be about the same as 6 hours watching.

Do you have electric heating or are you including car charging in that 20kw/h?

 

[Andy1733]

Nope no electric heating or ev charging.
I have a base line of 0.55kw though Ive noticed this has risen in the summer to 0.68. I guess the freezer/fridge are working overtime
I also run a koi pond and have a small freezer for the dogs which are raw fed. I've also a garage/workshop fridge for my beer

 

[Bob Smith]

Thanks Murdoch - that’s an interesting graph. And it highlights my problem. In total it will generate what we roughly consume. But not when we want it. Looking at those summer months, we would be producing energy that we have no use for. - can’t use it to heat water due to having a combi and car charger isn’t solar friendly although I guess I could charge it manually.

I take that is the purpose of the batteries to maximize the self-use of what has been generated.

 

[Richard-the-Ninth]

We also use smart plugs to make sure TVs are not in standby. I worked out that 18 hours standby can be about the same as 6 hours watching.

Are you sure about that?

Our TV uses 0.2 Watts in standby
We have a remote socket adaptor, it uses 0.8 Watts in standby

Smart sockets are great for big loads, but TV? no point.

 

[Andy1733]

Only thing I would add up north is. Whatever you expect your solar to do. 1/2 it.
A 3.6kw setup with 5kw battery I would say on a 20kw per day usage. Over 12 month I would save approx 1/3 on my usual usage.
Though come October 1/3 equates to about £1500.

 

[Murdoch]

Old fridges and freezers use much more power than you think.

 

[Andy1733]

Old fridges and freezers use much more power than you think.

Tbh m8 I have way to many. I have the American fridge freezer in the house. A small chest freezer for dogs. A under counter fridge for beer a big chest freezer for food. Pond uses about 300watt per hour....
Dilemma
Stop storing food. Stop keeping fish alive. Don't feed dogs or have warm beer

 

[binky]

Tbh m8 I have way to many. I have the American fridge freezer in the house. A small chest freezer for dogs. A under counter fridge for beer a big chest freezer for food. Pond uses about 300watt per hour....
Dilemma
Stop storing food. Stop keeping fish alive. Don't feed dogs or have warm beer

feed fish to dogs, use pond to chill beer

fortuately fridges and freezers use most leccy in summer when you Pv will be working most.