Solar diverter - flawed return on investment calculations

iotum

Screwdriva said:

Batteries still do make sense if you have a heat pump or drive alot in an EV but for the average household, Octopus Flux torpedoed any remaining financial rationale for a battery. 

The hot water diverter's best days are behind it I'm afraid, unless you're using alot of oil to heat hot water. 

Being on the old FIT scheme I've not paid much attention to the Octopus plans. After a quick look it seems that the Flux is aimed at households with a battery. So I don't understand how it "torpedoed" the financial rationale for a battery. Could you enlighten me please?

mickyduck55

Like some of the previous comments since moving to flux I have turned off my Solic 200 as its financially more prudent to export my excess rather than consume it heating water.  I have even started using my 10 kWh electric shower rather than heat a full tank of water. 

JKenH

iotum said:

Screwdriva said:

Batteries still do make sense if you have a heat pump or drive alot in an EV but for the average household, Octopus Flux torpedoed any remaining financial rationale for a battery. 

The hot water diverter's best days are behind it I'm afraid, unless you're using alot of oil to heat hot water. 

Being on the old FIT scheme I've not paid much attention to the Octopus plans. After a quick look it seems that the Flux is aimed at households with a battery. So I don't understand how it "torpedoed" the financial rationale for a battery. Could you enlighten me please?

The reason for purchasing a battery is to time shift, either store your surplus generation to use when there is insufficient solar or to import electricity at a cheap rate to use when import rates are higher. 

Octopus Flux doesn’t physically store energy like a battery but instead stores the value of that energy. The energy you get out of a battery is less than what you put in, perhaps 20% less. Similarly, the value of the energy that you get back from Flux is less than the value of using the energy at the time it was generated, on average closer to 30% less.

At the end of the day, you are buying a battery primarily to reduce your energy bill. (There may be a fun factor or the satisfaction of being more self sufficient but it is hard to put a value on these.) 

Imagine your energy usage is 5000kWh p.a. You generate 4000kWh and (without batteries) can only consume about 2000. The other 2000 goes to the grid for which you might be paid 5p/kWh. (£100). You pay, say, 33p/kWh for import so your bill is 3000 x 33p = £990. Your net bill is £890.

You decide to buy a battery to make better use of the other 2000 kWh which you are exporting. In summer you might have 25kWh generation on a good day but in December you will be lucky to have 5kWh. Your average generation is around 11kwh/day and your average consumption closer to 14kWh /day (less in summer, say, 8 kWh, and more in winter, say, 20kWh). So in summer you might have a surplus of 15kWh but if you put that into a battery what do you do with it? Around 6kWh of your summer’s day consumption will be covered by the solar you have self consumed leaving only 2kWh  to be covered by the battery . So in summer all you are getting out of the battery is 2kWh/day. In winter perhaps all you will be able to put in the battery is 2kWh but in spring and autumn you can perhaps save and reuse more. Unfortunately we can’t rely on averages and the days when you need your  battery the most, there will be less sun to fill it and vice versa). Depending on its size your battery might enable you to recycle 75% of your solar PV -say 1500kWh but you will only  get around 1200kWh out of it. Potentially you could save £396. (1200 x33p) but you will lose 1500kWh of export payment (£75) £45 so your net saving is £321 £351 and your net bill is £669  £639

Or, instead of a battery you switch to Octopus Flux. The import rates on Flux are (approximate figures) 3 hrs at 19p, 3hrs at 45p and 18 hours at 32p. Let’s take an average import rate of 32p. You self consume 2000 kWh so your bill for import will be £960 (3000x32p). Export rates are 22p and 35p but veer on the safe side and say all your export is at the lower rate of 22p so you receive £440. Your net bill is therefore £520. (Edit: plus deemed export lost of £120 total £640).

These are very rough figures but give an idea of why Flux might make a battery redundant.

You can of course make better use of a battery by importing electricity at cheap rates and reusing it later but I will leave someone else to do the calculations for this.

edited to correct figures

Reed_Richards

JKenH said:
The reason for purchasing a battery is to time shift, either store your surplus generation to use when there is insufficient solar or to import electricity at a cheap rate to use when import rates are higher. 

There is a secondary reason which is to give you more power than your solar panels alone can achieve.  Even boiling a 3 kW kettle is liable to require more power than my solar panels can provide if the sun goes behind a cloud.  But the panels working in combination with my battery, I don't have to wait until the sun shines to have my cup of tea.  And my heat pump sometimes wants more power than my panels alone can provide in full sun at noon. 

Alnat1

It can be hard to time your washer's/dishwasher's heating phase with the sun shining but with a battery that's not a problem. It also means you can cook your tea after sunset without paying premium rates.

Exiled_Tyke

I firmly agree with both of the above comments.  The battery smoothens out the 'free' supply electricy negating the need to time appliances precisely and allowing for higher consumption than the panels alone allow.  On top of that, even in summer, we are high users after the sun has gone down as we eat fairly late (at peak electricity price times) and often use the oven so the battery covers that too.

JKenH

In response to the above, I don’t see how that differs financially from using Flux. If you have surplus solar during the day you store up the value of what you export and spend it later. If you have spare generation between 4pm and 7pm you get paid 35p/kWh and buy the electricity back in the evening at 32p. 

The beauty of Flux is that there is no limit to how much electricity you can store or for how long. With a decent size array you can store 500kWh worth of solar PV in the high summer months to spend in winter. 

Edit: so far this month (and it hasn’t been a sparkling one so far) I have exported 142 kWhs and imported 69 kWhs so have stored 73kWh worth for future use. With a battery after covering the 69kWh imported those 73 kWh would have been lost 

orbit500

Interesting. I’m on old style FIT. If I switched then I would lose my deemed export of about 2.5p/kWh but keep the 5p generation payment.
I have 12kWp solar and 30kWh battery. I’m a heavy user at 16000kWh and a heated pool. Current setup is EDF goelectric35 with 4.5p night rate and 46p day.
I’m going to run some numbers!

JKenH

It will be interesting to see the numbers but my gut feeling is you’ll probable be better off staying with EDF.

Grandad2b

JKenH said:

so far this month (and it hasn’t been a sparkling one so far) I have exported 142 kWhs and imported 69 kWhs so have stored 73kWh worth for future use. With a battery after covering the 69kWh imported those 73 kWh would have been lost 

If you were off grid those 73kWh would be 'lost'. Surely if you remain on grid you are still able to export?