My estimates do not include SC.

Other factors:

We work from home and heat main rooms to 21 deg 

Secondary rooms and bedrooms much cooler

Half building is very old & inefficient. Half is modern and well insulated. We mostly heat the newer part to 21 deg plus kitchen in old.

SW England,.. Somerset .. probably milder than some.

Predicted generation/consumption:

Net export 2675kWh during summer on IOF at an average of ~26p per kWh = £695

Net import ~2070kWh during winter on Cosy (assuming we self consume all solar generation) at ~13p = £269

Our SC is 365 x 47.53p = £173.50

£695 export - £269 import - £173.50 SC = £252.50 predicted credit for the year

So we are predicted to be running a positive balance of around £250/year giving scope to crank the heating up a little if it gets very cold this winter. That £252.50 could buy us another 1942kWh of heat in winter, giving me confidence that we can probably crank the thermostat up to 20C having run at a miserly 19-19.5C last winter, or maybe turn the DHW up by a couple of degrees. I've already increased our predicted usage a little as I accept we ran on bare minimum during our first winter as we didn't know where the costs/usage would be. I'm not looking to make a profit, if we are net zero cost I'm very happy.

IOG is 7p+ vat overnight. Other benefit is ensuring I maximise pv solar prod via exports - not using myself or filling batteries .. say mid May to mid Oct

Having moved over the years via Fixed, tracker, Agile and now Intelligent Go... my usage has massively changed

1. Instead of heating setback at night we raise the heat at night- as well as filling batteries

2. We fill batteries at night in summer as well as winter.... and export as much as possible during the day from solar and emptying batteries.

Very roughly we use

5800kwh ashp

2000kwh EV

3000kwh other usage

2000kwh imported to fill batteries ... and export during day

Say importing 13000kwh. If I can average 8p import cost circa £1040

Target is to export 6000+kwh @15p

In my perfect world a net cost of under £150. But sooooo many assumptions and unknowns 

@toodles

Of course. I've also used Tim and Kat's Green Walk (YouTuber's) excellent spreadsheet* for different Octopus tariffs that predicts monthly and annual costs, which has been very useful and saved me from reinventing that particular wheel. You can update it with your own monthly usage and generation figures, and edit the tariff rates for those of your own geographical area (or any tariff rates you choose). As expected, Cosy is best in winter for ASHP/small battery owners (unless you have a battery large enough to meet your daily demand then cheap overnight tariffs look better), and IOF is best in summer (financially). I ended up using it to model IOF vs Cosy, and ignored/blanked out the other tariffs for simplicity. What is useful is it will allow you to predict exactly how much of a financial saving a tariff like IOF will give you over alternatives, allowing you to make an informed decision if you think £x hundred per year is worth it for giving up control / cycling of your batteries.

* Note if using this spreadsheet, do take particular note of the assumptions that are made, as results will not be valid where assumptions are not met. One assumption is that the battery is large enough to cover daily usage and all imported electricity is imported at the cheap rate. This means it gives incorrect results in winter months where import cannot be met from cheap rate or battery (e.g, high usage, small battery and a cheap overnight EV type tariff)

Now taking readings from my Tesla app for the year 2024, I find the following:

Home consumption 9.8 MWh.

Imported from the grid 9.4 MWh.

Exported to the grid 4.8 MWh.

Solar production 6 MWh.

Octopus app gives me the following figures:

Consumption 10.028 MWh. (£1,415-23p.)

Exported 6.349 MWh. (£952-39p.)

My Enphase app reports:

Consumed 10.6 MWh.

Imported from grid 9.9 MWh.

Exported 5.3 MWh.

Solar production 6.3 MWh.

Net imported 4.5 MWh.

By the way, sorry about my incorrect reporting of figures in my previous posting. I’ll try to do better in the future! Regards, Toodles.

Well by my reckoning based on the Flexible Octopus unit rate of 25.94 pence/kWh which I believe is relevant to your neck of the woods, that gives a spread of 82.3% saving to 83.63% saving, with the Octopus consumption figure giving the middle ground figure of 82.7%.

Given those figures, if any of my neighbours came up and asked my natural answer would be "I'm saving a little more than 80%", but I wouldn't feel in the slightest fraudulent talking about an 83% saving. I can also guarantee that, as @old_scientist suggested, the jaw would drop open at the 80% and the extra 3% wouldn't really even register because four fifths is already a VERY relatable - and attractive - figure.

I think people can relate to terms like our bills have halved or we have no bills, whereas a purely monetary value can be subjective. Some may consider £100/month a lot for an electricity or gas bill whilst others may consider it cheap.

Now taking readings from my Tesla app for the year 2024, I find the following:

Home consumption 9.8 MWh.

Imported from the grid 9.4 MWh.

Exported to the grid 4.8 MWh.

Solar production 6 MWh.

Octopus app gives me the following figures:

Consumption 10.028 MWh. (£1,415-23p.)

Exported 6.349 MWh. (£952-39p.)

My Enphase app reports:

Consumed 10.6 MWh.

Imported from grid 9.9 MWh.

Exported 5.3 MWh.

Solar production 6.3 MWh.

Net imported 4.5 MWh.

By the way, sorry about my incorrect reporting of figures in my previous posting. I’ll try to do better in the future! Regards, Toodles.