Without meaning to sound dismissive, @toodles, I think you may be overthinking it a bit. My assumption would be that all any visitors are really interested in is how much less your leccy bill is as a result of the solar PV setup you have; how much you are "saving" over not having the setup available.

• Running your home, you are able to use electricity generated for free. That's definitely a saving.
• You are able to store excess free electricity for use later. That's a saving.
• You are able to export excess generated electricity and be paid for it. That reduces your overall bill and so is a saving.
• If you need to import electricity from the grid, the battery allows you to do it at a different time from when you need to use the electricity - most likely when the cost is cheaper. As a result, you can get benefit from different tariffs. That reduces your overall bill, so is a saving.
• If you really wanted to get fancy, you have the capability to "play the market" by importing and storing cheap leccy and then export at a higher price, pocketing the difference. If you do that, it's reducing your overall bill and so is a saving. If you don't, it isn't.

As for how much those savings work out at in pounds, shillings and pence, I'd say a quick fag packet calculation would be quite adequate. You know how much leccy you use and the bills you pay, and you can see what the standard variable tariff would be in your area. Anyone asking an "in passing" question like that is only going to be interested in approximates so a percentage saving to the nearest 5% or so would be fine. For us, our usage is high since we have both a heat pump and EV. Our net bill for the last year was just under £2,000 and if we'd been on the current standard variable tariff rate for the last year we'd've had to spend a little over £3,300. All bar the shouting, that's a 40% saving.

We have several neighbours who, at various points, have asked us similar kinds of questions. If I start mentioning enough context to give an "accurate" answer, their eyes start glazing over before I've got anywhere near a figure. If I say that my bill's about 40% lower, they're instantly interested enough to start asking for the extra detail anyway.

@majordennisbloodnok Yes, I’ll try not to overthink it all! A saving is a saving, large or small, as you say, giving an approximate percentage is probably good enough initially and avoids the ‘MEGO’ effect! But…. how much am I REALLY saving???!!!😉😆 Frugal Toodles.

Posted by: @toodles

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But…. how much am I REALLY saving???!!! Frugal Toodles.

He, he.

If you really wanted to work it out, I'd suggest you take a look at all the tariffs available to you over the last year and remove all the ones you couldn't've used without solar PV and battery. Look at the historic prices for them and take the cheapest for each month (Octopus, at least, allow you to switch tariffs pretty regularly) to work out what monthly bills would have been, then compare with what you've actually spent (ignoring capital expenditure). It won't be much different but it will be more accurate.

For most people, the real clouding of the issue is the combination of adopting more electrical devices with the ability to generate the electricity to power them. Solar PV and battery plus EV is a far bigger potential saving than just the savings of moving from diesel to electric for the car. Even so, anyone chatting with you will still only want the caveats if they ask for them.

@majordennisbloodnok In my case, comparing energy costs over the last 4 or 5 years is fraught with ‘gotchas’ as we have changed suppliers with the previous supplier (Eon Next, I’m looking at you!) apparently being incapable of supplying sensible statements of consumption and in 3 years, never supplied a single statement that didn’t need correction! Once with Octopus, I have changed the tariff a few times and during this time I have gone ‘all electric’ including getting rid of the gas boiler and adding a heat pump; as a consequence, all  attempts at working out average energy consumption have at best been estimates. According to Octopus figures, my annual consumption for ‘electricity only’ amounts to ~£2000. after our export is taken into account, we have paid ~£450. During this time, we have made few changes to our electrical device consumption and the main variation will be that of how cold / mild the weather is! (Whilst writing this, the washing machine, the heat pump tumble dryer, the air fryer and a microwave oven are all running in addition to the background domestic load.)

Regards, Toodles.

Understand what you mean, @toodles, but I don't think you need to go that far back; I think the last 12 months would be fine. Are you able to get your last year's actual consumption (i.e. house load, not what was imported) from your inverter? I'm assuming so and would be rather surprised if not. On that assumption, you've got a grand sum total of kWh you used over the year and a total you paid for it (the £450 you mentioned).

The Octopus standard variable tariff (which we would have been on if timeshifting of usage wasn't possible for us) for our area - not too far away from you, I believe - is at the moment 26.39 pence/kWh and I could obviously have got historic prices too. Even if Eon, for instance, misbilled you the price per unit is still a fixed and advertised fact for a given time period. That price per unit combined with your year's total consumption (or prices per unit combined with monthly consumption figures over the last year if you wanted to be even more precise) is perfectly reasonable as a direct comparison. I understand your device consumption has varied over the year but the variation has been when, not if, that consumption was necessary. Even the washing machine, dryer, air fryer and microwave are things that need to be used even if the "when" might have been different under other circumstances; in fact, I'd argue that they ARE part of the background domestic load rather than in addition.

I don't for a moment claim this whole process will give you a comparison accurate down to the last penny or so but even for you wanting to be reasonably accurate I'd say it's going to be pretty damn close - close enough IMHO to justifiably describe savings in terms of single percentage points or even tenths of a percentage point. It'd certainly be far more difficult for someone on a standard tariff to estimate what they would've saved if they'd had all the kit you've got and taken advantage of all the available TOU tariffs.

@majordennisbloodnok Of course, you are right Major, I had been trying to use a number of years to obtain a meaningful average (and created much extra work for trying!).😉

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.

In answering your original question, we are fortunate to now be in a position where we expect to be net zero, both in terms of our generation meeting or exceeding our usage, and in terms of cost annually. So the simplest answer is we now have no bills. How much do we save? Before we began this journey of decarbonisation, we spent roughly £2,500 a year on oil and electricity (around £100/month on each) heating and powering our house.

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.

@old_scientist Quarterly updates would be much appreciated please! Toodles.

Posted by: @toodles

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@majordennisbloodnok Of course, you are right Major, I had been trying to use a number of years to obtain a meaningful average (and created much extra work for trying!).😉

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.

Posted by: @toodles

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@old_scientist Quarterly updates would be much appreciated please! Toodles.

@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)