Solar system savings - sense check

dai_bach

Hi all,

We had our solar panels installed in early 2023 but I've only just recently had a chance to sit down and look at what return we're actually achieving (not helped by some issues we had with our electricity meter). With that said, I was hoping you might be able to sense check my calculations, please? 

In terms of what we have:
11 panels 435w. 4 South facing and 7 West/South West facing. Solar iBoost. No battery. Based in NE England.

I'm looking at 2024, but this still involves a few estimates. I'm not concerned if it's 100% accurate but keen to understand if I'm in the right ball park.

So, my calculations I came up with are:

Generated  3,890 kwh (is this reasonable based on my set up?)

Exported to grid ~1,500 kwh at £0.15 per Kwh = +£225

Water heated from iBoost ~1,000 kwh at gas price of ~£0.06 per kwh = +£60 savings (this is a guess as the display on the iBoost isn't great for data)

Used (balancing figure) 1,390 kwh at avg. cost of £0.25 = +£336 savings

Total = +~£620 for the year.

On face value this seems fairly low, but maybe it is reasonable, what do you think?

One thing that did strike me when doing this was the iboost element - is it financially that straight forward that I would be better off selling at £0.15 and just buy the gas at £0.06 to heat the water tank? Appreciate this ignores environmental factor of not burning the gas if I don't need to.

Cheers!

Martyn1981

My first thought was answered later, that you'd probably be better off exporting leccy at 15p than heating the water. Totally get your environmental concerns, but remember your export will displace some grid generation, so hopefully balances out.

Worst case, in the summer, if you don't use much hot water, then the gas boiler (combi?) may be about 50% efficient. The boiler will be working from cold, and all heat lost from the pipework is not a benefit, but in the heating months, can be seen as space heating. So that has gas at 12p, and the reality is probably somewhere inbetween, perhaps 9p?

I think your calcs are correct, the theory is sound, as you asked. Do you know what you generated in 2023, even if only a part year, as 2024 was a really bad PV year. Worst I've had since 2011, by a wide margin, so at least that means you should hopefully (on average) earn a bit more, even if just as 15p export.

Edit - Just had a thought, if you have the 15p export tariff, do you have a cheap rate night tariff? Some folk have 15p export and 7p night rate, in which case, you'd probably be better off heating the water on night rate leccy, as the boiler efficiency will bring the 6p gas to a similar price, and in the summer, the gas will cost a lot more (as mentioned).

dai_bach

Thanks Martyn, good to know I'm on the right track!

Good point on the boiler efficiency point, hadn't considered that, but still overall seems better to just sell at 15p? Not really much benefit for the iBoost then? Only cost a couple hundred quid so not the end of the world..

No, don't have the cheap rate night tariff - do you know who provides that? I can take a look at moving if it makes sense.

2023 looks like I produced 3,310 kwh from mid April 2023.

Cheers!

Martyn1981

Hiya, Octopus offers that package, but it's for EV owners. Octopus Intelligent Go, and you need a compatible EV or fast charger. But might be worth you looking at packages to see what works best for you.

Cheap rate is an excellent addition to the 'electrify everything' package, but often doesn't work out unless you have a high leccy demand that can be shifted to night rate - the best example being an EV. Even with PV, it didn't work for us till we got a BEV.

Also, whilst adding batteries may be marginal at the moment for saving money, combining them with cheap rate allows you to charge the batts from night rate, for day use, during low PV periods. And, as odd as this may sound, might even be worth charging on cheap rate, and exporting PV, in that 7p / 15p example I mentioned.

Sorry to waffle, but another tip or trick some of us tried, was to install a modern air conditioning unit that works as a heat pump (an air to air unit (A2A)). So in the spring and autumn, you can use the excess PV to help heat your home, and reduce use of the gas central heating. At those times the COP (co-efficient of performance) can be 3 or more, so that one 15p unit of export, might displace 3 or more units of gas.

A2A isn't currently supported by the Gov for subsidy support, but the Gov should be announcing changes to the Warm Homes Plan early this year, which will include support for A2A units. But that might just mean loosening PP rules, not sure.

Solarchaser

Agree with Mart, figures look good.
Makes sense to switch off the iboost and double the savings from the export.
Also agree 2024 was pretty darn poor for solar.

You are a low electricity user household,  so battery return would be marginal, but selling the excess back is working pretty well for me as a high electric use household with no gas

Grandad2b

Your figures are about right - £600 saving for the year tallies pretty well with our setup (4.44kWp, south facing, West Yorkshire). We don't have battery or iBoost but an EV is on the horizon and that might change the economics if we can import a battery full overnight at 7p/kWh then export all our solar. Over 2024 we generated 3.7MWh, imported just shy of 2MWh and exported almost 3MWh.
We still have to get a handle on battery round trip efficiency - how much can you expect to get out for every kWh you put in?

 

Magnitio

dai_bach said:

Thanks Martyn, good to know I'm on the right track!

Good point on the boiler efficiency point, hadn't considered that, but still overall seems better to just sell at 15p? Not really much benefit for the iBoost then? Only cost a couple hundred quid so not the end of the world..

No, don't have the cheap rate night tariff - do you know who provides that? I can take a look at moving if it makes sense.

2023 looks like I produced 3,310 kwh from mid April 2023.

Cheers!

Your iBoost may become useful again when the price of gas increases or the income from exported electricity decreases.

Agree with many of the comments above that the last 12 months has not been great. This might just be due to El Nino, which should come to an end this year.

Reed_Richards

dai_bach said:

.... but still overall seems better to just sell at 15p? Not really much benefit for the iBoost then? Only cost a couple hundred quid so not the end of the world..

But a couple of hundred quid has wiped out your first year of export income.  If you did not ask for an iBoost yourself I would say it was mis-sold.  You could always have retrofitted one at such time when gas prices per kWh rise beyond the sum you receive per kWh for export.  

Screwdriva

dai_bach said:

In terms of what we have:
11 panels 435w. 4 South facing and 7 West/South West facing. Solar iBoost. No battery. Based in NE England.

Not sure what PGVIS forecasted for your location, but my initial gut instinct tells me 3900 kWh seems a bit low for a 4.8kW system, especially across S/ WSW aspects. For perspective, my system consistently generated the same with one less panel (10 bifacial panels spread across 3 aspects) and despite significant shading (2 panels shaded by a chimney for several hours a day). 

May I ask what brand the panels/ inverter system is?

As others have said, the iBoost is now financially redundant unless you're on FIT or have no other convenient way to heat hot water. 

Petriix

I've always said that iboosts or similar are a waste of money. Most people seem to pretend the diverted electricity would otherwise have been charged at peak rates to help with the confirmation bias. Slightly more complicated (and easier to pretend they're useful) for people on deemed export payments. 

I gave up deemed exports at the end of 2023. In 2024 we exported around 66% of our generation, all paid at 15p per kWh. We mostly just run the big loads overnight. If you can access a cheap overnight tariff then I highly recommend load shifting to maximise export. Some of the Economy 7 rates are quite competitive for those without EVs though I totally recommend getting one for anyone who can charge at home. 

SuzeQStan

We are on octopus intelligent go and we do as Mart mentioned - shift energy use to the cheaper tariff times 11:30pm-5:30am and we also charge our solar battery which we then use to power our home during the day. 

We do that all year round as it means we export all the generation we can in the warmer season and using the battery helps prevent us from using costlier energy during the day/evening in winter.

one thing to mention - if you have a hybrid plug in vehicle as opposed to EV you can still get Octopus Go which offers 4hrs of cheaper energy  9.5p per kWh at nite. Not as good a price as 🐙IO which is 7p per kWh for 6 hours at nite - but it still allows you to shift your energy use at a cheaper cost