Category Archives: Why I didn’t buy..

Why I didn’t buy .. spray foam insulation.

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I recently signed up for a free home energy survey. After various questions about my home – so not much of a survey – my visitor alighted on spray foam insulation in the loft as a technology that should be of interest to me. Claims included:

  • Spray foam insulation would dramatically reduce my heating bills (“save upto 50% on future fuel prices”) – a sum amounting to hundreds of pounds of savings annually.
  • Spray foam insulation would result in a better EPC rating and thus increase the resale value of my home ( “.. Eco measures are adding up to 14% on property values.. “).
Spray foam loft insulation

Given that the salesman attached some credibility to the UK’s national EPC scheme which requires an assessment of each home before sale as evidence for the buyer, then a good start point would be to see what the EPC said about my loft insulation. According to my most recent survey from September 2015 (so almost 4 years old) my home, at that time, had 100 mm of loft insulation between the floor joists with a recommendation to increase to 270 mm thickness by placing another layer of insulation across the top of the floor joists which would reduce my energy consumption by £219 over 3 years. £219 over 3 years amounts to £73 annually. In fact, since the survey was completed, I added the additional insulation recommended in October 2016 so that saving has been made and 1 point improvement on the EPC scale obtained. According to Which? magazine quoting the National Insulation Association “100mm of spray foam insulation is equivalent to around 170mm of loft insulation” so my 270 mm of conventional insulation may provide more insulation than the proposed spray foam insulation. My £73 annual saving from the EPC is far from the hundreds of pounds annually being claimed by my guest. Thus I don’t think the claimed savings are justified.

Floor level loft insulation

Then there’s the question about what the impact on my EPC rating would be. The extra 170 mm added post-2015 survey added 1 EPC point to my house rating. That’s not a big increase.

Then there’s the question about what the value of that 1 point on the EPC rating would be. According to the EPC certificate itself the indicative cost of adding 170 mm of insulation is £100 to £350 pounds. I actually paid £300. It’s not reasonable to believe that any additional value added to the home exceeds the cost of doing the work for such a small scale job. The salesman actually left a written claim that “Reports now showing that Eco measures are adding up to 14% to property prices” – I suspect that you’d need a lot of Eco measures (well beyond loft insulation) to add 14% to property prices which would be around £70,000 for my home!

Then there’s the overall question of the cost. Bearing in mind that my current insulation cost £300 and may be equally if not more effective than the spray foam then what was the spray foam quotation for? Discounted to £7,658 with marketing testimonials after the usual call to his boss to get “approval”. Which? reckons £2,500 – £3,000 for a 3-bed semi, whereas I am blessed with a substantial 4-bedroom house so it might conceivably be double the price; but for any investment on that scale I’d want to see competitive prices not just one.

Finally the question arises if I really wanted to spend £7,658 on energy efficiency would more loft insulation be best value? Our old friend the EPC has a proposal in that sort of price range, and one that it believes delivers greater savings. And that proposal is.. floor insulation for my solid concrete floors. The EPC suggests that this can save £132 annually – 81% greater savings than that extra 170 mm of loft insulation albeit at up to 20 times the price. However I already ruled out floor insulation on grounds of poor return. (My guest wasn’t persuaded as to the value of floor insulation, but a check of EPCs on similar neighbouring properties by different surveyors all identified floor insulation as a greater benefit than more loft insulation)

Recommended measures from EPC

So for me the answer is clear – I don’t see any value in spending £7,658 on alternative loft insulation. For you the answer might be different – you might be planning to convert the loft into dwelling space in which case you want insulation against the tiles/slates rather than at floor level. However if you just wanted storage space then other solutions like a shed, storage unit or skip (!) could easily prove to be better value.

Why I didn’t buy .. a battery

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This week for the first time a salesman ventured into my home to sell me a battery for my solar PV system. His company were not the first to discuss it on the phone, or make an appointment, but this guy actually turned up which is a first.

The principal of such a battery is straightforward – there will (often) be times when you generate more electricity than you can use immediately, so why not store it for later use?

imageFirstly, let’s think about the architecture of the system that was being presented. This system puts a battery and a charge controller on the DC side of the inverter i.e. between the panels and the inverter. This reduces consumption on the generation meter when you charge the battery, but registers on the meter when the battery is discharged – not in itself a problem just a difference.

This architecture creates a system in which the battery can only charge in daylight and only discharge in darkness but to my way of thinking that’s less efficient than it could be. Firstly during my day there are often load peaks that go beyond what the panels can cover. Events like boiling a kettle, or the heating periods on the dishwasher or washing machine often push electricity demand beyond the immediate panel output, and you you might hope that the battery would cover these peaks and then recharge later – but no this system cannot discharge the battery during daylight so you’d end up buying electricity for these peaks. No discharge in daylight also means that activities like cooking in summer evenings on our electric oven and hob would be on imported electricity (even though the battery has sufficient energy) because it’s still daylight. Finally my Economy 7 meter tells me that I regularly don’t use as much as 4kWh overnight but to get pay back you want to empty and refill the battery as many times as possible.

Of course I could shift electrical load to the nighttime to empty the battery, but that’s contrary to trying to move load to the daytime to maximise use of PV directly. Which is better? Hard for the user to tell since no tools are provided to tell the user how much energy is in the battery, or any history of usage. By contrast my immerSUN produces detailed graphs of usage to show you what it’s achieving on my behalf. If this can be done with one device, why not also with a battery costing 20 times more?

How can I make sure the battery is empty in the morning when my normal night time usage would only be 1-2 kWh? Well you might consider running a heater to warm up a critical room or rooms at the start of the day (once you’ve got the equipment the energy is free of course), or in my case I might want to top up the electric car, is there a switched output or other mechanism to signal when the battery is exhausted and switch off such a device? No there isn’t.

So it can be hard to ensure that the battery is empty. What about filling it, how easy is that? Remember that the proposed battery has 4 kWh working capacity and sits on the DC cables between the panels and the inverter. Well, my 4 kW PV system like many is actually 2 x 2 kW systems feeding dual input inverter. That means that only half my panels would feed the battery so to fill the battery I’d need to generate at least 8 kWh in a day (and not use much of that during the day as immediate usage is prioritised over battery charging). My average daily generation between November and February is well below 8 kWh so there are many days in which the battery wouldn’t be charged fully (and may be not at all).

Then there’s the kicker – the price. If I look at a battery with 5,000 cycles life (that’s over 10 years although the brochure also quotes 10 years as the life) where each cycle is 4 kWh (which it will only be when new) that’s a lifetime throughput of 20,000 kWh. Electricity at today’s money is about 12 p/kWh so those 20,000 kWh are worth roughly £2,400 in savings. And the price of the unit? More than double that.

I did a detailed analysis including my actual ability to charge the battery based on real daily generation and usage, declining battery capacity with age and use (down to 80% of original capacity after 5,000 cycles), and energy price inflation; and concluded that energy price inflation would need to be around 30% annually each year just to get my money back over 10 years – without actually generating a return on the investment. While I do expect energy prices to rise faster than inflation in the long term I think 30% annually is unrealistic.

In the next few years I expect battery prices to fall as more cells are made for other purposes like electric cars, and also as used cells/batteries become available from electric cars at the end of their lives. I also expect revenue opportunities to arise through grid stabilisation and support where some access to the domestic battery is made available to support the local network.

However a battery on the DC side of the invertor which only charges in daylight and only discharges in darkness will be of little use to support the grid. I think a battery needs to be on the AC side of the inventor to flexibly interact with the house and grid based on charging to eliminate export and discharging to eliminate import in microcycles (unless the grid overrides that).

So I do expect to get battery storage eventually – just not now, not at this price, and not this solution.