Living with Solar: part 4

In this final part, I discuss the economics of solar, the various optional extras you can choose, and whether it’s worth the investment.

How much does it cost?

The total cost of the system will depend on a number of factors, most obviously the number of panels, but also the optional extras you buy. Interestingly, the panels themselves will likely not represent the largest proportion of the total. This how my quote broke down:

A system with batteries and other optional extras can easily cost 50% more than a plain panel-only system. So which options should you take?

Batteries

Until a couple of years ago this was a difficult decision, but in my opinion it’s now easy.

Over the last few years, the capacity has doubled and the price has halved. A 3.5kWh lithium-ion battery is now under £1,000 (Pylontech US3000c). This has a claimed life of 6,000 cycles, or around 15 years, with “deep discharge” that lets you use 95% of the stated capacity.

Choosing batteries at the time of installation means you will need a hybrid inverter, which will add around another £500, but this is cheaper than getting a separate battery charger later. Furthermore, any batteries you buy when the system is installed are now free of VAT in the UK.

Adding £1,500 to your system to get 3kWh of extra energy every day could nearly double the solar electricity you can use instead of returning to the grid, making it a no-brainer in my opinion. The capacity will reduce as the batteries age, but even if the average is only half, 6,000 cycles amounts to about 10,000kWh of electricity stored and used. At current pricing, that’s a return of about £3,000 (versus, at best, £750 for returning that energy to the grid).

The next question is, how many batteries should you buy? These Pylontech batteries allow up to 5 to be connected together. But it’s not easy to know in advance how many will give you the best return.

The system designer recommended one battery, but I went for two, giving a 7kWh total capacity. Why did I make this choice? I made a simple calculation: if I use 10kWh per day, and let’s say there’s only 8 hours of usable sunlight, then I need to power the house for the remaining 16 hours, meaning I need to store two-thirds of 10kWh for use overnight.

This calculation is overly simplistic. Overnight usage is lower than average; and in winter, I expect there will be many days where there won’t be 8 hours of daylight sufficient both to power the house and charge that amount of battery.

However, based on the initial usage patterns I see in late spring, I already think I made the right decision.

Firstly, when the battery fills to 100% in the daytime, overnight it drops to around 45% by the time is starts charging again the next morning¹, or 35% if the oven or hob have been used for cooking. This means that if I’d bought a single battery, it would be completely empty and I’d be using grid power for part of the night.

Battery State Of Charge (%) on 13th May 2022, with 6 panels. There was enough sunlight on this day, and the previous one, to charge to 100%. Times in UTC. The drop at 17:15 (6.15pm local time) is due to electric cooking.

Secondly, when the battery drops to 20%, it stops discharging. So in practice, the usable capacity is not quite as high as I’d thought. (This is something that I believe can be configured on the inverter; I suspect it has been set to keep some charge for emergency use).

11th May 2022 — discharging to 20%, again with only 6 panels.

Thirdly, each battery has a limited rate at which it can charge and discharge. By having two batteries, I can discharge at double the rate — meaning that when there is peak usage, there is less that needs to be pulled from the grid.

Here’s an example where there are several appliances running in the kitchen, and the sun is obscured by cloud:

High power consumption, mostly covered by the batteries

Looking at the specifications of the US3000c, I see:

  • Continuous charge/discharge: 37A (= 1.78kW at nominal 48V)
  • Peak for 60 seconds: 74–89A (= 3.55–4.27kW)

Hence one battery may be able to support a short burst for kettle and toaster, say; but it won’t be able to power the oven and hob together for an extended period.

With two batteries, the continuous discharge rate is doubled to around 3.5kW, which matches what I see in the graphic above. It means less power is required from the grid under peak loads.

Three batteries could increase this further, but then I’d hit the inverter’s own limit of 5kW, and such high peaks are rarely reached anyway.

For me, in a household that has an electric oven and hob, two batteries appear to be the sweet spot, and I estimate that the second battery will save me 1–2 units per day, at least in the sunny parts of the year; I believe I’ll recoup the cost of the second battery in a similar timeframe to the rest of the system. But if you cook with gas, there may be less benefit.

Eliminating every last drop of grid electricity can become an obsession, but unless you’re building a completely off-grid system, it doesn’t make sense to buy more battery capacity than you need. In the next few years, I expect the technology to improve and prices to fall further.

Bird netting

This one is a no-brainer too, and my panel installers said I had made the right decision to install it.

Bird netting at edge of panels

If birds decide to nest behind your panels, it will get very messy and expensive to clean up. Although my neighbour has had a solar PV system for many years, and hasn’t suffered from birds, I would rather be sure there’s no chance of this happening.

Optimizers

My system has two strings of PV panels: one of six panels and one of eight. This matches how the panels are laid out across two roof surfaces.

Since they are connected in series, the same amount of current passes through all the panels in a string. If one panel is generating less power than the others — due to partial shading, or bird muck on one panel, or even just manufacturing variations — then the current through the other panels is reduced too, which reduces the total power they can generate.

Afternoon tree shading—taken before the panels were fitted

The solution is to install electronic “optimizers” behind each panel, which adjust the panel output to match the others. Some manufacturers claim that even in normal operation, optimizers can increase the whole system output by up to 5%. That’s nice to have, although it would be cheaper just to add another panel (if you have the space). They cost £40-£50 per panel.

I decided to fit them because of a high tree line to the west, which means shading is an issue in the late afternoon. Without them, power could drop off more rapidly when some panels are shaded and others not. In practice, the shade covers the whole roof within the space of about half an hour, so I suspect it doesn’t make a great deal of difference. I still like the idea that I’m getting maximum output from the panels though.

If you have panels oriented in different directions across multiple roof surfaces, then they should be on different strings, feeding separate inverter inputs. If you have to combine them onto the same string, then you really do need optimizers.

Micro-inverters have also provide this benefit, as well as some other features relating to monitoring and system safety. But they have their own downsides: for a system with battery, power is converted from DC to AC and back to DC again, which is less efficient. Also, if they fail, they can be harder to replace individually than a monolithic inverter.

Solar diverter (iBoost)

If you have a gas boiler with a hot water storage tank, and that tank also has an electric immersion heater element, you can divert your spare solar energy to heat the water instead of exporting it to the grid².

iBoost controller (image from Marlec)

The iBoost has a sensor which clips on the main power cable into the consumer unit, and signals wirelessly to a main unit connected to the immersion heater. When it senses that energy is being exported to the grid, it energises the immersion heater. It varies the power delivered into the heating element in response to the amount of energy being exported. When the tank reaches its maximum temperature, the immersion heater’s thermostat turns it off, so any more excess power is exported again.

The iBoost has its own energy usage meter, which is important because otherwise you can’t tell the difference between your regular household usage and the iBoost’s opportunistic consumption.

iBoost in action on a sunny day. Green = battery charging; Orange = consumption, including iBoost; Blue = grid export

It certainly works, and is relatively cheap at £300-£350 when installed with your system, but I’m not sure whether I’d take this option again.

If it picks up most of my excess generation, I reckon the iBoost could pay for itself in 3 or 4 years via reduced gas usage. However the cost of 1kWh of gas, currently 7.32p for me, is much less than 1kWh of electricity³. If I get a smart meter⁴ and can get a good SEG tariff like Octopus Fixed, I could earn 7.5p per kWh just by selling it back to the grid.

An iBoost (or similar device) certainly makes sense if you have a very large PV system that runs completely off-grid, and you have nothing better to use the excess power for.

There is one undeniable benefit though, which is that it reduces my personal carbon emissions. Every kWh of PV electric water heating is one less kWh of gas that I burn. One kWh fed back into the grid may reduce carbon use somewhere else, but it depends on how much is being generated by renewables that day.

Emergency consumer unit

Given a PV system with battery, you might expect that it can power the house in the event of a grid power outage. However, it turns out that many systems cannot do that. They are designed to shut down completely in the event of a power outage, so as not to energise the outside power cables and create a safety hazard for workers trying to fix the lines.

If you do want to power your house this way, you need to have a system which can isolate itself from the grid — with either a manual or automatic changeover switch. However, it seems to me that this is not as useful as you might think. After switching over to battery power, you are likely to end up running around your house switching off as many devices as possible, just to preserve the battery energy.

There is another option though: the Solis hybrid inverter in my system has a second AC output which remains continuously powered. By hooking this up to a second consumer unit, you can have one socket ring (16A) and one lighting circuit (6A) which are continuously powered.

I took this option and used it to power a double socket next to my Internet router, acting as a large uninterruptible power supply. The low power draw of the router should result in many hours of run time in the event of a power outage, without the need for a separate UPS.

EV charger

I don’t have an electric vehicle, so I didn’t take an EV charger. I was told to expect to pay around £700-£750 for a basic car charger, or just over £1,000 for an intelligent one (such as “Myenergi Zappi”) which I believe can divert spare solar power to car charging, similar to the iBoost for hot water. Note that a home EV charger will require a 32A dedicated breaker from the consumer unit, which might have to be upgraded.

If you have an EV and it can be connected at home during the day, then this could be a valuable way to use spare solar generation capacity. If you drive to work, and the EV is only parked at home in the evenings, then this doesn’t help you much. You will still most likely want a home EV charger, but you’ll probably rely on an EV tariff which lets you charge your vehicle from the grid cheaply overnight.

Installation and scaffolding

You can’t avoid professional installation, and this is likely to be the largest part of the cost — but is also why it’s worth getting quotes from several suppliers.

My property is a bungalow, and in the end the installers decided that scaffolding was not necessary. They deducted it from the final bill.

No VAT!

Until recently, solar installations were subject to a reduced rate of VAT of 5% (except for batteries which were still 20%), with the proviso that the installation charge had to account for at least 40% of the total cost.

From April 2022, the VAT rate has dropped to zero, including batteries installed at the same time as the rest of the system. This gives a small but welcome reduction in the cost.

Fixing your roof

Finally, your roof needs to be in good condition, because repairing it after the system has been installed will be expensive. So if you’re not sure, get it checked by a professional roofer, and do any necessary repairs first. If you want to get moss removed, this is the time to do it as well.

How long will it last?

On my system the panels have a 12 year warranty, but should easily last 20 or 25 years. They do degrade over time, and their generation capacity is expected to drop by 0.5%-1% per year. The manufacturer warrants that after 20 years, they will still produce at least 80% of their original output.

The inverter has a warranty of 10 years (5 years manufacturer warranty plus 5 years extension purchased by the installer). I’m expecting to replace it after 10–15 years, at a cost today of around £1,250 plus installation.

The batteries have a design life of 15 years and 6,000 cycles, and a 10 year warranty. When it comes to replacing them, they are likely to be cheaper and better than today.

There shouldn’t be any ongoing maintenance costs. A slight increase in output may be obtained by professional cleaning, although the cost is likely to outweigh the benefits.

In addition there is a risk of unexpected repair bills, for example if a panel or optimizer completely dies and disables an entire string.

Will it pay for itself?

Electricity usage

Subtract two meter readings one year apart, to determine your annual electricity usage. You can get this information from your bills. You’ll need this figure to work out your potential savings, and the designer of your system will also want you to provide it.

My household annual usage is about 3,600kWh.

Note that usage in winter tends to be higher than in summer, due to increased use of lighting, indoor entertainment like TVs, and other appliances like clothes driers. I have LED lights so this effect is less pronounced.

System yield

There are a number of things which can affect the system yield, most importantly:

  • Location. I am in Kent, which is one of the sunniest parts of the UK; if you live further north then yields will be lower.
  • The type and number of panels, which in turn is limited by the size and shape of your roof, and what you can afford.
  • The slope of your roof (“elevation”) and its orientation (“azimuth”). A south-facing roof is optimal; mine faces south-west which is still good.
  • Shading due to hills, nearby trees, or other obstructions.

Your system designer should take all this into account and give you a predicted system annual yield figure in kWh, separated into self-use (electricity you generate and consume yourself) and grid feed-in values. Multiply the first by your electricity unit cost to get your annual saving through reduced electricity bills. If you are eligible for the Smart Export Guarantee then multiply the second figure by your SEG rate to see how much income you will earn from that. (Beware: SEG values are tiny, in some cases as low as 1.5p per kWh)

My predicted figures, for the initial design with 4kWh of battery storage, were:

  • Total yield: 4,902 kWh
  • Self-use: 2,257 kWh [includes energy stored in battery and released later]
  • Grid feed-in: 2,645 kWh

These figures are estimates, based on modelling. Until I have run the system over a full year, especially over winter, I won’t know the true results.

Calculating the savings

Given that my annual usage is 3,600 kWh, the prediction is that 2257/3600 = 62.7% of my electricity should come from solar. At the current exorbitant rate of 29.5p per unit, this amounts to £665 per year saved.

When I get a smart meter, on Octopus fixed SEG tariff I could receive 7.5p per unit for grid feed-in, bringing in potentially another £198 per year.

Since I have an iBoost, much of this may get diverted to hot water heating instead, but my current gas price is very similar at 7.32p per kWh.

What about environmental benefits?

This is hard for me to quantify.

Firstly, any electricity you generate and use or export means a corresponding reduction in centrally generated electricity. A proportion of that, however, is already renewable. You can find the UK’s power mix figures here, updated in real time. Over time, the benefit of a personal PV system will decrease as the UK moves towards more non-carbon based central generation.

Secondly, the system itself, particularly the panels and batteries, will have generated CO₂ from their manufacturing process, together with the mining and refining of the raw materials.

Estimates I have found online suggest that the panels will pay back their initial carbon debt in around 2.5–3 years.

Finally, with a solar PV system, there’s little incentive to conserve energy. Wash the laundry at 40°C instead of 30°C? Sure, why not — as long as the sun is shining or the batteries are full. This won’t change until metered export tariffs rise from their current paltry rates⁵.

Conclusion

It will be a year before I’ll know the true performance and benefits of the system; and in that time, the price of grid electricity may have gone up or down.

But I’m very happy to have a system which can reduce my reliance on the grid, and still very excited to watch all that “free” energy I’m generating.

Based on my experience, if you decide to go with solar my advice is:

  • Get as many panels as you can fit. Panels are a mature technology and a relatively small proportion of the total cost; nobody complained about having too much power. (However, above 6kWp you may need a more expensive inverter, and anything above 7.5kWp may be difficult to integrate into a domestic wiring system).
  • Get a battery system with 3–8kWh capacity. Go for the higher end if you use electricity for cooking, or you tend to do your laundry in the evenings.
  • Get bird netting.
  • Think carefully whether any other extras are useful enough to you to justify the cost.

¹Those figures were with only 6 panels in early May. With 14 panels in late May, more power is generated in the late afternoon and early morning, and I found the battery only dropped to 55% without cooking. Still, the experience with fewer panels shows what things might be like when the days are shorter.

²My hot water tank has a 200 litre capacity. Each 1°C rise in temperate requires approximately 200 x 4.1 kJ of energy, or 0.23 kWh. To heat from 20°C to 60°C will use approximately 9.2 kWh. I have now set my gas timer only to heat water in the evenings, so that after our morning showers, the iBoost still has work to do.

The heating element draws power up to 2.85kW, so anything above that, plus normal household usage, is still exported to the grid.

³Gas meters don’t count kWh: they measure the volume of gas used. The conversion factor varies depending on your location, and you can find it on your gas bill.

⁴I’ve requested one, but am still waiting. Apparently they install them on an area-by-area basis.

⁵Those people who were lucky enough to get the old Feed-In Tariff scheme are paid handsomely for their generation. But under that scheme, they were paid for their entire solar generation, whether they used it or exported it. So they also are incentivised to use as much as possible.

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