PV solar panels and battery - strategy

Get a quick return on your investment

For many years the approximate payback period was 25 years. About the life of the panels. As the uptake of home PV solar panels increased, and accordingly, the cost decreased, so did the government incentives. The incentives kept falling, but the calculation remained the same, 25 years payback. That is not a good return on investment, ROI, as at the end of the period, although you have paid off all the costs due to the savings and any income, you have an asset which is at or near to end of life. 

However, things have changed, considerably, recently.

Even when we bought ours, the estimated payback time was, according to my calculations, about 16 years. The installers calculations were slightly more optimistic, but not by so much as to be farcical.

Currently, from the Energy Savings Trust,

How much do solar panels cost to install?
The average domestic solar panel system is 3.5kWp and costs around £7,000.

 Just a quick look at the above table shows a much improved payback period, provided you sell your excess power generation. More on that later.

Also of note, is the impact of how you occupy your home. Which equates to how much of the self generated electricity you can use directly. Again, more on that later.

The criteria for a quick payback appear to be a big array of PV panels and as simple an installation as possible. I.e. Save as much as possible at the same time as spending as little as possible on the installation. Excluding, in my opinion, all schemes that involve not owning the PV panels, but renting part of your roof to third parties.

Maximise the green benefits

I imagine if environmental considerations are your top priority you will want to reduce CO² emissions, from whatever source, as much as possible. Generating your own electricity is a good way to do that, whilst part of the National Grid mix is still carbon intensive. See extracted live National Grid data here.

There are a number of arms for reducing CO² emissions;-

1. Reduce your own consumption of energy.
2. Generate as much of your own energy as you can.
3. Use as much of that generated energy yourself.
4. Even renewable energy from the grid has transmission inefficiencies, so it is better to use it locally, without the distribution losses.

Reduce your own consumption of energy

Reduction, being careful of what energy you use, and what time of day, is something of a given, but is not part of the scope of this article.

Moving swiftly on.

Generate as much of your own energy as you can

If we generalise, a home may have four sides, or walls. Each facing North, West, South, and East, with a few degrees of rotation away from these points of the compass. The sun rises in the east, pops round to the south, where it is at it's strongest, and highest, then falls back down to set in the west. If you have a large plot of land and a lot of money you could set up a miniature Goonhilly Earth Station, covered in PV tiles, that could track the sun each day, in both rotation and elevation.  However, it is not very likely, and nor is our ideal house, but continuing with it. On a sunny day, all day, with three equal arrays of PV panels, installed on three of the four elevations of roof, your power generation would be a very nice looking bell curve.

However, it may be cloudy in the morning which would depress the start of the bell curve. Conversely, in the evening, which would give the bell curve something of a tail.

Given that you want to maximise your energy generation, you will not want to miss any of the precious sunshine whichever part of the day it comes out to play.

Hence all elevations will be covered.

In reality, it is more probably that the array sizes are different, and that on an individual array calculation, one of the elevations, is less economically viable. However, as the primary consideration is educing CO² emissions, and the associated objective is to maximise generation, all of the three elevations would have PV solar panels. If the cost of PV panels keep going down, I have heard some say that even north facing elevations can now be considered, as just daylight produces some generation with modern PV panels.

Use as much of that generated energy yourself

When do you normally use energy? Does that coincide with when it is being generated by the proposed PV solar panels. Of course not. So some form of energy storage is required. There are different ways to do this, but in a domestic situation batteries are considered the normal option.

Your objective is still to reducing CO² emissions, so anything you generate, that is not used instantly, will need to go to the batteries for storage.

The capacity of those batteries will have to be sufficient to store all of your excess energy generation, in the hight of summer, on a sunny day. How much power do you use on the average day, and how much of that is after the sun has gone down? Is that sufficient to drain the batteries ready for another sunny day tomorrow? How many sunny days do you expect in a row?

However, there is  balance between how much energy you can usefully store and the cost of another battery.

At least one high capacity battery is required so as to meet the objective, but perhaps more.

Maximise your level of self-sufficiency

 Self-sufficiency is when you generate and use your own electricity. The percentages are considerably different with or without a suitably sized battery. 

Much of the requirements for self-sufficiency are the same as 'Maximise the green benefits' above.

Reduce your ongoing energy costs

In this scenario, ongoing costs are the key driver, the reduction thereof.

There may have to be a trade off regarding installation costs and ongoing costs, but otherwise it is also similar to 'Maximise the green benefits' above, apart from a significant contributor.

In this instance ongoing cost is a factor. Therefore selling your electricity for a profit will aid the objective substantially.

This is dependant on how much you can get for the electricity you want to sell. At one time, in the not too distant past, you may have considered yourself lucky to have received anything for your surplus energy. After all, you could not use it. Hence, along comes power diverters. We use an Eddi.

Next in the stable, is a high capacity PV battery combined with a time of use tariff from your energy provider.

A lot of people will know this, but I will explain it here anyway.

Consider, for a moment that you don't have any PV panels, so there is no generation.

Your time of use tariff may include a cheap period, normal period, and expensive period. The expensive period would normally coincide with the peek demand upon the National Grid. The time when it is most under stress and is sometimes close to capacity. The peak period may be between 4pm and 7pm. The rate you have for buy and sell is much greater in peak time than at either normal or cheap periods. 

Hence there is a typical case of buy low and sell high. Making a profit every day. The profit is not sufficient to fund a bank of ten PV batteries, now that the price of electricity has gone down a little, but if batteries continue to get cheaper, that calculation may change favorably.

As an aside, you might not get permission from your local DNO to have a bank of batteries attached to the grid.

Add the PV generation back into the mix. Some of the batteries are smart enough to check the weather forecast. For bright sunny days, the battery limits the amount of over night (cheap) charge, leaving it at less than 100% at the beginning of the day. However, during the day excess energy is used to further charge the battery, ready for the commencement of the 4pm selling to the Grid. That increases profit, as part of the capacity has been charged for free instead of even the cheap period costs.

This can, in the summer months, totally offset any electricity used from the Grid, giving monthly bills of zero or less.

It is not as if you have to do much to make this happen. At least some battery's have settings to choose between an emphasis for green or cost saving, together with linking to your electricity providers tariff, and doing the buy and sell automatically. You can also set the amount of reserve you want to keep. So the discharge or selling at peak time does not leave you short.

Become off-grid or zero grid dependency

This is a whole new kettle of fish. It is unlikely that just PV panels on the roof are going to generate enough energy all year round to power your home. You may need a ground array as well. You will definitely need batteries to store energy form the day, unless you are going to adapt a dawn to dusk lifestyle, which is a long time in bed during the winter. It is unlikely that you will be able to store summer energy for use in the winter, due to capacity issues.

There is another area of consideration, that we don't normally think about. Peak usage. When you have a battery and you are drawing down on it to run your home, you don't have to think about turning the microwave, the lights, the kettle, and the toaster on all at the same time. Perhaps for breakfast. I have not done the calculation for this particular example, but there are combinations of devices which either through startup loads, or constant draws, require more peak load than the battery can provide. Or even the PV generation plus the battery can provide. Connected to the grid, and you don't even have to think about it. The grid provides additional power, so the battery does not have to strain itself.

This is not the case for an off-grid dwelling, or if you have chosen to be independent of the grid in terms of consumption. There is no automatic top up. Hence your battery calculations have to include both capacity and peak load calculations. Otherwise, every time the system trips, is another reminder to change your habits, and be aware of which items you can put on together. Obviously, any single device also has to be within the battery's peak load capacity. Similar problem with motorhomes.

Provision for power cuts

If you are excessively prone to power cuts, or are concerned about same, then it will change both the kit required and how you use your battery. Perhaps you have some essential piece of kit, maybe medical, and therefore need power, all the time. Here you will still need to consider an UPS, Uninterruptable Power Supply. Battery backup does not click in quickly enough to provide that function. The good news is you will only need a small UPS as it won't have to support your computers or whatever for very long.

Firstly, if a battery is to be used as a backup power source in the case of a power cut, there needs to be a special piece of kit that disconnects the system from the grid instantly. My understanding is that under normal circumstances, if there is a power cut you loose power to your home and PV generation ceases, irrespective if it is a sunny day or not. 

The idea of the battery back feeding the grid whilst a linesman is working on a supposedly dead line is not a good one.

This kit needs to be in place even if you are only holding 20% of battery capacity, for that short power cut.

However, for more prolonged power cuts, you may want to retain 100% charge in the batteries, or near 100%. This will of course preclude a lot of the cost saving benefits of a battery. No more buy low, sell high.

Black and silver PV panels, or all black

The first choice is whether to go for the normal PV panels which are black and silver, or the more expensive all black ones. However, having said that, I believe that all black are becoming more of the norm as their price decreases.

 A fairly typical look of a few years ago, with on roof black and silver PV panels.

I think the alternative all black PV panels are visually more pleasing, again an on roof installation.

Unfortunately, according to some, all black panels are marginally, about 3%, less efficient than black and silver. 

On roof fitting

Fitting PV panels on roof, above the existing roof tiles is both the most common and cheapest method for homes in the UK. It is also the least disruptive as little has to be done to the roof to accommodate the installation.

Both of the images in the previous tab are on-roof installations.

In roof fitting

In roof fitting, or integrated roof is a lot more disruptive than on roof.

 I think the in-roof is better looking than on roof. it has a lower profile, sits with the roof better, and does not need bird netting to stop the pigeons nesting behind the PV panels.

However, the roof under the panels has to be stripped, probably by a separate roofing contractor, and the edging tiles cut. Additional coordination required between scaffolder, roofing contractor, and solar installer.

Possibly only worth considering, from a cost perspective, if the roof is being refurbished or it is a new build.

Ours was installed by Spirit Energy, and this is a link to their article about roof integrated solar panels

Whole roof PV panels

We first saw whole roof PV when researching our own PV installation, at the GB-Sol website. Following their link to installers lead Spirit Energy. GB-Sol's whole roof product is now called infinity. They also have some case studies with more images.

When we looked at their website I seem to recall that they had some case studies that looked more like our home. Ours is a more complicated look that the current set of case studies, with more triangles and less plane areas. Someone said that the triangle panels fill the roof but don't generate power. GB-Sol priced the panels for us and Spirit Energy gave us and insulation price. Looking at the figures and with the uncertainty of whether a whole roof look would be acceptable to the local planning department, we decided to go for the in-roof option, without panels on the front of the house.

Whole roof PV tiling

GB-Sol also provide whole roof PV tiling, looking like slate tiles.

Tesla are another company providing a PV tile solution.

 We have not explored this option so don't know much about it. Our area is more clay tiles than slate roofs. So again something of a stand out form the crowd.

I suspect the are plenty of other companies which provide this type of solution. The above are just examples to show you the form.

How are you going to measure value

Measuring value is a whole subject on it own. When I was working I was a member of the Institute of Value Management (IVM). Value can be described as the equation function over cost, or from IVM website;-

Value is based on the relationship between satisfying needs and objectives and the resources required to achieve them, see subclause 3.1 in BS EN12973:2020 and Figure 2 therein. The aim of Value Management is to reconcile all stakeholders’ views and to achieve the best balance between satisfied needs and resources.

Hopefully you now have a clear idea of your needs form the 'Generation' and 'Aesthetics' segments above.

When the prices come in for your preferred option, you may find that for everything you want, the cost is too high. The it is a case of thinking which bits can I let go to achieve a price I am happy with. It is a case of finding a balance between function and cost. Without loosing so much function that it no longer meets your needs.

On the whole, in industry, Value Management is too often seen as a cost cutting exercise. When it is just that it is abuse of the system. It is being incorrectly applied. If you want an escalator and you get a flight of stairs, does that meet the needs of the brief, the requirements? It will cost less and provides the function of changing level within a building, but it does not cater for the same number of people, so will not satisfy the requirements, is say a shop or a tube station. Similarly, in a tube station, there is a choice of normal or commercial or heavy duty escalator. A heavy duty escalator costs about 10 times as much, but lasts a lot longer and copes but better with the traffic. Less maintenance and disruption. A failed escalator could have a significant impact on the operation of a station, perhaps temporary closure. Cost cutting could suggest the cheaper option, but whole life costings dictate the heavy duty escalator.

I know this article is not about escalators, but this is a real life example, which may help with your deliberations.

How much can you afford

In my opinion, stretch your budget as much as possible. It is a long term investment, and as with the purchase of a house, the now pain of a big mortgage, goes away over time, as inflation, cost and earnings, erode that effect. Energy is likely to trend towards more expensive, and the planet needs all the help it can get.