Eight years of solar power. How much of our own generation do we consume ?

As of today our rooftop solar panels are eight years old. Here's the latest graph of electrical generation vs. consumption. You can see the eight years which have passed, the data neatly making sinusoidal shapes due to changing seasons caused by the rotation of our planet around the sun:

The blip in the graph is due to the inverter failing in 2018. It also failed in January but I got my soldering iron out sooner this time and because it was January and not the middle of summer far less was lost. Therefore no visible second blip.

The red bars show our consumption of electricity each month. The blue bars show the generation of electricity from our solar panels each month and the yellow line shows our total generation vs. consumption. Since the beginning we've generated more electricity each year than we used and in the last four years we've reduced our consumption making this difference between generation and use much greater. This led to the yellow line rising steadily higher. If you live in the Netherlands or anywhere in Europe connected to the same grid then it's possible that during the last eight years you've used some of the electricity generated on our roof as what we don't use goes into the grid.

Over the last eight years the system has produced 27039 kWh of electricity. That's 3380 kWh per year, very close to the estimated 3478 kWh per year predicted before they were installed. But this total over eight years was impacted quite severely by the period when the inverter failed two years ago. Over just the last 12 months we generated 3438 kWh, which is remarkably close to the prediction.

But while it's nice to see that we generate much more electricity than we use and the system is working very nicely, the graph also exposes an obvious problem. We never manage to generate even close to enough electricity in the winter, and of course there's never any output from our panels over night, which is exactly when we turn on our light bulbs. The installation of a smart meter a year ago has given me more data to work with and so now I can see how much of the electricity that we use ourselves comes from our panels and how much of our own electricity we use (these are not the same thing...). When we're not using our own electricity we're using what comes from the grid, and the Dutch grid is far from 100% renewable (we have of course signed up to a 100% renewable contract, but the effect of that is mostly only to make the paperwork say its renewable).

Data from the beginning of January 2019 until the end of March 2020.
Blue: our generation vs. consumption. Any month where this is above one we generated more electricity than we consumed.
 Red: the proportion of our consumption which was from our panels, varying between 0.12 in winter and 0.6 in summer.
Yellow: proportion of our generation which we consume, varying between 0.25 in summer and 0.75 in the winter.

This new graph which makes use of the smartmeter data shows a more complicated picture. Though each year we generate far more electricity than we consume (blue bars are often well above 1), our pattern of usage doesn't match the generation very well. We over-produce in summer, under-produce in winter and we keep putting our lights on when it's dark outside rather than in the middle of the day. We come closest to being independent of the grid in the summer when our panels generate more than twice what we consume, and though we don't use much of our own electricity (yellow bars) it does make up nearly 2/3rds of our consumption (red bars). On the other hand, while in the winter we consume most of the electricity that we generate, there is very little of it so it accounts for only about an eight of our consumption, the other 7/8th coming from the grid.

As you can see, having solar panels on the roof, even if they generate far more electricity than we use, does not make us even close to being independent from the grid. What's more, there's no way we could make ourselves so.

Batteries
It is of course possible to buy a relatively small domestic battery which has enough capacity to store about one day's supply. If we had one of those it would allow us to consume exclusively our own electricity for March through to September, pushing the red bars for those months up to 1. However in the winter months it would be nearly idle because we then generate on average only about a quarter of our consumption, falling to a sixth in the worst month, and we are already using a high proportion of our generation (the yellow bars reach as high as 0.9), so the battery could do almost nothing to help in winter.

To be independent from the grid through the winter we'd need to store electricity from June, July, August and September to fill the trough in our production between October and February and there is no possible storage system which could do this. At present we could buy a small unit which we could fit into our garage, costing a not inconsiderable sum, to cover ~24 hours usage but to store enough to go through winter we'd need something about 120 times as large and 120 times as expensive. It's just not a sensible idea on any level.

Quite apart from the cost and I have reservations about the reliability of batteries for home storage. We already know that, as I expected in advance, inverters are not entirely reliable. It's a box of power electronics which works hard and so we should expect less than perfect reliability. A domestic back-up battery adds far more complication and is almost certainly less reliable. Why ? Because such a battery installation would require not only its own inverter circuitry similar to what we already have in association with the solar panels, but also a battery charger and the battery itself. More power electronics means more to go wrong.

Doubling the number of panels on our already full south facing
roof isn't an option. North facing panels won't work so well.

Such a battery would more than double the price of the solar panel system, more than double the embedded carbon cost, more than double the chance of it going wrong and requiring costly maintenance or replacement as a result. None of that is good news for us or for the planet. Even given all those constraints, it seems that the manufacturers' somewhat optimistic assumptions still suggest that a battery could only provide us 100% usage of our own solar power if we also at least doubled the number of solar panels that we have. As our roof is already almost full this would require us to annex our neighbour's roof entirely for our own use. That's not going to happen.

Let us remember that the excess that we generate and do not use is not lost. Investing too much in trying to use our own electricity ourselves is almost certainly counterproductive. A good proportion of our excess is consumed by other electricity users, both domestic and industrial. This leads to those consumers having lower emissions from their electricity consumption.

Reduce consumption!

A domestic battery makes no practical sense and doesn't necessarily make sense for our environment either. The best thing for us to do, and I would suggest for everyone else as well, is to reduce energy consumption as much as possible. We're continuing on that path, making our home more efficient every year. The less we consume, the easier it will be for our grid to operate without fossil fuel input. Any additional loads will keep the fossil fuels burning for longer.

Domestic and commercial
Our domestic solar panel setup is not actually 100% domestic. We operate our business, Dutch Bike Bits, out of our home, so the business is also powered from our solar panels. We try to run our business in the most ethical way we can. Therefore there is no extra energy consumption in another building and we don't use any form of motorized transport so our parcels, all of which contain goods to support people who cycle and are therefore assisting other to also use the most efficient vehicles on the planet, begin their journeys by bicycle.

Low carbon footprint hummus (super fast and economical to make)

Ingredients: Chick peas, garlic, tahini,
lemon juice, salt

Hummus is one of the tastiest things to eat for lunch on bread. It's also super easy to make yourself, even from a store cupboard if you're trying not to go outdoors (the COVID-19 pandemic is a very big problem at the moment in the Netherlands).

So if you're stuck for something tasty to put on bread for lunch, consider making hummus. It takes far less time than going to the shops, it's cheaper than commercial hummus and the result is tastier.

The ingredients required as all easily stored except for the garlic. The only fresh ingredient needed is garlic, though you can use garlic from jars if required.

In this example I used a 400 g tin of chick peas, one table-spoon of olive oil, a couple of tablespoons of tahini and small squirt of lemon juice, four cloves of garlic and a tea-spoon of salt.

Drain the chick peas, but keep the liquid. You'll find you need to add some liquid while liquidizing and this is the best thing to use. Keep any which is left over for use as liquid in other recipes (it'll go off if left too long - I put seal on the tin and use within a day or so).

The exact proportions of ingredients are not important. You can vary them to suit your taste buds and also to suit what you have in stock. Even the main ingredient, chick peas, isn't really necessary. OK, so real hummus may be a chick pea dish, but exactly the same procedure can be followed with any kind of tinned or pre-cooked bean and it you get a similar tasty result. So if you need something to spread on your bread and don't have any chick peas, try some other bean. For instance, black-eyed beans make a great spread as well.

Super simple - just put everything in a liquidizer. No cooking is required. Note that everything is shown "dry" here. It's always necessary to add some of the liquid from the chick pea tin to make it liquidize properly. You can also vary that according to taste - some like their hummus to be more liquid than others. If you don't have a liquidizer you can simply mash all the ingredients together. 

After 30 seconds or so it'll look like this. Stop when you like the consistency. It's immediately ready to serve.

One 400 g tin of chickpeas and the other ingredients together make about enough hummus to fill two average commercial retail hummus containers. We buy these only occasionally and when we do that we keep the containers which can be re-used many times. The red specks are due to an optional extra ingredient - I included a red chilli this time around. Other spices or herbs can be added in the liquidizer at the start.

Lunch. This recipe always makes delicious hummus.

Carbon footprint

As before when I made a pizza, I wanted to calculate the carbon footprint of this meal. It won't be very high for the calories because vegan food never is. And in this case it'll last for a few days of lunches so the cost per day will be low anyway. Unfortunately, I couldn't find accurate figures for the chick peas or tahini, so I warn you in advance that the following figures are to a large extent guess work. Perhaps you can help with this.

The first part is easy. The liquidizer consumes 1000 W but it's required for only a very short time. Less than 30 seconds in total. Therefore the total amount of energy consumed is very small. This works out as about about 8.3 Wh, or 0.0083 kWh. Due to the full sun today the only effect that it really had was to make our electricity meter run backward less quickly but I've calculated here as if we were using the average carbon footprint for electricity across Europe. Even so, the liquidizer doesn't do much harm:

Ingredient	Quantity (g)	CO2 equivalent (kg/kg)	Total CO2 (g)	kcal
Electricity	0.0083 kW	500 g/W	4.2
Chick peas	400	0.7	280	468
Tahini	30	2.3	69	170
Olive oil	10	1.5	15	80
Lemon juice	10	0.5	5
Salt	1	0	0
Total carbon footprint / calories			373.2 g	718 kcal

As explained before, I cannot claim this time that the figures above are in any way accurate. The carbon intensity of tinned chickpeas and of tahini are both based on figures that I found (here) for similar ingredients. I picked "ready to eat meals" for the chick peas and "Nuts and almonds" for tahini (which is ground up sesame seed). As those two items dominate the total this makes the entire result questionable. However I have picked replacement values which are on the high side so I would be surprised if the carbon footprint of hummus made this way is greater than what is shown above.

I hope that readers can contribute better sources for those ingredients.

How much did it cost to make ?

These cheap chickpeas cost about 60 cents a tin, the small amount of Tahini is worth about 20 cents. The other ingredients perhaps 10 cents at most between them. That makes for a total cost of probably less than 90 cents for an equivalent amount of hummus as is found in two commercially made pre-prepared packs which cost about €2 each.

What about dried chickpeas ?

Dried chickpeas have a lower carbon footprint when sitting on your shelf than does an equivalent quantity of tinned chickpeas. However a lot of energy is required to boil them at home. I therefore would expect a higher carbon footprint for chickpeas that I boil myself than is the case for those bought in a tin where economies of scale will make the process more efficient, almost certainly more energy than is consumed in making the small amount of steel in a can. This seems to be confirmed by various sources.