The 90% Rule

A design principle and critical assessment of our needs as human beings

Over the course of the last 4 years, whilst building our strawbale house, installing the systems and becoming accustomed to them, we have consistently applied a thorough design framework.

This has come to be known between us and in various conversations with friends, family and volunteers as The 90% Rule which goes something like this:

A system which provides 90% of your needs involves minimal sacrifice when objectively and critically examined and leads to an exponentially simpler design and end result, leading to greater freedom, empowerment and agency.

Daniel Colman 2021

The 90% rule has various positive feedback loops which I’ll go into a little later as well as negative feedback loops when not applied, which will perhaps resonate with those readers with conventional homes and lives. For now, let’s look at a couple of examples from our own lived experience.

Hot Water

Most hot water systems in modern homes provide piping hot water, under pressure, at all times, 24 hours a day, 365 days a year. In other words we demand 100% provision which has to be catered for through design. In the case of hot water this takes the form of either gas boilers or electric heaters of some description. These are incredibly complicated devices with countless components, designed obsolescence, a large amount of embodied energy (energy and resources used in their production) and of course consumption of gas and electricity with the obvious detrimental effects this entails both in terms of emissions and expensive infrastructure. Perhaps putting it into questions will make this clearer:

  1. Do you know how to fix your boiler?
  1. Will you have the same boiler in 15 years time? In which case where will it end up and how much will you have spent on a new one?
  1. Will gas or electricity suddenly become more abundant, cheaper or suddenly carbon neutral (of course wind and solar are an improvement on this front, but still, energy companies are unlikely to suddenly become benevolent)?

So we have here an open-ended commitment to maintaining, replacing and consuming in the name of satisfying this requirement for hot water, 100% of the time. Let’s assume the average person showers for 15 minutes every other day, more than adequate to keep you smelling fresh, that works out as 45 hours per year, add in washing up, let’s say 5 minutes of actual hot tap running per day (I am excluding dishwashers, you’ll see why later!), which comes to 30 hours. So 75 hours in total. That’s the amount of time in an entire year that you require hot water, not much right? But you’ve got a system which in theory could provide you with hot water the entire time, not very sensible.

Let’s look at this now through the lens of The 90% Rule and see what the outcome is. Ok, so I want to have access to hot water, under pressure, 90% of the time, and I am happy to either improvise, with a kettle on the wood burner for example, or simply do without for the other 10%. Well, the requirements of my design just became much simpler.

Now hold your horses here, we have to ask some pertinent questions and avoid knee jerk answers or culturally normalised false standards. Will I die, suffer or sacrifice in any way shape or form if I boil a kettle 4 times a year and pour water over my head in the shower? What’s your first answer? Think about it again, will you really suffer? Has anybody ever walked up to me and said ‘Wow Dan you look like hell, you’re really sacrificing with those 4 bucket showers every year’ – of course not, it’s a ludicrous proposition. The movements required to have a bucket shower are not dissimilar to movements we pay good money for in yoga classes! 

Ok, this all seems self evident, but where’s the benefit? Well, we have a solar water heater which has no moving parts, and requires no electricity. Inside the house we have a small 12v pump to provide pressure for the shower which is easy to fix, or would cost 80 pounds to replace. This system provides us with our hot water needs for the entire year apart from the month around the Winter Solstice, but even then, we usually just plan our water usage, if it’s going to be sunny on Thursday, we top up the tank and conserve the water until the next sunny day. This year I took precisely 3 bucket showers using a kettle warmed on the stove, which was heating the house and often cooking dinner at the same time.

Let’s take a look back at those preliminary questions again:

  1. Do I know how to fix this system? Yes I do
  1. Will you have the same boiler in 15 years time? In which case where will it end up and how much will you have spent on a new one?  It has a 25 year guarantee, replaceable tubes and the entire system cost about 300 euros.
  1. Will gas or electricity suddenly become more abundant, cheaper or suddenly carbon neutral (of course wind and solar are an improvement on this front, but still, energy companies are unlikely to suddenly become benevolent). Of course not, but this now doesn’t concern me and I will never receive a bill for hot water.

Having seen through and critically examined culturally conditioned responses: ‘I simply couldn’t do without a hot shower’ or ‘Boiling the kettle sounds like a hassle’, I defy anyone to say that we sacrifice anything. I would in fact argue the exact opposite, we appreciate hot water when it again becomes abundant in early Spring and we are grateful for sunny days in the mid-winter. I will never have to replace a boiler, call an engineer or pay a bill. Critically examine your learnt assumptions and drop your false standards and a whole world of freedom opens up in front of your eyes. 

On a side note, we could work backwards and argue that a kettle and soap would provide you with most of your needs and be even simpler than the system outlined above, but the point is by imposing unreasonable (by historical standards) expectations, we create exponentially more complex designs.


Let us now turn to electricity. We will assume that our designer, Sarah, is determined to use solar to power her house, to have a 100% renewable energy resource and even the ability to store energy in batteries. However, Sarah wants to literally recreate a conventional house with a renewable energy source. In other words she wants to have pretty much unlimited power, 100% of the time regardless of the weather, and the freedom to buy all the appliances she (thinks) she needs.

Sarah has made a critical error here, as have most of us when we talk about the green energy revolution. Sarah will need an unbelievably complicated system which she has no hope of installing herself. A roof covered in solar panels, endless wiring, expensive inverters to provide 240v power for her mod-cons and an enormous battery system. All of the devices will have obsolescence built in, any errors will need to be fixed by a professional engineer at great expense and no doubt Sarah will be paying off the loan she took out for the system for many, many years. Not much autonomy, empowerment or freedom for poor Sarah. But at least she can sleep easy knowing she’s doing her bit for Mother Earth, avoiding more bills and can carry on happily as before right?

Sorry Sarah, wrong again. Her consumption will continue, appliances will be bought, used and chucked. She may save money but the cost of both buying and maintaining this system makes even that assumption questionable; most solar systems take 15 years+ to pay for themselves and don’t even include battery storage, so you have to buy it back from the grid anyway!

Applying The 90% Rule would have helped Sarah and also had some profound knock on effects that we didn’t anticipate in our own project. The initial motivation to go down the renewable route was two fold, to avoid bills and to be kind to the earth. With this in mind we built a small system with five 100w panels and 2 batteries. We knew that power would be tight over the winter but were willing to make do. Basically we decided we were happy to have endless power 90% of the time. Our system was easy to install and, because our demands at the outset were modest, I was even able to follow a youtube video and build my own battery pack from recycled laptop cells. 

This is where The 90% Rule really takes off. By adopting this approach from the off we simply designed, adapted and grew around the reality of a 12v system with limited power. As such we don’t have any appliances whatsoever in the house apart from a battery powered vacuum, a hand-held battery blender and a 20 litre 12v Fridge. Look around your house, how many flashing lights are there in your kitchen, how many plug sockets, how many beeps, buzzes, things to be replaced, things on the blink? Are you any happier than me for these things? It’s highly doubtful. Furthermore, when it came to designing a kitchen, it became exponentially easier and cheaper because we literally have a hob, a sink, cupboards and drawers. No dishwasher, microwave, oven etc., etc., etc.

Oven Baking

It might be useful to home in on one area, the oven. Most people would never compromise on this, but by compromising just 10% it all gets 90% cheaper and easier. We have a wood burning stove with a small oven. Between October and April, more often than not there is something baking while the fire roars and heats the house, neat right?! I am by no means anti-tech in any way, but I am anti-unnecessary design and being imprisoned in the name of convenience. With this in mind, we found a nifty solar oven using the same solar tube technology as the water heater. It costs about 260 euros, has no moving parts and when the sun shines, you can bake, roast and cook. There will be days when we can’t bake, but does that really matter? Or let’s put it another way: Is it worth designing based on the erroneous principle that you need to bake every day, even through 35°C+ summer heatwaves?

Listening To Your Heart

So much of this comes back to examining our assumptions about what really makes us happy, about what actually affects our well-being. I have the healthiest diet of anyone I know, I am fitter than I have ever been, I work 4 days a week doing a job I love and, despite earning less than £1000 a month, have plenty of disposable income, no debt whatsoever and bills which amount to about 150 Euros a month. I mention this not intending to show off or brag, they are just the parameters of our life. Much of this we owe to the rigorous application of The 90% Rule and a willingness to answer honestly, from the heart, questions relating to our needs. When people see our life and come out with questions like ‘How do you manage without a dishwasher?’ my heart sinks. They fail to see the cumulative effect of falling into all the little traps around them ‘I must have this’, ‘I couldn’t do without that’ – is this true? Or have you had these mantras drilled into you? I guarantee that if you fail to examine these assumptions you will end up enslaved to maintain a lifestyle you think you need but in reality, far from ensuring your well-being, leaves you powerless to make meaningful changes.

On top of the benefits you’ll reap personally, the burden you place on the earth will be exponentially eased.  Our house consumes about 4% of the energy of a conventional house, without taking into account all of the junk we don’t produce as noted above. We are far from perfect and we constantly ask ourselves awkward questions and try to confront our own often contradictory behaviour. Earth is crying out for enlightened action, but without confronting the assumptions, beliefs and standards that guide us, we will simply create more chaos.

Be courageous, ask simple yet profoundly bold questions of yourself and others and the freedom which appears in your life will blow your mind. The 90% Rule is not a mathematical formula but an invitation to reexamine the choices which shape your brief existence on this planet. 

2 Comments on “The 90% Rule

  1. Sorry I haven’t commented for a while, although it could be seen as a blessing, but I keep reading your post and and always inspired. I have bamboo plants on order 🙂
    I hear you and what you are saying, but I will make a point about the stage of life that you are at and if you have dependant children or care for someone, both of which have a profound affect on how conveniences soon become a must, unless you want to spend 16 hours a day working 🙂
    Feed four or more people on a schedule, clean and dress them in a reasonably presentable manner and deal with the mess created by them, and an oven, dishwasher and washing machine are essential. If you want to keep the food that you harvest then whilst lots can be preserved and pickled (the dishwasher also comes in handy sterilising containers), a freezer (or two) soon becomes a must if you have 50kg of soft fruits and berries left after you have made jams and juices. Not to mention the 100 kg of peas, beans and other vegetables.
    And whilst you may not be able to appreciate it yet, as you get older things will become harder and a little bit of extravagance is appreciated from time to time 🙂
    Keep it going for as long as you can, the planet certainly needs all the help it can get, lets just hope that some of the choices that you have mentioned become the norm for more people in the future. We just need governments to help make the change easier for it to take hold.


  2. Pingback: Energy Update: Rethinking Luck, Privilege and Abundance | Les Vignes Basses

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