Thinking Solar - Low Power Kettle

Okay, so a kettle is a kettle - you fill it with water, plug it in, switch it on, and a few minutes later it's boiled. Fine, for the majority of people that's the case, even more, for convenience it's quite usual to have a 'fast-boil' facility based on a 3kW, or even larger, heating element: however, in homes with microgeneration technologies at hand the use of higher power kettles simply results in unnecessary energy imports.

Kettles with Solar PV - A Change in Mindset

Prior to the advent of microgeneration technologies, such a Solar Photovoltaic (PV) systems, energy was energy, so doubling the power input into a heating element in an appliance such as a kettle simply resulted in halving the time to boil the same volume of water, however, when looking to maximise energy efficiency & self consumption with a limited current power source it makes perfect sense to reduce the power input and deliver the same energy over a longer timespan.

The case for low powered kettles, it's as simple as drinking a coffee!

Basically, leaving solar aside, it's not power that needs to be considered - it's energy. If a given volume of water is heated from a given start temperature to boiling point with either a 3kW or 1kW kettle then, within reason (kettle mass, cycle heat-loss etc), the same amount of energy will be needed, it's just the timescale which varies. If looking to minimise the use of imported electricity when microgeneration or storage technologies are involved, an assessment of the average ratio of self generation vs imported power is required to be made.

Taking an appropriate sized PV array in the UK as being 4kWp, the proportion of annual generation hours above 3kW is pretty low, much lower than that where above 1kW is available and considerably lower than the number of hours where only 500W is being produced, which is more representative of daytime generation when a cup of tea/coffee is required, that's morning/evening/winter/dull/raining etc, not just around midday if it's sunny.

Example - Constant Generation Model

Of course, everyone will be different, but let's look at a theoretical example based on 500W of generation and compare energy consumption of a 'standard' 3kW kettle to a low power alternative rated at 1kW. For simplicity, let's assume that there is a constant 500W of generation available, therefore a 3kW kettle would be importing 83%((3-0.5)/3) of it's power with a 1kW kettle importing 50%. Of course with time being a relative variable, the percentages also apply to the total energy, resulting in an imported energy saving of of 33% (83-50).

Example - Variable Generation Model

As mentioned, it doesn't apply for everybody due to varying usage patterns, but let's compare the 'simple' example above which is based on a constant level of generation to generation in a real-world environment.

Here we have a representation of a 4kWp Solar PV array in the UK generating on a reasonable early February day, showing a comparison of energy import requirements to boil a given volume of water in a 3kW kettle and a 1kW alternative.

Solar PV Energy self consumption - Comparison of Low Power Kettle (1kW) to Standard (3kW)

Assuming a kettle is required to be boiled at 90 minute intervals during generating hours starting at 10:00am and allowing for an energy efficient domestic baseload, the chart clearly shows the difference in energy import requirements relative to power generation throughout the day.

In addition to consuming power generated by the Solar PV microgeneration system, the 3kW kettle required an additional 263Wh of energy to be imported against 103Wh for the 1kW alternative, describing a 60% reduction in energy imported over the daylight period for this representative day through using the low power alternative.

Low Power Appliance Usage - All of the time?

It's dark now and I've just used the 3kW glass jug-kettle because we're not generating, we currently don't have energy storage, and I was simply in a hurry. The functionality is no different as it heats water to the same temperature, it just aesthetics, capacity & time - two kettles, well isn't that the height of extravagance!.

The 'standard' (3kW) kettle is often used at night, when generation is high or when we're simply desperate for a quick cuppa, the rest of the time it's normally the 1kW kettle. Sometimes during the winter when the logs are burning and the kitchen seems like a cold & far distant place, either a stove-top kettle or cast iron teapot & trivet are used, but at least the availability of choice of source to boil water adds a little individualism into our existence.

Insulation & Additional Efficiencies

There are a number of energy saving kettles available which tend to be normal high power units with either a double wall construction or additional external insulation, however, these 'specialist' kettles normally come at a premium price. We've had a small (1kW) kettle for a number of years and take the age-old & low-tech solution of using a simple tea-cosy to push the energy savings a little further. A standard tea-cosy tends to fit over a small kettle quite snugly when it's not being used and through simply topping up before the next boil, provides an initial temperature boost to the next cycle. For safety we also ensure that the kettle is switched off at the wall before covering and would recommend that anyone looking at this option to do the same.

Further efficiencies resulting from using small kettles soon become apparent, it's surprising how much easier it is to estimate the correct volume of water required for 1 or 2 cups (or mugs) of tea or coffee in a small kettle, but like many other energy efficiency measures, the potential for savings are as much related to the individual as to the appliance being used.

Availability

Low power kettles are available from many sources, but they're normally well hidden, often being referred to as 'travel', 'camping' or 'caravan' kettles as opposed to what they are - low power.

Some of these units are rated as low as 500/600W and are therefore more likely to heat from solar than even a kettle drawing 1kW, however, they often have a considerably reduced capacity, so are unlikely to be able to be used with more than a couple of mugs, let alone a teapot, also taking an extraordinary time to boil.

Many kettles fall into a lower power band around the 2.2kW mark. Although achievable savings when used in conjunction with microgeneration technologies such as Solar PV certainly aren't as good as they will be with kettles rated around 1KW, these may offer a reasonable compromise for those either unwilling or unable to use smaller capacity appliances for various reasons.