Is A Slow Cooker More Energy Efficient Than An Oven?
It's getting to be that time of the year again, so I've taken the slow cooker out of it's estivation.
It's not just that it makes low-effort, warming meals, but with an energy crisis overshadowing the UK it's also potentially a lower energy way to prepare meals.
But, how much energy do they actually consume? And how does that compare to cooking the same meal in an oven?
Although the appliances don't draw very much power, they do tend to be on for a long period of time (relative to an oven or an airfryer), which can lead to the energy using adding up.
I plugged our slow-cooker into a TP-Link P110 smart socket, and used my new simplified docker container to collect usage stats (and write them to InfluxDB) whilst cooking a meal. The Flux used to query stats back out is exactly the same as in my earlier posts.
The Meal
For no particular reason, other than wanting one, I cooked (and ate) a cottage pie.
Although the recipe's preparation stage involves some hob browning (entailing additional energy usage), those steps are exactly the same when oven cooking, so that energy is discounted in the comparisons made in this post (as we care about usage of the appliance, not of this particular meal).
The Slow Cooker
We've had our slow-cooker for quite a while (at least 4 years by the looks of it), it's a Morphy Richards Accents "Sear and Stew" thing:
It's cheap and cheerful (£20.00 at the time) and sports an entire three settings (gasp) - low, medium and high.
The sticker on the underside gives the rated power consumption as 130-163Wh.
For those wondering, "Crock-Pot" is a brand name, so the difference is the same as calling a vacuum cleaner a hoover (or, Googling with Bing).
The Cook
I cooked for longer than in the recipe: I was going out that morning, so either needed to start the cook (too) late, or a little early. The total cook time was 10 hours (0800 - 1800), with the slow cooker on low throughout
We'll account for that additional time later, although it's probably still reasonably representative of how people tend to use slow cookers.
Heating Element Behaviour
A heating element is basically just a big resistor, it's either on or it's off, and it outputs energy at a (more or less) fixed rate (fluctuations in voltage can lead to fluctuation).
Because an element's state is binary, to support multiple heat settings, an appliance generally needs to
- have at least two heating elements (so
lowmight use one,mediumanother, andhighboth), or - use a thermostat to turn the element off when a given temperature is reached (so
low,mediumandhighwould all correspond to a specific temperature, but when the element is on, all would have the same draw), or - Use a single element, run in duty cycles of different lengths, or
- A combination of the above
Because a slow-cooker relies on a gradual build up of temperature over time, it's unlikely to use the thermostat approach (though I could be wrong - we'll try and check that later).
Usage
In total, the 10 hour cook consumed 1.32 kWh of electricity
Which at current prices, cost us £0.46, just 14 months ago I was on a tariff where the same cook would have cost £0.17.
It'd seem wrong not to take this opportunity to reflect on that change in cost and send a big "Fuck You" to those who's action and inaction put us in the position of needing to even care about this level of usage.
It's easy to say "Fuck You" to the energy supply companies: their pricing has been utterly predatory. But they're also not actually the root of this.
An even bigger "Fuck You" is due to the current UK Government: members of whom, were too busy fart-arsing around in a vanity leadership contest (which, you didn't even have to have links to the UK to vote in, Tory "patriotism" laid bare) to work together and address the energy crisis early.
Even when they did finally deign to act, they've delivered "help" in the form of taking out debt in order to give more tax payer money to (ex-employer) energy companies rather than taxing their record profits. Not only have they managed to damage the economy in the process (again, making money for some ex-employers - though there is some question over whether "ex" is accurate), but in the midst of an energy crisis are seeking to ban a cheap, sustainable route to national energy security instead pursuing fracking, which even the boss of the UK's fracking company says won't help the energy crisis.
And that's just the last month or so... UK energy strategy has been woefully neglected for far too long. Closing gas storage facilities like Rough because "we can just buy in" is the very epitome of short-term profit-centric thinking.
Although they're all bell-ends, there are more than a few in this (and the last) government who probably shouldn't be trusted with a loaf of bread, much less governmental responsibility.
/tangent
Average Usage
The 10 hour cook time makes some of the maths very easy. On the low setting, the hourly averages are
- Consumption:
132Wh - Cost:
£0.0461
Cooker Equivalent
The slow-cooker's rated draw is low, but it's also maintained for a long period of time. So, the question is, does that time ultimately lead to it consuming more than the oven's shorter cook with a much more powerful element?
Even I can't eat two big cottage pies, so for this we'll rely on the average oven usage I calculated previously.
- Warmup (to 180c) consumed 137Wh
- Post warmup, the oven consumes 6.4375 Wh per minute
I've checked usage for a few cooks since I first calculated that number, and it does seem to be fairly consistent, with subsequent checks coming in within 0.002Wh of that per minute figure.
As noted in my previous post, though, the Oven's usage does assume that you put the food in pretty promptly after it's warmed up, rather than wandering off and coming back 20 minutes later.
My Oven Cottage Pie Recipe calls for 1 hour at 180c, so we can calculate usage as
warmup + (mins * per-min-avg) = usage
137 + (60 * 6.4735) = 525.41Wh
So, interestingly, it looks like the Oven is the more efficient route, consuming about 40% of the energy consumed by the slow cooker.
That advantage doesn't change much, even if we account for the fact I cooked the meal for a couple of hours than normal by subtracting the usage for those hours.
1320 - (132*2) = 1056 Wh
The Impact of Other Settings
Because we know the Slow Cooker's rating, we can try to predict what the impact of other settings is likely to be.
That way we can hypothesise whether cooking on a higher setting, for a shorter time is likely to make a substantial difference (we'll test these assumptions later).
The maximum stated draw of the slow-cooker is 163Wh, so we can reasonably assume that the high setting draws this.
This recipe calls for a four hour cook on high (although the recipe doesn't put the mash into the slow cooker, so this is probably too little time in practice)
163 * 4 = 652 Wh
This is still higher than the oven, but by a much smaller margin than when cooking on low.
We can expect that medium should perform even worse - although it uses a lower wattage than high it also requires a longer cook time, which should translate to higher overall consumption.
The medium setting, logically, should fall roughly in the middle of the range - so we'll call it 147 Wh.
The next challenge is working out how much longer a medium cook needs to be. No recipe ever seems to use the medium setting and I'm increasingly convinced that it exists just so they can call the other setting "high".
Again, we'll split the difference - the recipe linked above calls for 4 hours on high, 7 on low, putting medium at 5.5
147 * 5.5 = 808 Wh
Our total consumption is, indeed, headed in the wrong direction.
Testing Assumptions
It makes sense on paper, but does it apply in practice? For example, the manufacturer might have included a margin of error in the rating, and high might draw less than we assume.
So, it's worth running some test "cooks" - not being ready to make and eat more cottage pie, I filled the slow-cooker with water instead.
Consumption actually peaks a little above the stated max draw - as noted earlier, that's likely to be caused by fluctuations in voltage.
Even allowing for that, though, our assumptions around consumption on high look to be correct: the appliance was on high for just under 4 hours and consumed 652 Wh, giving an average draw of 163 Wh.
I turned off, emptied and refilled the slow cooker (with the same amount of water) and then switched to Medium to see what the draw of that setting is
Initially, it's exactly the same as high - it's using the same heating element as the high setting.
A little later, it switches to the low heating element and the draw drops accordingly (to 132Wh). It looks like medium doesn't have it's own element and instead works by switching between the two used for each of the other settings.
Multiple tests show that this seems to be driven by a thermostat rather than a timer - with a smaller volume of water, the high draw only lasted for 1h 47m (compared to 2h 28m in the graph above).
Once it's switched to the lower element, it doesn't appear to switch back (it's possible taking the lid off for a while might cause a switch, but that's not really representative of an actual cook).
I left the appliance on medium a bit longer than planned, but over the first 5.5 hours of that, the average usage was 150Wh (roughly where we'd predicted it would be).
Our earlier calculations, then, were correct, and neither medium nor high is able to outperform the oven either.
Adjusting Behaviour
Could we, perhaps, make sacrifices in order to break even (or better) the oven?
If we're willing to sacrifice some of the softness that slow-cooking gives, we could cook on high for a shorter period
163Wh * 3.5h = 570Wh
163Wh * 3h = 489Wh
163Wh * 2h = 326Wh
Of course, this is only a safe option for recipes where meat is browned in a pan first.
The savings really are quite meagre: reducing the cook time to 3 hours uses 489Wh, saving just 36.4Wh against the oven's 525Wh. Even at today's inflated prices, at less than a penny it's not a financial saving you're likely to notice and your food may be much less enjoyable to boot.
Conclusion
It's not actually the result I was expecting, but our slow cooker is less energy efficient than our oven.
Although the max draw is much lower (0.163kWh vs 2.25kWh), the slow cooker (true to it's name) requires much longer cooking times, leading to a higher overall power consumption (much higher in the case of cooking on low).
That relationship between time & cost was also the reason that the oven worked out to be slightly less energy efficient than an air-cooker - the oven used less energy per minute (once warmed up), but needed to do so for longer than the air-fryer, so using more energy overall.
So, despite it's much lower power rating, a slow cooker is not a viable means to reduce energy bills, and might even use double the energy required by your oven.
That doesn't mean that a slow-cooker isn't worth having - they're relatively inexpensive to purchase, and can be used to make some delicious meals (as well as things like mulled apple juice).
