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Comment by FabHK | Hacker News Reader

Why talk about 2.6 kWh/day (power*time/time = energy/time = power) when there is perfectly fine unit for that, namely the watt?

2.6 kWh/day = 2.6 kWh/24h = 108 W, on average.

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Electric bills aren't calculated by the Watt, you pay per kWh. The expected cost of running the fridge is the salient information.

0.108 x 168 hours/wk x 4.4 wks/month gives a good approximation for kwh/month. Demand over time gives consumption just fine.

A 75% drop is nice and much improved.

Yet the service into my home and each individual circuit has a maximum on received power so the Watt is usually more pertinent anyways. Taking an average Watt usage value and extrapolating that into kWh/month is both incredibly easy and completely standardized.

I've had this thought before, when seeing labels that talk about kWh/day. The answer is very simple: you pay per kWh. When people want to know power efficiency, what they really want to know is "how much will this cost me to run?". That answer is most easily expressed in kWh per unit time.

In the US, at least, there are some utilities that charge based on maximum kW (demand) and total kWh used (energy). ComEd in Chicago is a utility with a demand rate plan option.

That tends to be commercial rates since businesses can have larger spikes in consumption, so the "pipe" needs to be larger. Industrial rates are similar.

There are some like ComEd that you call out that can apply the model to residential rates, though my (now dated) experience is that they are rarer.

Knowing the average of 108 W wouldn't help with knowing your peak demand, as fridges vary significantly from off to startup to running, so knowing the average isn't useful in that situation either.

It would be completely wrong for peak demand. I had to learn this the hard way. While the small fridge I bought only uses 80 W while running the compressor uses 800W+ for a second on startup which was too much for my off the grid inverter.

Sweden just mandated kW prices along with the kWh. I think because we are starting to see the extra strain on the grid with EVs.

That strain does not seem to be reflected in the usage, which has been in a shallow decline since the 90s. Maybe they could consider using smart demand management, which is becoming popular with a lot of utilities to move usage away from peaks and into the quieter times.

I think these tariffs are meant to encourage exactly that. Note also that there are many levels of bottlenecks. One could be in your neighborhood, if all your neighbours have EVs.

Perhaps it will work. I'm just a bit skeptical because it seems unlikely to be a widespread problem. The average driver in Sweden will only need perhaps 6 kWh per day, which at L2 means charging for 35-40 minutes. A bit of demand management from the utility and everyone in the neighborhood can get what they need without stressing the local grid. Or just knock down the rate to something inconsequential and let it trickle all night.

> when seeing labels that talk about kWh/day

That's at least kinda reasonable. I'm always amused when I see TV energy labels that state

xx kWh/1000h

To.m be fair I want to know how much a fridge costs to run when it’s on 24/7, my tv not so much.

Also giving an averaged power drain would be misleading. If the device uses 2.4kW but only for half an hour per day, that's not a 50W device as far as cabling, fuses and other electrical considerations.

Although this is totally informal, in a normal conversation if somebody gives me the wattage of a device, I assume they are peak power draw. For kWh/day, I assume they’ve accounted for some reasonable duty-factor.

Possibly because it gives better intuition for the approximate cost per unit of time. Similar to how fuel consumption can be written as volume/length = area, but is still usually presented in the former way, since that shows the actual amount of resource being consumed.

Because the most familiar anchor for scale is the monthly meter reading, which is in kWh.

Watts measure power, kWh measure energy - and they are a more convenient unit than J.

If you have natural gas connection, you can be charged for both kWh and GJ on the same bill!

Or your utility may use freedom units like Therms

Ccf is also common because it's the directly measured quantity by your gas provider and you don't need silly charts like https://www.eia.gov/totalenergy/data/monthly/pdf/sec12_5.pdf

Seems odd to sell a gas by volume though... are these understood to be at a standardized pressure/density?

My gas bill each month lists the correction factor used for that month to compensate the meter reading to the volume consumed (and thus billed) based on the average weather.

I thought watt is a DC unit, so often avoided for AC measurements, since it is ambiguous which watt you are talking about, and the device is always consuming more or less than the average, and very rarely consuming that actual amount. Often the alternate unit is called VA, even though that too seems like a watt

Watts work fine for AC - multiply your RMS voltage by current. The RMS takes care of the fact that AC isn't steady like DC.

VA (takes power factor into account) is relevant for sizing transformers, breakers, wiring, etc but usually only affects your bill if you are a large industrial customer.

That is just what I said: that is the formula for VA, not P, even though both have the same dimensional units: https://en.wikipedia.org/wiki/Volt-ampere and https://en.wikipedia.org/wiki/AC_power#Apparent_power. And they'd be roughly the same for something like a toaster, but if you happen to be interested in, say, the billed power consumption of a 30 year old fridge with a motor

Can't edit any more, but yes I see your point now, and my earlier comment was wrong; you can't get the watts of an AC device by multiplying RMS voltage and current. That will indeed give you VA. Watts = VA * power factor, to get real power. VA is always apparent power.

Because that's not what they're interested in! Really what they care about is that the fridge is consuming about 9.36 MJ/day, because really kWh is just a convenience unit for joules. But since everyone gets charged in kWh that's the unit they use.

It is bit too derived unit. But on other hand it does make calculations pretty simple. Say 0.14 per kWh and then cost in month is simple multiplication 2.6300.14 . Or a year is 2.63650.14...

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