Saturday, September 03, 2011

Powersavings and water heater.

I have switched to Powershop, a New Zealand power company that provides intelligent meters with daily readings. This makes it really, really easy to see the impact of changes you make to your power usage. Not only that, but they provide time of day billing, where power from 7AM to 11PM is 50% more expensive than night time power.

That convinced me to look at how we used power. First, I shifted the laundry and dishwashing. I made it either the last thing I did before bed, or the first thing I did when I got up. That made a moderately substantial change in our bill, but nowhere near what I was hoping for. It was time to track down the big user in our power bill.

We are currently using about 40-50kwh of power per day. In New Zealand, we have expensive power prices ($0.25/kwh), so that's $10-$15 per day. Not cheap. The power meter we've got will tell us how much power we are currently using, which allowed me to zero in on the big users. First, just sitting there doing nothing, we use ~200w. That's for the server (with six HDs), networking and other vampires. Not bad, and not the main power user, they are constant background use, so they only end up being 5kwh/day. 10%, but not bad, bottom of the heap to fix. Next are the heat pumps and dehumidifiers. The dehumidifiers pull about 400w each, so they're a biggy if they were running all day. Heat pumps, also not a biggy, although they can pull about 5kw each. We're going to have to look at passive dehumidifing for the house if we keep it.

The biggest power user in the house is the hot water heater. It looks like it is using about 20kwh per day. However, we don't need a lot of hot water. We don't use it for laundry, the dishwasher uses cold water and our showers all center around 6-7AM.

I looked into ways to save power. The first and easiest is to turn off the hot water heater when you don't need it, so I did that. Our power usage collapsed. From 45kwh to 24kwh. That's a savings of $5/day. Not only that, but we had plenty of hot water for the rest of the day. If we were worried about running out of hot water, we could turn the water heater back on, and it would be at temperature within an hour.

However, there's a big scary monster out there in water heaters - Legionella. The bacteria loves warm, stagnant water between 20C and 45C. That means it loves electric hot water heaters. The recommendation is that water should reach your tap at at least 50C to prevent legionella from colonising the pipes and fixtures.

Oddly, it seems that legionella is found in 40% of electric water heaters, but the disease doesn't seem to be that prevalent (1 case per 100k population/year). It only appears to be a concern for people who are immune compromised. If it does take hold though, it is very dangerous if not caught quickly.

Because of that, we decided to turn the water heater back on during the day. I decided to attack the problem from another direction. Depending on how quickly the water heater loses heat, I can keep it above 50C while still saving on power. If the water heater doesn't drop below 45C during the day, it can be left off safely, bringing it back up to 60C at night, quickly killing off legionella and making use of night rates.

I will conduct some experiments to see:

  1. What the temperature of the water is in the heater.
  2. What temperature water is reaching the taps at.
  3. How quickly the temperature drops off.
  4. How quicly the heater drops into the danger zone.


Bruce Hoult said...

Hot water is using 20kwh/day? That is crazy!

In the summer I use about 12 kWh, total. We've probably got a similar number of computers going. And (for the benefit of readers elsewhere) we're about 250m apart so our weather isn't very different.

I think you may have a leak. Check that the ballcock on your heater is working, and that it's not overfilling and spilling hot water out the overflow pipe onto your roof, or out the side of a wall, or wherever it is.

Bruce Hoult said...

Also, are you really still paying that much per unit? The most I've paid, averaged over a month, so far this year is 19.67.

(the Aug number will be lower once it stops being provisional. The Sep number will be *much* lower as I'll be buying that power in October)

Here are my current prices:

The 20.71c for Standard Power is made up of 48.5% night @17.31c and 51.5% day @23.91c.

Not that I'm going to actually *buy* anything at that high a price! That's crazy talk!

I picked up a special at 19.08c a few days ago that padded me out to my next end-of-month (Sep12). I've got nothing in the bank after that, but can run the rest of September and the start of October in arrears and buy whatever is necessary on October 15 when it should be under 20c.

Jason Pollock said...

Yeah, my hot water isn't on ripple control, so I pay higher per kwh. Are you on ripple control? My "Summer Power" package is currently 20.65c/unit, which is 23.02c/unit day and 14.2c/unit night. My day/night split is currently about 66/34. When I turned the water heater off during the day, it went to 50/50 (or better), which was nice to see. My current standard power is 25/16. I guess I'll get in touch with powershop and see if they can explain the difference in pricing between us.

I've looked under the house, and I can't see a leak, but that doesn't mean that there isn't one! However, if there was, I would have expected to run out of hot water with the heater off, which we didn't. At 36 hours, it had dropped down to "warm", about 36C (I think).

Jason Pollock said...

Hey, how big's your water heater? Mine's 180l.

Bruce Hoult said...

My Summer Power is 21.89 + 15.36.

I'm not on ripple control.

My hot water is currently 46 C at the tap. A little cool, but that'll be mostly because I've already used 20 l for dishes and I guess another 25 - 30 l for a shower this morning, and the hot water I used has been replaced in the tank by cold water which has simply mixed with the hot water.

The tank is 200 l. It's been off since around 1 AM – 19 hours at this point.

Bruce Hoult said...

I was getting 51 - 52 at the tap at 1 AM, after the heater was on for two hours. It was still the same at 9 am, after being on all night, so I guess that's the temp and two hours per day is plenty.

It's 2 kW so clearly you wouldn't want it actually drawing power for more than a couple of hours a day.

In summer i use about 12 units/day. If I go away and leave the hot water off then I drop to 5 units per day. That's with most PCs off but still running the UPS, cable modem, 3x router/WIFI, one MacBookPro with the screen off, and a P3/650 linux box doing traffic accounting. And the fridge. The MBP and P3 each use about 20 - 25 W.

I've never adjusted the hot water temp here.

Jason Pollock said...

It seems that the rules have changed, and water heaters now have a lower limit of 60C.

I lost 10C in the tank between 8:30AM and 5PM, so I'm leaking heat somewhere.

Jason Pollock said...

It seems that my actual standing losses, instead of just mixing of the cold at the bottom of the tank are actually 4C for 7hours.