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:
- What the temperature of the water is in the heater.
- What temperature water is reaching the taps at.
- How quickly the temperature drops off.
- How quicly the heater drops into the danger zone.
