Can I afford not to waste electricity heating up my showers?

August 7, 2006 5:21 AM

My old electric hot water service has sprung a leak, and I'm researching replacement options. Is there anything greener and more cost-effective available to this Australian than off-peak electric?

I've signed up for a 100% green electricity plan, so CO₂ emissions are not a factor any more; but I don't much like the idea of using high-quality energy like electricity to generate below-boiling-temperature heat. It just seems wrong, especially in a country that gets so much sunshine.

The thing is, it's also cheap.

I've been using AU$270/year worth of electricity to run a 315 litre off-peak heater at 8c/kWh. An equivalent replacement will cost me $1100, so over its expected ten year service life I'd be up for $380/year to support our hot water habits.

I've looked at both heat pump and solar thermal options, and both of these tend to cost about $4000 up front; which even if they cost zero to run and/or maintain, means I'd still be behind at the end of their expected service lives.

In fact, heat pump units are generally not designed to run on off-peak electricity, because the air is cold at night and they don't work as well; so even if I used a heat pump and consumed only 1/3 of the energy, at 14c/kWh for untimed power I'd still be paying $160/year to run it. I can't justify this cost.

We get occasional nasty frosts here, which doesn't bode well for solar collectors - at least, not the type where the output water, as opposed to an isolated loop of glycol mix, runs though the panels. AFAIK the glycol types can't be retrofitted to an existing tank, either.

My town doesn't have reticulated natural gas, and I'd rather burn Queensland bagasse than Bass Strait gas or LPG in any case, so whatever I get is going to be at least somewhat electric.

Do I need to toss my energy-efficiency-advocate cred overboard to come out ahead, or can the hive mind show me options I've missed?

posted by flabdablet to technology (11 comments total)

Putting the economics of the heat pump another way: running a heat pump instead of a heating element will reduce my electricity bill by $110/year. Over ten years, that's $1100. So if I could get a heat pump unit for no more than $2200, I'd be doing that.

$4000+ upfront is too much.
posted by flabdablet at 5:28 AM on August 7, 2006

Its a real shame you don't have piped gas, because instant gas water heaters are really very efficient and cost effective - they come closest to "only use the energy you need". Bottled gas is an option, but it tends to be more expensive than piped gas, and I've never run a water heater off bottled gas so I don't know how quickly you would go through the bottles.

And yes, Gippsland probably isn't the world's best place for solar hot water heaters. Here in Darwin they work perfectly. Down there, you would be running on the booster a lot of the time, pretty much constantly in winter.

Have you done calculations taking into account adjusting the thermostat on the electric storage heater? Reducing the water temperature can significantly reduce the amount of electricity used. That's about the only positive advice I can give you. What do you use the water for? Just showering and dishes? I hope you don't warm-water wash clothes, because that's a waste of time. Water efficient shower head? Short showers? I assume you're on top of this stuff, because reducing the size of the heater might be your only option.
posted by Jimbob at 5:43 AM on August 7, 2006

(I don't know anything about heat pumps for water heating by the way. Never heard of such a thing, and it has piqued my interest...heading off to Google...)
posted by Jimbob at 5:45 AM on August 7, 2006

I disagree about the solar preheating. The panels are really cheap -- if you're handy, you can build it yourself -- and you can just shut down the valve that runs to it come winter. During the portion of the year where the sun exposure is sufficient to raise the temperature of the water, you'll be saving a good bit of energy.
posted by waldo at 6:28 AM on August 7, 2006

Jimbob, I'm pretty happy with my present usage controls; hot water service salespeople I've already talked to seem surprised when I tell them $270/year - apparently the average for my size household is closer to $400.

Yes, the thermostat is on the minimum legal limit (60°C). Most of it goes on showers. We're all too attached to long relaxing showers to cut them back much, so I've just turned down the heater's inlet stopcock to control the flow.

We have a dishwasher that heats its own water, which we run every two or three days. Most laundry is done cold; we occasionally hot-wash truly stinky loads (and it does make a difference).

I think the 315 litre off-peak unit is correctly sized for this household. We occasionally run out of hot water when we have more than three visitors.

I don't want to use any kind of gas, because that would mean my hot water service was responsible for significant CO₂ emissions.

Waldo, the question is not so much whether I am able to save energy; clearly, straight electric heating is the most energy-wasteful option available. The question is whether I can actually afford to do so. If you can show me where in Australia I can buy cheap, mains-pressure-rated solar collector panels that won't rupture if they freeze, I'll be grateful.

My current plan is to buy the standard off-peak electric unit, install it outside the house, and retrofit solar collectors and a little circulation pump if I can find panels that will actually pay for themselves over their expected service life.
posted by flabdablet at 7:04 AM on August 7, 2006

I've been looking into a tankless water heater myself. I don't know if this is the same thing that others have been referring to as a heat pump or instant water heater, but it does heat the water only as you're using it. This saves alot of energy, as you're not paying to keep the water warm.

It also means that you don't run out of water. You choose the model you want based on how much water you might be using at once rather than how much you need to have in reserve (e.g. one question is how many showers are in the house -- rather than how many people are likely to take showers in the morning). Assuming you've chosen correctly, you can shower forever and never run out.

Here in Canada, it looks like for our needs (e.g. only one shower) this will cost us about $1000 (not including instalation). What I've found so far is that the electric models are so much more energy-efficient than the gas models that it offsets the higher cost of electricity.
posted by winston at 7:41 AM on August 7, 2006

There has been advertised a method of pre heating your hotwater by running the cold water through a coil of pipe wrapped around the drain so you recover energy after first use. Or you could build a masonary heat sink based on solar input and run the cold inlet through that. It would allow you to use the off-peak electric heater and probably cut your hot water electric usage by a measurable percentage. Google on pre-heaters for some options.
posted by ptm at 10:30 AM on August 7, 2006

Winston, heat pumps are not the same as tankless heaters. A tankless heater still has an electric heating element, so the only energy it saves is whatever would be lost through the tank insulation in a storage heater.

Because tankless heaters needs electricity right when I'm using the hot water, I wouldn't be able to run one on 8c/kWh off-peak power; I'd need to run it on 14c/kWh day rate. So unless the energy loss through storage tank insulation amounts to more than (14-8)/14 = 43% of total energy consumed, and I don't believe it would be anywhere near that, the tankless is going to cost me more to run.

Energy use in a hot water service is proportional to temperature change, for a given volume of water. If I've been raising the water I use from an average of 10°C input to 60°C holding temperature, and I'm losing say 5°C over the course of the day through insulation, then a tankless would save me only 10% energy or so. I'm pulling these figures out of my arse but they seem about right.

ptm, I like your preheater thinking. I can't see a masonry wall working well for us, because most of our hot water use is in the morning and the wall would have cooled down by the time the hot water service was drawing cold water through it. A drainwater heat recovery unit, though - now that's an interesting option, and should work well for showers. I'll see if I can find some price/performance info on those things.
posted by flabdablet at 4:16 PM on August 7, 2006

Have you looked at the available government rebates for green/solar hot water heaters? I haven't gone through all the options, but Sustainability Victoria has what seems to be an easy-to-follow set of information. Rebates are up to $1500, I think, which would put the heater in your price range.
posted by jacalata at 7:21 PM on August 7, 2006

Jacalata, the maximum rebates tend to apply to maximally-priced systems. However, there are a couple of retrofits listed on this page that have my attention: the evacuated-tube pumped collectors from Apricus and Endless Solar.

These firms both talk about energy savings of the order of 80%. My unassisted energy cost over an estimated ten year service life would be $2700; 80% of that is $2200, so if I can pick up one of these retrofit units for around $1500 including rebate and get it installed for less than $700, I'm ahead!

I've asked for more information from both manufacturers, and I'm marking yours as best answer. Thanks!
posted by flabdablet at 8:44 PM on August 7, 2006

wow, my first best answer! good luck with the research.
posted by jacalata at 12:03 AM on August 8, 2006