Hot water
There are 5 main ways to heat hot water.
- Gas Instant
- Gas Storage
- Solar - Boosted by either gas or electric
- Electric Resistive
- Heat Pump
Lets go over the pros and Cons of each
Gas Instant
Pros
- Lost cost for supply install.
- Low gas usage
- No Standing losses (no tank)
- Unlimited hot water
Cons
- Still burning gas (not the cheapest)
- Unlimited hot water can lead to dramatically high usage
- Power Point needed (though not dedicated), can make install cost higher to switch to
Gas Storage
Pros
- Low cost for supply and install.
- Typically no power needed, so no sparky needed
- High recovery rate, means you can have smaller tank. My old gas storage could do 140L/hour heating
Cons
- Smaller tank means lower all at once usage, e.g. filling bath.
- Standing losses from tank
- Burning gas (not the cheapest)
Solar
Pros
- Cheapest hot water when theres sun
Cons
- Most expensive to install
- Needs a backup source (either gas or resistive)
- Large tank needed
- In most parts of Australia only works well a few months a year
Electric Resistive
Pros
- Cheap Supply cost for unit
- Very basic, can last very long time with little to no issues
Cons
- Most expensive way to heat hot water
Heat Pump
Pros
- Cheapest way to heat water year round
- High recovery rate, my Reclaim is 110L/hour
Cons
- Medium supply install price (between gas/resistive and solar)
- Complicated system
- Larger tank size, often not an issue, but is for some houses
- Best to run once per day, with high ambient temp, hence the larger tank
System comparison
So thats the basics of the 5 common ways to heat hot water. Overall Heat Pump always comes out as the winner in every situation I've looked at. But lets go through some common reasons people choose various other situations, why heat pumps are still a better choice.
Instant gas never runs out of hot water
While this is true, a properly sized heat pump dosen't run out of hot water either. A water efficient shower head is 7L/minute, which is mixed water, shower water is about 50/50 hot and cold (from a 70c tank), meaning its only 3.5L/min hot water. Thats 210L/hour, with the 110L/hour recovery you are only using 100L/hour. So a standard 270L tank size means you could shower for almost 3 hours non-stop.
Solar is free hot water
While solar hot water is almost free, minus a small circulation pump, however it obviously only works when the sun is shining. Meaning when its not you effectivity have a gas or resistive system. Plus a high install cost for the solar system. The other downside is when the sun isn't shinning you won't have PV solar to run the electric booster. Meaning your cost to heat hot water goes to the highest way. If you use gas then you get the very high install cost of solar, plus the high cost to heat from gas. Even in Brisbane theres not enough consistent sun, its only places like Darwin that get enough consistent sun. Even if you are in Darwin or similar I still don't think its the best option, as roof space is a limited resource that is better used for PV panels. You can get much better results by putting your savings by going a heat pump into more PV panels on that roof space, which can end up with lowest total monthly bills.
Heat pump install cost is to high
While yes heat pump is the 2nd most expensive hot water system to install, this can easily by offset by having the lowest running cost, meaning Heat Pumps almost always end up with the lowest Total Cost of Ownership, basically the cost to install, and make hot water every day for its lifetime.
Heat pumps are terrible and die quickly
Yes, there are many terrible heat pumps on the market. This is because of all the government rebates that are out there for them. This has lead to a flood of very low quality heat pumps that can be almost free to install, they were $33 here in Vic for a long time. And as you would expect these are made at the lowest possible cost, and priced to perfectly claim every cent of government rebates. The companies pop up and disappear overnight so theres no warranty to speak off. This has been stopped recently in Vic (at least from the rebate side). Theres a very easy fix to this, just don't get the to good to be true terrible systems installed, get a good quality unit.
I can just install more PV and use electric resistive
While this can be a good option it will cost more to install enough PV to cover hot water in winter and install something like a catch power to limit hot water usage to your excess solar. E.g. my 6.6kw solar system often only produces 8kWh in winter, with no excess from regular house usage, so I would need about 4x the panels to run hot water from close to 100% solar year round. The only time this should be considered is if you already have a dramatically over-sized PV array and install a catch power. The catch power can add about $1,000 installed to an existing system, which is quite a significant cost.
What Heat Pump to Get
I personally got a Reclaim, because it was the only one that would physically fit in a cramped space I had central to the home. I think iStore units are better value, a good unit thats built well and priced reasonably. They have been sold over 10 years and most of them from 10 years ago are still running. If you need a split system type unit like I did Emerald now also sell a cheaper split system unit.
Heat pumps are typically sized at 50L/person, to go with efficient shower heads and average showers. Go with how many people live there, or how many the house is designed for, whatever is higher. E.g. a 3 bedroom house is typically designed for 4 people. But you may have 3 couples living in one as a share house and have 6 people.
You can get away with a smaller tank, but it will mean you may have to leave it running 24/7 which can increase running costs. You may also need a larger tank, if for example you have not very efficient shower heads and refuse to use efficient ones.
Costs
Lets look at the cost to make hot water, and what the payback will be. All the default values in the calculator below are the prices and values for me, you should of course tweak them as needed. First lets go over all the assumptions I've made.
Hot Water Usage
A standard water saving shower head is 7L/min, there are ones down as low as 4L/min that people love, and a regular shower head many people use can be as high as 20L/min. The easiest way to figure yours out is to get a large bucket, get the shower running on cold at what you shower at. Collect the water for 15 seconds in the bucket, then multiply those litres by 4. This will get you the Litres per minute number of your shower head.
Hot water mixing and ratios
In my calculator below I'm assuming a heating cost of 0.0314kWh/L. This assumes heating from 15c (average year round temp here in Vic), to 42c, which is an average shower temp. In a tank system the water is typically heated to 70c, but then mixed down with cold water to 55c at the tank, then mixed again at the shower. There would be losses in the pipes for all system types, so lets go with 0.035kWh/L to account for those pipe losses.
Standing Losses
Tank losses are the losses you have in a tank from keeping the water hot. Im accounting for 6kWh/day, which is the approximate year-round losses on my Reclaim tank. I've accounted for 4kWh/day of losses from the gas storage tank since they typically have smaller tanks due to their 24/7 operation and high recovery rate.
100% Solar
Running your hot water on 100% solar obviously isn't applicable to gas, and as above is hard with resistive, however I've added it into the calculator for resistive and heat pump, as you may already have oversized solar, and heat pump is easy to run off 100% solar, even on limited solar days, mine typically runs for 2-3 hours at 1kw.
Heat Pump Efficiency
I have defaulted the calculator to a COP of 4, as this is what the recommended iStore does as a year round average. My Reclaim says they have a COP of 5, but this is probably in warmer weather, not as an average.
Gas Efficiency
I have defaulted to 75% as this seams to be an average for gas hot water heating, there are some units getting close to 90% however these seam to be mainly in the US and Europe for boilers used for hydronic heating as well, and are drastically more expensive.
Solar
I'm not including solar as it varies drastically from location to location. You could look up how many sunny days you get at your location per year and take that as a % and compare to either gas storage or electric storage. E.g. if you had sunny days 40% of the year,and you had electric boosted then you cost is roughly 60% of the resistive cost.
Yearly Costs
Gas Instant
Gas Storage
Resistive on CL
Resistive on 100% Solar
Heat Pump on Grid
Heat Pump on 100% Solar
Conclusion
Depending on your exact numbers you will get different results, but you can see using my numbers the heat pump comes out at the cheapest cost to heat hot water with, even if its 100% grid, and drastically cheaper with solar. This is also not taking into account the daily supply charge for gas, if hot water is your last gas appliance that can easily add another $300 or so per year to the cost of keeping your gas hot water. With my numbers in my calculator I'm saving $535.65 per year by switching from my gas storage hot water to my heat pump running off solar. With the install cost of a iStore 270L for about $3,000 that would give a payback of under 6 years, thats as good as solar PV! If you currently have resistive HWS it would be a saving of $913.64 per year. Thats a payback just over 3 years, thats a massive saving.