Hot Water Storage - Energy Losses

A bit over 20 years ago we replaced a failing electric storage HWS with an electric boost Solahart. Now the Solahart is in its twilight years and I’m looking at options to transition to a reasonable arrangement for the future.

For the last few years I have had a temp sensor against the tank underneath the insulation and temp logged every 15 minutes by an RPi. From looking at the data I know the tank will drop 8 degrees overnight (without water drawn) and requires about 1 hr of boost to raise the temp 8 degrees when there is negligible heat radiation around. So, the tank loses about 2.5kwhr each night and is electrically replaced probably 6 months of the year.

If I look at rough capital costs I could replace with a Solarhart (at recent service tech said around $4400) and it looks like a replacement electric storage unit is around $1500) but I can’t seem to find any info on losses of current models which I assume is fairly critical in selection criteria these days. Does anyone have any data on losses?

My thoughts are now is the time to do the numbers on a PV system with a ~$3000 head start to power an electric storage HWS rather than solar HWS.


I would look at that combination and do those sums. The benefit you get from solar PV power is strongly related to the proportion of power generated that you can use on site. The more you have to sell to the grid and buy back at a higher price the less you save. Storage HWS gives you a destination for your excess solar power in the middle of the day when the sun is high. Unless you are home then and using aircon, doing the washing , running the dishwasher, your excess will go to the grid.


Why not keep the Solarhart a bit longer and use the proposed new solar PV to offset or supply the boost? Do you really need to boost for an 8C drop? Our old Solarhart which was 15 years old only ever needed to boost when it was very wet and cloudy.

For a guide to the losses from a storage electric system asking for the standby power usage might be a better question. IE how much power will the system consume over 24 hrs when you are not at home. It would seem a straight forward question, answers unknown. The temperature drop is less likely to be directly assessed. It is a function of the insulation quality and daily environmental variables.

If you have not found these resources yet:

We have Solar PV for power and gas for HW. We had gas before the PV. If we had to upgrade and cost was not a consideration it would be a large HW storage tank with flat plate solar panels and gas boost. The lowest capital cost would probably be $1,000 for a 250l electric storage HWS with the solar PV configured to heat the storage HWS in the middle of the day. As per @syncretic comments.

@gordon is likely to offer some advice on the best way to mix and match Solar PV and HW. It can also depend on where in Aus you live.


Which component is failing with the Solahart?

I don’t know your location, but a PV system with heat pump on a timer for middle of the day heating may be the best option. If your existing tank is ok, then a bolt-on HP may be the least expensive way to go.

Here where it is quite sunny, we (2 of us) manage with a 30 evacuated tube system + 250l tank and generally don’t need any boosting (the system doesn’t have any). With judicious use of water in cloudy periods we’ve managed to go well over a year with no lack of hot water. When it was wetter and we had longer showers, there were still only a few days per year when the water was a bit cooler than desired for showering.


The house is not flush with northerly aspect roof space. I’m not saying I specifically boost to offset overnight loss but in winter on a moderate cloudy day with losses and hot water draw the water will be below 40 degrees within 24 hrs without boost (after starting at thermostat cut out ~65 degrees) . Even on a clear sunny day in winter heat increase is very low and certainly not enough to recover temp from usage and losses.

I have looked through the links posted and there are statements about needing rating systems that are not in place now.

The concern is the tank. There was a minor leak detected 5 years ago and it was flange corroded through. This service just gone the anode head was corroded on to the tank. The Tech showed me a pic and he needed to alert me to the anode issue as trying to remove it may have caused a failure of tank and I needed to make a call. Anode was changed and old anode still had about 5mm of (zinc?) along 50% of wire. Tech also showed pic of the bung which I assume is at other end of tank from the flange. There was precipitation like from a slow leak, he said it wasn’t leaking at that moment but if it develops then the tank is unserviceable. So I asked how much to replace tank or how much to replace whole system. Tank was 3k, whole lot 4.4k. Tech said at this stage it could go years or may fail soon.

Located in Perth.

Yep, that is it.


What sized tank do you have and what hot water use do you have (large househood or small, short or long showers, using hotwater for clothes washing etc)?

Do you have a tempering valve installed as this can make outlet water cooler in winter…even though tank storage temperature is still hot. In winter cold water is colder making the mixed water substantially cooler. It might mean adjustment to the tempering valve in winter to reduce amount of cold water mixed at the outlet. In summer, adjustment to increase mixing levels.

We have a 315L Aquamax booster storage tank and find it will have sufficient hot water for about 3-4 days of cloudy weather (3 winter and 4 in summer). We have also made insulated covers (foam wrapped in duck tape) for the connection points to the tank as these can be a major source of yeat loss. We slso wind back the tempering valve in winter so that less cold water is added at the outlet.


Tank is ~300l but I think the issue is heat loss from tank over 24hr cold period, I could extrapolate that it would drop 16C without draw. Medium household with medium showers, some cold some warm washing machine cycles, dishwasher (internal heater) and some manual dishwashing.

No tempering valves, my temp measures are fixed sensor against tank under insulation. Here is screen of last 3 days mix of stormy and sunny periods.


Where has is the thermometer installed? Ideally, it should be near the tank outlet to the house mains. If the thermometer is installed elsewhere, it could be affected by cold water replacing hot water used within the house.


Looking at the end of the tank it’s at about 4 oclock, I can’t remember specifically the location of other piping in relation to it. I see the recharge effect impact the short term result but I tend to look for longer period changes when there is no water drawn.

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I put a query through the Rheem website and promptly received info I was chasing.

After seeing the data (looking at line for zero usage) I would expect a market for premium insulated lower loss units and aftermarket insulating blankets (if there is no technical reason it shouldn’t be done). Also my sarcastic thought, wheels on bottom and put it inside in winter and outside in summer :wink:


I initially found installing hot water services outside to be curious. In the US gas or electric are either in the garage when it is part of the main house, or in a dedicated closet or the attic/roof cavity (sometimes 2 units, one at each end of the house) in upscale houses. Insulating blankets on the hot water services are common and insulation on the hot water pipes the norm.


Our house was built in 1952, and the original water heater was still in the roof space when we moved in in 2005. Having it in the roof was essential as it relied on gravity feed to the taps. The previous owner had superseded it with an off-peak storage unit installed outside.

The only reason we had the original removed was that we wanted to access the space to put in an attic.

The tank of the off-peak unit rusted out, and we replaced it with an on-demand natural gas unit.

We are now looking at replacing that with a heat pump system after we get additional solar panels installed. Every heat pump system I have seen, even those with the heat pump able to be located remote from the tank, have a vertical tank, so there is no way one of those could go into the roof space.

The delivery/outlet piping from our existing water heater is lagged (insulated), which is just as well, as it is quite distant from the bathroom.


We have a split solar HW system with the storage tank under the house/in the garage area (house is on stumps).

Mains pressure is used to force the hot water from the storage tank to household faucets, while a small electric pump is used to move water from the storage tank to the roof top panels. It seems to work well without any issues.Piping between the panels and storage is insulated to reduce losses as the distance is about 5 metres.


Some people have been using added insulation to their HW tanks for years in order to reduce losses to the environment.

I think Rheem are a bit optimistic regarding energy losses per day in their table. For the 315l (I assume this is the one you have) I calculate over 3.5kWh/day loss in the best case above (~5C loss over 12hours), whereas the table says 2.5kWh. That’s for summer, if the tank is outside the winter loss would be greater.


It all sounds very technical and the points made are relevant in many ways. However, the real question is what you want to get out of the HWS. Do you really need ‘really’ hot water all the time? I never checked the temperature of our HWS output and our booster is turned on and off at fuse in the meter box if needed. This only happens a few days in the year and only for 2 - 3 hours as we are happy to have water that is just warm enough for a comfortable shower at night. This way we prevent a high load on our Solar system which would most likely switch automatically to mains power anyway.
We installed the 1st 300ltr Solarhart HWS with electric booster in 1996 and replaced it in 2017. We recommend Solar HWS to anyone.


We are the same. We only turn on the booster when the water isn’t sufficiently hot enough (possibly 3-10 days per year and mainly during periods of cloudy or wet weather).

I did mention this technique to a plumber and they indicated that it is not something their industry supports (the industry position is the booster is to remain on at all times). The plumber argued that not having the booster on all the time may result in pathogens such as Legionella flourishing in the storage tank. When I suggested that that tank temperature regularly exceeds 70oC (the thermostat cutout is often triggered by the sun heated water and needs to be reset), and at such temperatures any pathogens would be killed instantaneously, the plumber just shrugged his shoulders.

One has to however realise that there is a possible risk especially if the storage tank is not very hot regularly.


That was also our norm for the years we lived in central Qld. Even on cloudy winter days the roof mounted solar did the trick. Family of five. The system faced NW on a standard pitch roof (22deg?). Our booster was connected to off peak night rate. It was easy to monitor when the booster was in use.

Not every roof is the same, shading may be an issue, how cold it gets at night, and how older systems were lagged on the connecting pipe work, all become factors. The further south you go plus weather effects it is easy to see why boosting every night becomes the norm.

There are also closed loop and open loop rooftop systems.


I suspect that risk is grossly exaggerated, for whatever reason. The legions haven’t been seen here in nearly 30 years of no electric booster action. The tank does reach over 90C in sunny weather, but there have been numerous periods of a week or more of cloudy weather over the decades when the water is below 50C.

Rooftop evaporative air conditioners on the other hand, are a well known and frequent source of Legionella outbreaks, yet they are allowed to proliferate.


If you do go for the solar PV system & a standard hot water system make sure your solar is feeding in to the right tariff. Here in Tasmania our solar was feeding in to Tariff 31 - lights & power not to Tariff 41 - heating & hot water, so we had no benefit from the excess power we generated reducing the cost of boosting our solar hot water system. This didn’t matter for the first 5 or 6 years as our feed in rate was the same as the rate we paid for power used, but that system stopped about 15 months ago. We’re about to install a battery & change to an off peak tariff to optimise the power we generate.


You got me started, rant incoming!
Here in WA the State Government split the state owned power corporation into the individual units back in 2006. This was done to allow competitors to provide energy to domestic customers but it never happened, but we now have 2 government corporate structures in place that have expensive tastes (like spending 50k on farewell for boss when he retires). We are locked into dealing with one supplier and the rate is something like 8c into the grid, 29c out of the grid. If you have more than 5kW system I am reading there is no credit for PV energy into grid.