Hi

I'm looking for any help on how to structure the water heating for a client who wants to go off the grid. (I'll get installers to do the work, but need to check the structure/system is correct first!)

 

House Requirements:

• Couple Aged 60 Living There - shower 95% of the time!
• 2 BNBS with a King Bed Each
  • in peak season maybe 4 extra people.
  • However, only for the major hot season tourist months (Sep to Jan)
  • Min rental will be 1 month so can enable/disable geysers etc. 
• Probably have gas in the kitchen.
• TAKE NOTE: We want to get the house off-grid as well!

 

NOTE: You can't tell BNB guests they can only get hot water at certain times, and they only might be there for 3 to 4 months of the year in peak season! This is where it's complicated.

What I'm trying to work out is...what's the best way to structure the water heating requirements for the house going off-grid as there are such different load requirements!

 

I've read this great article (from ITS Solar), but it's assuming 1 geyser (As a case study, let’s assume a three-bedroom house with a family of four that uses 28kWh (Eskom units) on average per day. The PV system is a typical 5kW inverter with 4kW of PV panels and 10kWh of battery storage. The house has a standard 200L geyser with a 4kW element in).

 

What would you recommend for the water heating side of things?

• Could we have 3 geysers (150L) and 1 large heat pump that does them all? 
• or do we require 3 geysers and 3 smaller heat pumps?
• or 3 geysers and 2 heat pumps (one for each side of the house)
• or do we use a heat pump on the one side and then on the other use a flat plate connected to the geyser? (there's lots of roof space)
• or do we use a heat pump & geyser on the one side, and then gas for the AirBNB (as they're only occupied 3 to 4 months) - we'd only rent for a month at a time, so we could turn geysers on/off if not in use.

The house hasn't been built yet, so we're deciding on everything for the client for now.

Here is a layout of the rooms, roof space and sun direction in case it's needed!

Any help and guidance is appreciated!

 

1. Primary Considerations

Base Load: The couple’s daily hot water needs (~150L geyser should be sufficient).

Peak Load: Additional 4 guests in peak season (~300L extra needed).

Energy Source: Solar PV, solar thermal, heat pumps, or gas backup.

System Automation: Ability to turn off/on Airbnb geysers efficiently.

2. Recommended Water Heating Setup

Option 1: Centralized Heat Pump + Geysers (Best Efficiency)

1x 5kW heat pump supplying hot water to three separate 150L geysers

Each geyser is pre-heated by solar thermal collectors (flat plate or evacuated tubes, depending on budget).

The heat pump runs mainly during solar hours (linked to PV system) for best efficiency.

Airbnb geysers can be isolated (turned off) when unoccupied.

Pros: Highly efficient, uses free solar heat, centralized maintenance.

Cons: Initial setup cost higher, but long-term savings are significant.

Option 2: Hybrid Approach (More Redundancy)

1x 5kW heat pump + 2 geysers (one for the couple, one for Airbnb).

1x Flat plate solar thermal system for the couple’s daily use (lowers heat pump workload).

Gas backup for Airbnb units (since they're only used 3-4 months/year).

Pros: Gas is only used seasonally, solar thermal works year-round.

Cons: Slightly more complex to manage but reduces off-grid battery strain.

Option 3: Fully Decentralized (Easier to Manage)

3 smaller heat pumps (1 per geyser, e.g., 2.5kW each).

Solar thermal pre-heating for the couple’s geyser.

Each Airbnb unit has independent heating, reducing strain on a single system.

Pros: Simple to scale, Airbnb geysers can be isolated.

Cons: Multiple heat pumps might cost more upfront.

3. Key Considerations

Roof Space & Sun Direction: Use available space for flat-plate collectors where possible.

Energy Management: If using solar PV + batteries, time heat pump operation with solar peak hours.

Geyser Placement: Keep geysers close to points of use to reduce heat loss.

Backup Plan: Gas backup for Airbnb might be a good failsafe in case of extended bad weather.
 

12 hours ago, TaliaB said:

1. Primary Considerations

Base Load: The couple’s daily hot water needs (~150L geyser should be sufficient).

Peak Load: Additional 4 guests in peak season (~300L extra needed).

Energy Source: Solar PV, solar thermal, heat pumps, or gas backup.

System Automation: Ability to turn off/on Airbnb geysers efficiently.

2. Recommended Water Heating Setup

Option 1: Centralized Heat Pump + Geysers (Best Efficiency)

1x 5kW heat pump supplying hot water to three separate 150L geysers

Each geyser is pre-heated by solar thermal collectors (flat plate or evacuated tubes, depending on budget).

The heat pump runs mainly during solar hours (linked to PV system) for best efficiency.

Airbnb geysers can be isolated (turned off) when unoccupied.

Pros: Highly efficient, uses free solar heat, centralized maintenance.

Cons: Initial setup cost higher, but long-term savings are significant.

Option 2: Hybrid Approach (More Redundancy)

1x 5kW heat pump + 2 geysers (one for the couple, one for Airbnb).

1x Flat plate solar thermal system for the couple’s daily use (lowers heat pump workload).

Gas backup for Airbnb units (since they're only used 3-4 months/year).

Pros: Gas is only used seasonally, solar thermal works year-round.

Cons: Slightly more complex to manage but reduces off-grid battery strain.

Option 3: Fully Decentralized (Easier to Manage)

3 smaller heat pumps (1 per geyser, e.g., 2.5kW each).

Solar thermal pre-heating for the couple’s geyser.

Each Airbnb unit has independent heating, reducing strain on a single system.

Pros: Simple to scale, Airbnb geysers can be isolated.

Cons: Multiple heat pumps might cost more upfront.

3. Key Considerations

Roof Space & Sun Direction: Use available space for flat-plate collectors where possible.

Energy Management: If using solar PV + batteries, time heat pump operation with solar peak hours.

Geyser Placement: Keep geysers close to points of use to reduce heat loss.

Backup Plan: Gas backup for Airbnb might be a good failsafe in case of extended bad weather.

Very thorough plans and options @TaliaB and I also like your important consideration for redundancy (fall-back-plan). Not enough consideration is given for fall back. 👍🏽👍🏽👍🏽

Edited March 1, 20251 yr by Moffat

48 minutes ago, Moffat said:

Not enough consideration is given for fall back

Thanks @Moffat

What power source is still available for off-grid application as fall back option - 1.Generators 2.Wind power generation.

1.Generator Considerations:

A. Fuel Costs: Running a generator to power a heat pump or electric geyser is expensive. A typical 5kW generator burns 1–1.5L of fuel per hour, which adds up quickly.(depending on generator size and load)

B. Inefficiency: Generators are best for short, high-load events (e.g., power loss due to inverter breakdown or running out of gas) but not for continuous heating.

C. Noise & Maintenance: Running a generator frequently increases wear, needs regular servicing, and can be disruptive.

If a generator is absolutely necessary, it should be small and used only to charge batteries.

2.Is Wind Power a Good Fit?

A.The site has consistent wind speeds of 4.5 m/s (16 km/h) or higher.

B.There are few obstacles (trees, buildings) blocking the wind.

C.You have space for a 10m+ high mast (higher is better).

D. Budget and maintenance cost is high.

E. Potential for lightning strikes and damage.(high masts)

 

19 hours ago, AlexanderR said:

Any help and guidance is appreciated!

Re-looking at this, let me give you my thoughts... you are not indicating where the hot water cylinders are located, nor where this house (which province) is located... here in the Green Kalahari/not so green Kalahari, one would be running with low(er) pressure water than at most locations, since it isn't from a municipal source, 1 bar or slightly less, from a 6?/8?m high Jojo tank. This has us using a 100l solar hot water cylinder, which has 12, I think, evacuated glass tubes and has been running like this now, coming up to 5 years. This time of year the hot water is 70+degrees C and you need to be careful not to burn yourself. In winter, in the afternoon the water is also hot and by next morning still warm enough to do the dishes, by not adding cold water. We do not heat the water, other than the sun via the evacuated glass tubes. This may be a cost effective router to consider, since this hot in our case, has cost us nothing, since its been in place.

The layout of the house seems less than ideal, can you maybe swing/rotate it such that you have better roof areas facing the sun which may allow the off-grid aim easier, possibly? If you look at the solar HWC as we are using it, you also will want to/need to mount it on the roof facing the sun... not sure about high pressure variants, these would probably need a pump, lots of plumbing, but also allow the cylinder to be under roof, ours is on top of the roof and facing slightly east of due north (between 15 an 30degrees), didn't build the house, would have also built the house facing north, otherwise.

Coming back to location/province, if they have more sunshine than not in summer and not too many clouds in winter, then an evacuated tube hot water system is probably the most cost effective/least troublesome setup, I believe, certainly the heati9ng efficiency is the highest with no impact on your PV solar/electric system.

15 hours ago, TaliaB said:

1. Primary Considerations

Base Load: The couple’s daily hot water needs (~150L geyser should be sufficient).

Peak Load: Additional 4 guests in peak season (~300L extra needed).

Energy Source: Solar PV, solar thermal, heat pumps, or gas backup.

System Automation: Ability to turn off/on Airbnb geysers efficiently.

2. Recommended Water Heating Setup

Option 1: Centralized Heat Pump + Geysers (Best Efficiency)

1x 5kW heat pump supplying hot water to three separate 150L geysers

Each geyser is pre-heated by solar thermal collectors (flat plate or evacuated tubes, depending on budget).

The heat pump runs mainly during solar hours (linked to PV system) for best efficiency.

Airbnb geysers can be isolated (turned off) when unoccupied.

Pros: Highly efficient, uses free solar heat, centralized maintenance.

Cons: Initial setup cost higher, but long-term savings are significant.

Option 2: Hybrid Approach (More Redundancy)

1x 5kW heat pump + 2 geysers (one for the couple, one for Airbnb).

1x Flat plate solar thermal system for the couple’s daily use (lowers heat pump workload).

Gas backup for Airbnb units (since they're only used 3-4 months/year).

Pros: Gas is only used seasonally, solar thermal works year-round.

Cons: Slightly more complex to manage but reduces off-grid battery strain.

Option 3: Fully Decentralized (Easier to Manage)

3 smaller heat pumps (1 per geyser, e.g., 2.5kW each).

Solar thermal pre-heating for the couple’s geyser.

Each Airbnb unit has independent heating, reducing strain on a single system.

Pros: Simple to scale, Airbnb geysers can be isolated.

Cons: Multiple heat pumps might cost more upfront.

3. Key Considerations

Roof Space & Sun Direction: Use available space for flat-plate collectors where possible.

Energy Management: If using solar PV + batteries, time heat pump operation with solar peak hours.

Geyser Placement: Keep geysers close to points of use to reduce heat loss.

Backup Plan: Gas backup for Airbnb might be a good failsafe in case of extended bad weather.
 

Thanks. I like the idea of option 2. I wasn't sure 1 heatpump could do 3 geysers, especially if on a different side of the house. I thought there was a distance to geyser limit.

Why not 3 of these...Solar geysers

 

https://www.builders.co.za/Plumbing-Bathroom-and-Kitchen/Geysers-and-Water-Heaters/Solar-Geysers/Apollo-Solar-Technology-AS-LP-12-Integrated-Low-Pressure-Solar-Geyser-100-L/p/000000000000744582?region_id=ZANP00&gad_source=4&gclid=Cj0KCQiA_Yq-BhC9ARIsAA6fbAiejpfDOj_jKl0t0ex8qNzWKUJfIz9BUQ8cOb3fbZRTFxs0wxBoFJ0aAm1EEALw_wcB

 

R10K/each, with 1 gas geyser as backup for the bnb?

1 hour ago, TaliaB said:

Thanks @Moffat

What power source is still available for off-grid application as fall back option - 1.Generators 2.Wind power generation.

1.Generator Considerations:

A. Fuel Costs: Running a generator to power a heat pump or electric geyser is expensive. A typical 5kW generator burns 1–1.5L of fuel per hour, which adds up quickly.(depending on generator size and load)

B. Inefficiency: Generators are best for short, high-load events (e.g., power loss due to inverter breakdown or running out of gas) but not for continuous heating.

C. Noise & Maintenance: Running a generator frequently increases wear, needs regular servicing, and can be disruptive.

If a generator is absolutely necessary, it should be small and used only to charge batteries.

2.Is Wind Power a Good Fit?

A.The site has consistent wind speeds of 4.5 m/s (16 km/h) or higher.

B.There are few obstacles (trees, buildings) blocking the wind.

C.You have space for a 10m+ high mast (higher is better).

D. Budget and maintenance cost is high.

E. Potential for lightning strikes and damage.(high masts)

 

Thanks. It's in a nature reserve. Think like a Kruger (near Orpen). No generators l, no wind.

 

Lots and lots of sun!

Edited March 1, 20251 yr by AlexanderR

11 minutes ago, Kalahari Meerkat said:

Re-looking at this, let me give you my thoughts... you are not indicating where the hot water cylinders are located, nor where this house (which province) is located... here in the Green Kalahari/not so green Kalahari, one would be running with low(er) pressure water than at most locations, since it isn't from a municipal source, 1 bar or slightly less, from a 6?/8?m high Jojo tank. This has us using a 100l solar hot water cylinder, which has 12, I think, evacuated glass tubes and has been running like this now, coming up to 5 years. This time of year the hot water is 70+degrees C and you need to be careful not to burn yourself. In winter, in the afternoon the water is also hot and by next morning still warm enough to do the dishes, by not adding cold water. We do not heat the water, other than the sun via the evacuated glass tubes. This may be a cost effective router to consider, since this hot in our case, has cost us nothing, since its been in place.

The layout of the house seems less than ideal, can you maybe swing/rotate it such that you have better roof areas facing the sun which may allow the off-grid aim easier, possibly? If you look at the solar HWC as we are using it, you also will want to/need to mount it on the roof facing the sun... not sure about high pressure variants, these would probably need a pump, lots of plumbing, but also allow the cylinder to be under roof, ours is on top of the roof and facing slightly east of due north (between 15 an 30degrees), didn't build the house, would have also built the house facing north, otherwise.

Coming back to location/province, if they have more sunshine than not in summer and not too many clouds in winter, then an evacuated tube hot water system is probably the most cost effective/least troublesome setup, I believe, certainly the heati9ng efficiency is the highest with no impact on your PV solar/electric system.

Hi. This sounds like a great plan!

It's in a hot area near Kruger in a nature reserve. Tons of sun!

Are these the geysers you're meaning...

https://www.builders.co.za/Plumbing-Bathroom-and-Kitchen/Geysers-and-Water-Heaters/Solar-Geysers/Apollo-Solar-Technology-AS-LP-12-Integrated-Low-Pressure-Solar-Geyser-100-L/p/000000000000744582?region_id=ZANP00&gad_source=4&gclid=Cj0KCQiA_Yq-BhC9ARIsAA6fbAiejpfDOj_jKl0t0ex8qNzWKUJfIz9BUQ8cOb3fbZRTFxs0wxBoFJ0aAm1EEALw_wcB

5 minutes ago, AlexanderR said:

Are these the geysers you're meaning...

Yup, that's the kind of thing we have, but like I stated, low pressure system, like the link you have, this would not work with a municipal 4+ bar pressure water source... unless you can reduce the pressure overall to the house, I'd think...

some of our neighbours have put in pressure pumps to have higher pressure water in their homes, we have a low pressure shower head and thus I can be in there for 15 to 20 minutes and probably not even consume 20l of water... the dishwasher and washing machine regulate water with a pressure sensor. so if it takes 5 minutes to reach desired water level instead of one minute or 90 seconds, so what, it functions as it should...

either way, I can certainly recommend the evacuated glass tube solar hot water cylinder based on personal experience and if the water isn't used regularly in summer etc. you may need a 'afdakkie' for the glass tube to prevent the water from boiling... after all, water at 70degC, none used, next day it may start with 60degC in the morning and boil by evening, possibly, we haven't had this, but in some of the shacks around the Upington lower income neighbourhoods, apparently this has happened, I've been told, but I would think these hardly are low pressure systems in the municipal area...

5 hours ago, TaliaB said:

Thanks @Moffat

What power source is still available for off-grid application as fall back option - 1.Generators 2.Wind power generation.

1.Generator Considerations:

A. Fuel Costs: Running a generator to power a heat pump or electric geyser is expensive. A typical 5kW generator burns 1–1.5L of fuel per hour, which adds up quickly.(depending on generator size and load)

B. Inefficiency: Generators are best for short, high-load events (e.g., power loss due to inverter breakdown or running out of gas) but not for continuous heating.

C. Noise & Maintenance: Running a generator frequently increases wear, needs regular servicing, and can be disruptive.

If a generator is absolutely necessary, it should be small and used only to charge batteries.

2.Is Wind Power a Good Fit?

A.The site has consistent wind speeds of 4.5 m/s (16 km/h) or higher.

B.There are few obstacles (trees, buildings) blocking the wind.

C.You have space for a 10m+ high mast (higher is better).

D. Budget and maintenance cost is high.

E. Potential for lightning strikes and damage.(high masts)

 

Greeting all. I believe @TaliaB has done very well & put fire to the conversation and my last comment about backup was meant to highlight that “not many proposals”, take into account backup or fall back. The more questions are asked and answered the easier it is to come to a “near-perfect” solution, especially in regard to loads, budget, accessibility to support (shops/installers) and gas as well as the location of the property and house orientation (north etc). This helps the contributors cover blind spots. But this thread is fleshing out very nicely, thus far. 

On 2025/03/01 at 1:22 PM, AlexanderR said:

Thanks. I like the idea of option 2. I wasn't sure 1 heatpump could do 3 geysers, especially if on a different side of the house. I thought there was a distance to geyser limit.

Why not 3 of these...Solar geysers

 

https://www.builders.co.za/Plumbing-Bathroom-and-Kitchen/Geysers-and-Water-Heaters/Solar-Geysers/Apollo-Solar-Technology-AS-LP-12-Integrated-Low-Pressure-Solar-Geyser-100-L/p/000000000000744582?region_id=ZANP00&gad_source=4&gclid=Cj0KCQiA_Yq-BhC9ARIsAA6fbAiejpfDOj_jKl0t0ex8qNzWKUJfIz9BUQ8cOb3fbZRTFxs0wxBoFJ0aAm1EEALw_wcB

 

R10K/each, with 1 gas geyser as backup for the bnb?

Yes sure but how is the water quality (boreholes)scaling and sediment buildup in hard water areas can reduce effieciency. Also with lots of sun in your ares if hot water demand is low (e.g., when Airbnb units are unoccupied), the system can overheat, leading to:

Pressure buildup and potential system damage.

Increased maintenance needs due to thermal stress.

The need for heat dissipation methods (e.g., heat dump radiators or circulation loops).

Edited March 2, 20251 yr by TaliaB
Spelling

13 hours ago, TaliaB said:

Yes sure but how is the water quality (boreholes)scaling and sediment buildup in hard water areas can reduce effieciency. Also with lots of sun in your ares if hot water demand is low (e.g., when Airbnb units are unoccupied), the system can overheat, leading to:

Pressure buildup and potential system damage.

Increased maintenance needs due to thermal stress.

The need for heat dissipation methods (e.g., heat dump radiators or circulation loops).

Hi @TaliaB - thanks for the detailed explanation! I really appreciate your effort, time and insight.

• water quality and pressure is fine.

• good point about unoccupied issue! I've moved the solar geyser to the side where a couple will always be living. It also faces the sun and has a roof it can easily be installed on.

---

RECENT UPDATES: We want to try totally go off the grid. The cost of the 3 phase mains cable is R75 000 to connect to grid (due to distance), so we can rather put this towards a full off-grid system.

---

Here are some suggestion and locations...

Option 1: Heat Pump & Geyser + Solar Geyser

What about this setup?

Option 2: Solar Geyser + Gas

What about this?

Questions:
1 - what size gas geyser is best?

2 - which off the above is better for the off-grid?

3 - can we put in a gas backup if the solar geyser isn't working well enough in winter? (I'd rather do this later after testing)

4 - would the gas geyser next to the dishwashers, help with reducing the energy use on the dishwashers (main kitchen each cottage) by heating the input water to 70 deg? (Thus, if the main house and 2 cottages all used their dishwasher at the same time, it would drop the wattage by quite a bit as the gas would do the major heating?)