A Plan for a Cheap PV Heating System

[Achilles]

I’m looking to build a low-cost PV system to help with heating. I found a deal on Solarwatt P210-60 GET AK panels—German-made, 10 years old, 240W each for €10 ($10.42). Seems like a solid option for a budget-friendly setup.

The plan is to connect these panels to a 500L (132 gal) or 1000L (264 gal) buffer tank and ideally use a DC heating element to warm the water (skipping an expensive AC inverter). The stored warm water (30-70°C) would be used in two ways:

1. Preheating water for my condensing gas boiler, so it runs more efficiently.
2. Circulating it through my house radiators on milder days to reduce gas usage.

If I buy 25 panels, that’s €250. A used 500L buffer tank with coils costs €350. Adding cables, aluminum rails, a DC-to-DC charge controller, and other components, I estimate a total cost of €800-900. i've tried to search the forum for this but


You might be thinking, Why not just use solar water heaters? Since their 40-50% energy efficiency is way better than the ~10% I’d get from PV panels, that would make sense, right?

The problem is:

1. I can’t find used solar water collectors in my area, and new ones cost €90 per square meter. (if my math is correct watt/€ is higher but maybe i'm wrong)
2. If I install them on my roof, there’s a risk of leaks while I’m away in the summer. If even a small plumbing issue happens, I might not notice until it’s too late.

Why I’m Not Connecting This to the Grid to save on electricity
Let me give you a quick tour of Greek bureaucracy:

• No Feed-in Tariff – Individuals can’t sell excess power back to the grid anymore.
• Net Metering Was Just Abolished – I can no longer offset my electricity bill with solar production.
• Net Billing (the new system) – I can only use the power in real time (while I’m at work). Any extra power—which would be most of it—gets dumped into the grid for the market price of just €0.01–€0.02 per kWh, while I pay €0.09 per kWh to buy electricity.
• Strict Permit Rules – Even a single solar panel connected to my home’s electrical system (even with a battery and no grid export) requires a €450 permit—with no guarantee of approval due to substation capacity limits.

If I added batteries, I’d need to account for battery costs + 450 for permit + an inverter + (probably they wont approve me)
That would blow my budget, and I’d still have to deal with battery limitations, while excess energy would still be sold for next to nothing.


EDIT: after a bit of research in here i probably realized i'm probably over simplifying things maybe i should consider converting to ac which then the questions becomes since a heating element is not a sensitive device can i just get something reliable but cheap to convert the dc to ac something like a EASUN 6.2KW Solar Inverter would work its under 300euros

Let me know your questions thought objections

and thank you all in advance for helping me save a buck or two

 

[Zwy]

The plan is to connect these panels to a 500L (132 gal) or 1000L (264 gal) buffer tank and ideally use a DC heating element to warm the water (skipping an expensive AC inverter).

Greater efficiency of the electric power using a heat pump might be a better idea.

The yield from the panels will be 200W or less, 25 panels = 5000W at peak sun hours, the rest of the time it will be less. These are used panels and possibly might output only 150W or less. 5 Kw equals about 15K btu's (commonly used in the US) but if you understand either Kw or btu's you get the picture. This is why a heat pump is used to gain efficiency, with a COP of 3 or more, the yield is multiplied by 3 so the 5 Kw is now 15 Kw of heat energy.

The stored warm water (30-70°C) would be used in two ways:

1. Preheating water for my condensing gas boiler, so it runs more efficiently.

Don't preheat water that is on the inlet side of a condensing boiler. The lower the temp of incoming water, the greater the condensing efficiency.

A buffer tank should be installed on the outlet of the boiler which if done correctly makes for a low loss header. Any condensing boiler should have a low loss header which is a primary/secondary loop.

1. Circulating it through my house radiators on milder days to reduce gas usage.

High temp emitters require higher boiler temps which leads to higher boiler inlet temps reducing condensing efficiency.

Low temp emitters such as radiant floor heat allow for greater condensing efficiency. If you are currently running something like old cast iron radiators or other high temp emitters, you might be better off first switching to a low temp emitter to increase condensing efficiency of the boiler. Conservation of energy usage always pays even if installing a solar system as the system won't need to be as large. Look for inefficiency first and ways to improve conservation.

If I buy 25 panels, that’s €250. A used 500L buffer tank with coils costs €350. Adding cables, aluminum rails, a DC-to-DC charge controller, and other components, I estimate a total cost of €800-900. i've tried to search the forum for this but


You might be thinking, Why not just use solar water heaters? Since their 40-50% energy efficiency is way better than the ~10% I’d get from PV panels, that would make sense, right?

The problem is:

1. I can’t find used solar water collectors in my area, and new ones cost €90 per square meter. (if my math is correct watt/€ is higher but maybe i'm wrong)
2. If I install them on my roof, there’s a risk of leaks while I’m away in the summer. If even a small plumbing issue happens, I might not notice until it’s too late.

If you want hot water from the sun, then solar water heaters are the way to go concerning efficiency. The units do take up roof space however that could be used for PV in the future. PV allows for flexibility on how to use the electric power generated while a dedicated hot water solar collector is a one purpose use of the space.

Why I’m Not Connecting This to the Grid to save on electricity
Let me give you a quick tour of Greek bureaucracy:

• No Feed-in Tariff – Individuals can’t sell excess power back to the grid anymore.
• Net Metering Was Just Abolished – I can no longer offset my electricity bill with solar production.
• Net Billing (the new system) – I can only use the power in real time (while I’m at work). Any extra power—which would be most of it—gets dumped into the grid for the market price of just €0.01–€0.02 per kWh, while I pay €0.09 per kWh to buy electricity.

If panels and grid tie inverters are cheap enough, it can still pay. At $10/panel, it might work out. However you will need a mounting system (which costs money) and the grid tie inverter/s.

• Strict Permit Rules – Even a single solar panel connected to my home’s electrical system (even with a battery and no grid export) requires a €450 permit—with no guarantee of approval due to substation capacity limits.

It all depends on usage and the rates whether the cost is effective. In summer, my shop utility bill can be $900 USD/month. My PV array for the shop cost $7K USD. Not hard to do the math if 3 or 4 months of the year the bill would be $900/month. The remaining months can be anywhere from $250 to $400/month.

If I added batteries, I’d need to account for battery costs + 450 for permit + an inverter + (probably they wont approve me)
That would blow my budget, and I’d still have to deal with battery limitations, while excess energy would still be sold for next to nothing.

I'd look into energy efficiency first. Conservation always pays.

EDIT: after a bit of research in here i probably realized i'm probably over simplifying things maybe i should consider converting to ac which then the questions becomes since a heating element is not a sensitive device can i just get something reliable but cheap to convert the dc to ac something like a EASUN 6.2KW Solar Inverter would work its under 300euros

Let me know your questions thought objections

and thank you all in advance for helping me save a buck or two

 

[efficientPV]

It all depends on how many heating elements can be installed to better match the panels. These can be switched in as sun level increases. It can be done efficiently and without batteries with some technical background. You are likely not serious enough at this point to invest time in learning.

 

[Solar Guppy]

DC is hard to switch on and off so that's a hard no with standard HW heater elements, then you have the issue of your solar may do nothing for hours as tank has met it's maximum temperature, so in the end for all the effort and materials, you would have been better off doing some odd-job work and pay the electric company for the heated water.

 

[Achilles]

Greater efficiency of the electric power using a heat pump might be a better idea.

The yield from the panels will be 200W or less, 25 panels = 5000W at peak sun hours, the rest of the time it will be less. These are used panels and possibly might output only 150W or less. 5 Kw equals about 15K btu's (commonly used in the US) but if you understand either Kw or btu's you get the picture. This is why a heat pump is used to gain efficiency, with a COP of 3 or more, the yield is multiplied by 3 so the 5 Kw is now 15 Kw of heat energy.



Don't preheat water that is on the inlet side of a condensing boiler. The lower the temp of incoming water, the greater the condensing efficiency.

A buffer tank should be installed on the outlet of the boiler which if done correctly makes for a low loss header. Any condensing boiler should have a low loss header which is a primary/secondary loop.



High temp emitters require higher boiler temps which leads to higher boiler inlet temps reducing condensing efficiency.

Low temp emitters such as radiant floor heat allow for greater condensing efficiency. If you are currently running something like old cast iron radiators or other high temp emitters, you might be better off first switching to a low temp emitter to increase condensing efficiency of the boiler. Conservation of energy usage always pays even if installing a solar system as the system won't need to be as large. Look for inefficiency first and ways to improve conservation.



If you want hot water from the sun, then solar water heaters are the way to go concerning efficiency. The units do take up roof space however that could be used for PV in the future. PV allows for flexibility on how to use the electric power generated while a dedicated hot water solar collector is a one purpose use of the space.



If panels and grid tie inverters are cheap enough, it can still pay. At $10/panel, it might work out. However you will need a mounting system (which costs money) and the grid tie inverter/s.




It all depends on usage and the rates whether the cost is effective. In summer, my shop utility bill can be $900 USD/month. My PV array for the shop cost $7K USD. Not hard to do the math if 3 or 4 months of the year the bill would be $900/month. The remaining months can be anywhere from $250 to $400/month.



I'd look into energy efficiency first. Conservation always pays.

Thank you for the detailed response.

The house is old, built with bricks and concrete, and currently has no insulation. I'm using a 25kW diesel boiler.

Since these expenses are part of a government-subsidized program, my options are somewhat limited. I'll check if I can opt for a heat pump instead, though I assume I would need at least a 14kW unit.

Replacing the radiators would be a significant expense. However, with 10cm XPS insulation around the house and new double-glazed PVC windows, I might be able to keep the existing radiators while running them at 50-55°C due to reduced heat loss. most of the time exept heavy winter There are probably more radiators than necessary for the house, which could help take advantage of the condensing efficiency of the gas boiler.

 

[Achilles]

It all depends on how many heating elements can be installed to better match the panels. These can be switched in as sun level increases. It can be done efficiently and without batteries with some technical background. You are likely not serious enough at this point to invest time in learning.

Thanks for the reply

The whole point of this was to learn while saving a buck or two and, of course, not spending too much on the entire project.

While reading about impedance matching is something i can get behind (computer engineering background) , having to modify or create a custom buffer, figure out plumbing, drill holes, and ensure everything is properly set up on the water side is not something I'm very fond of.

probably a project of this scale if out of the water but I'm might get 3-4 panels at this price to see if i can power directly with dc a heating element to heat maybe a room in the house as a way to learn

 

[Achilles]

DC is hard to switch on and off so that's a hard no with standard HW heater elements, then you have the issue of your solar may do nothing for hours as tank has met it's maximum temperature, so in the end for all the effort and materials, you would have been better off doing some odd-job work and pay the electric company for the heated water.

The whole point was that the tank would never be able to overheat because it would be very large, as a fellow member pointed out. However, the condensing gas boiler I’m considering will not be efficient if it receives warm water at the intake.

I agree with you. Don’t blame me I got really excited when I saw the prices of used solar panels and thought I had the space. But then I realized setting up a normal grid-tie system would be bureaucratic hell just the application would cost 40-50% of the entire system cost

 

[shadowmaker]

Air to water or ground source heat pumps are not wise with conventional radiators. They lose efficiency the higher circulating temperature goes. Four leaf radiators can reduce circulating temp significantly but are expensive. Maybe installing few small computer fans to your existing radiators could help and keep costs low?

For your climate common minisplit could be wise heating (and cooling) system. Just install plenty of those cheap solar panels with grid tied inverter or even do it right and get a hybrid inverter (48V) with some lifepo batteries. This way you could reduce your heating bill in winter and get cooling totally free in summer.

EDIT. You can keep this completely separate from your electrical system if you want.

 

[BarrieM]

@shadowmaker is right. At 0.09c per kWh to buy from the grid it's a no brainier to install mini splits. Adding solar panels and battery would be an added bonus. Mine are remote controlled so I can check house temperature from anywhere and adjust accordingly.

 

[Geocom]

The whole point was that the tank would never be able to overheat because it would be very large, as a fellow member pointed out. However, the condensing gas boiler I’m considering will not be efficient if it receives warm water at the intake.

I agree with you. Don’t blame me I got really excited when I saw the prices of used solar panels and thought I had the space. But then I realized setting up a normal grid-tie system would be bureaucratic hell just the application would cost 40-50% of the entire system cost

Sorry for my English, but in Bulgaria they sell two types of controllers specifically for heating water from photovoltaic panels.

Photovoltaic water heater controller

German Votronic SR Duo solar controllers, for battery charging through a photovoltaic panel. With output for a second battery.

solarhouse.bg

Voltic - Καινοτόμο σύστημα θέρμανσης νερού από τον ήλιο - Voltic

Καινοτόμο σύστημα θέρμανσης νερού από τον ήλιο Εξοικονομήστε έως και 70% απο τον λογαριασμό ζεστού νερού χρησιμοποιώντας καθαρή ηλιακή ενέργεια VOLTIC σε δράση Ο ειδικός ελεγκτής VOLTIC επιτρέπει τη σύνδεση φωτοβολταϊκών πάνελ απευθείας σε θερμάστρα χωρίς αλλαγή του θερμοσίφωνα. Φωτοβολταϊκά...

volticbg.com