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From a gas furnace to a heat pump powered by solar panels

RBEL

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Dec 12, 2022
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Hey guys!
I am thinking about going from a gas furnace to electric heating by installing an electric heat pump and a few solar panels.
I am merely interested in your opinions whether my calculations are correct.
This is what I am thinking:

1) Yearly gas use: 1200 m3
2) Yearly gas use converted to electricity: 1200 m3 * 10.55 = 12660 kWh
3) Heat pump must deliver: (yearly electricity use) / COP = 12660 kWh / 4 = 3165 kWh
4) Let's say I have 400Wp panels, so I'd need 8 panels to satisfy the demand for the heat pump.

What do you guys think?
 
Need to factor in seasonality-- less sun in winter when you need the heat.

Recommend going to pvwatts.nrel.gov and putting in your system data and seeing how many kWh you generate in the heating season.
 
Need to factor in seasonality-- less sun in winter when you need the heat.

Recommend going to pvwatts.nrel.gov and putting in your system data and seeing how many kWh you generate in the heating season.
So for me it will be roughly the 3165 kWh * 1.2 (for the inefficiencies / seasonality) = 3798 kWh pump needed.
Thanks!
 
From a mathematical exercise to get to "net zero" you appear to be correct although I'd probably use a COP of 3 for my initial calculations but that's just me.

If you're talking off grid then your calculations need to focus on the coldest days of the year when your heat pump's COP could get pushed to below 2 and then finally to 1 when the electric resistance has to kick in and take over.

EG: 10 kWH/day average to run the heat pump could turn into >30kWh/day on the coldest days of the winter and you have almost zero solar production or it might be sunny but your array is covered in snow. Many of us struggle for weeks on end in the winter.

You may not live in area where it gets that cold but you likely live in area where its cold and dreary for several days in a row so you'd better have something to back that heat pump up.
 
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So for me it will be roughly the 3165 kWh * 1.2 (for the inefficiencies / seasonality) = 3798 kWh pump needed.
Thanks!
I think that adding twenty percent is way too low to account for the seasonality.

Here, in upper Michigan, I do very little heating in May,June,July, and August, yet almost half my annual potential power production occurs then. It’s of no value for heating.

Do what shimmy said- go to pvWatts and look at its estimate of your production for the winter months.
 
So for me it will be roughly the 3165 kWh * 1.2 (for the inefficiencies / seasonality) = 3798 kWh pump needed.
Thanks!
Dense cloud cover kills solar... I went 11 days straight in December and my 5 kW array averaged 2 kW per day. BTW my inverters consume 3 kWs per day just at idle.
 
If would be helpful if you indicated where you were located. Cold Climate minisplits do not have a great reputation in climates that on occasion drop below zero or lower. Yes some are rated to put out nameplate output to -12 F but the heat produced is poor "quality" and there is actually heat removal from the space during defrost cycles that can occur quite often during damp but cold weather conditions. With respect to the "quality" of heat, a BTU is BTU on paper but most people will tend to increase the room temperature setting to compensate for the high air flow and drafts required to delivery the nameplate BTU. In many cases people end up buying a backup. The other issue is COP, as the temps drop the COP plumets down to near 1.0. It is a major surprise to someone when they first get their cold weather power bill. A manual J calculation doesnt care where the heat comes from and in many cases heat pumps are just plain undersized for the maximum heat demand. They are great rigs, I have been heating my house from net metered surplus power for a month or so in NH and use it anytime the night time temps are above 30 F. At a minimum, the systems work best when set at one temp and left there. They can take a long time to reheat a space so any savings from temp setback is lost.
 
I am currently located in Europe - the Netherlands to be exact.
 
You could look at air to water heat pumps. Install a large buffer tank and use it as an additional battery to carry the load during non production hours.

Additionally Europe has some really great solar thermal products that integrate into your roof directly. Combination systems are possible with indirect buffer tanks.
 
I am currently located in Europe - the Netherlands to be exact.
dan kan ik je wel van een praktijk voorbeeld voorzien :

3 kw warmtepomp.. afgelopen februari maand verbruik 1132 Kwh, opbrengst zonnenpanelen ( 31x545w panelen), 70 kw
je zult dus echt naar je jaarverbruik moeten kijken, en in de zomer genoeg salderen om nov-dec-jan-feb goed te maken


<English)

i'll give you a practical example :

3kw heatpump.. usage last feb 1132 kwh , panel delivery 70 kw
look at your annual usage, make sure you have enough panels to make up nov-dec-jan-feb by net metering
 
dan kan ik je wel van een praktijk voorbeeld voorzien :

3 kw warmtepomp.. afgelopen februari maand verbruik 1132 Kwh, opbrengst zonnenpanelen ( 31x545w panelen), 70 kw
je zult dus echt naar je jaarverbruik moeten kijken, en in de zomer genoeg salderen om nov-dec-jan-feb goed te maken


<English)

i'll give you a practical example :

3kw heatpump.. usage last feb 1132 kwh , panel delivery 70 kw
look at your annual usage, make sure you have enough panels to make up nov-dec-jan-feb by net metering
Thanks!
Let's keep this in English, so that everyone can follow along.
I am merely asking: how many solar panels do I need to cover the demand made by the heat pump.
I understand that in the cold months it is not easily done, but perhaps some solar batteries could help?
 
Thanks!
Let's keep this in English, so that everyone can follow along.
I am merely asking: how many solar panels do I need to cover the demand made by the heat pump.
I understand that in the cold months it is not easily done, but perhaps some solar batteries could help?
currently have 900+ ah, but, if you only make 2 KW a day, and use 30+ that will be gone soon..
and as you know, the netherlands can have crapping gray days for weeks at a time ( including this spring for most part :-( )
 
For Amsterdam you would only average 0.69kWh/kW x kW PV.

For 12,660kWh / 4 heating months / 31 days = 102kWh/day. To be able to handle an average December day you would need 150kWh or (375) 400W panels.
 
For Amsterdam you would only average 0.69kWh/kW x kW PV.

For 12,660kWh / 4 heating months / 31 days = 102kWh/day. To be able to handle an average December day you would need 150kWh or (375) 400W panels.
You're not factoring in the heat pump cop which will average 3:1 atleast. So 12660kwh turns into ~34kwh/day, which isn't impossible.
 
Maybe a graph will help, maybe it won't?

Below is the production from my grid tied array from the most recent January. It's around 16kW and not that steep but true south with no shading. You'll notice that from the 18th on things were pretty shaky. Keep in mind I've also got water pumping, a LOT of refrigeration (my wife is a prepper and I love her), lighting, etc and a hot tub (don't judge) to keep going. Anyway... my attempt at a point is that there wasn't anything left over for the heat pump for somewhere around 8 out of 10 days. l was relying on the wood boiler for all of our space heating during that time.

View attachment 145574

Here's my usage. Sorry my REC's usage dashboard is not cooperating. Hopefully I'll get it later.
 
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