For those of us that are considering solar panels for a potential Net-Zero home, I have a question regarding usage/utility conversion. For a dual fuel home, (gas and electric for me) I believe in generating enough electricity to make up for the gas usage as well in order for a home to be true Net-Zero.

That being said, I looked up the conversion factor for therms to kWh from the all knowing internet and found 1 therm = 29.3 kWh. I've been tracking my electric and gas usage. For a particular month, I used 578 kWh and 56.65 therms. So this means I used 2238 kWh total. (I can now see why people go gas)

Now for the questions:

1. Is this the correct method in calculating the amount of electricity I would need to generate to make my home true Net-Zero?

2. Let's say I were on electric heat for the house, does this also mean for that same month I would use simply an extra 1660 kWh for that electric heat? Or is it not that simple?

I would recommend looking at sites such as:
http://www.energystar.gov/buildings/facility-owners-and-managers/existing-buildings/use-portfolio-manager/understand-metrics/difference
and
http://www.buildingscience.com/documents/digests/bsd151-understanding-primary-source-site-energy

These sites discuss the difference between site energy, the energy used at the building itself, versus source energy that includes all the energy from the wellhead to the building as well as what is used in the building. The biggest difference is for electricity, which according to EPA requires 3.34 or 3.365 (depending on reference) times energy used in the building when accounting is taken for the efficiency of generation, and line losses. So when you generate your own electricity with PV panels, you save more than three times that amount of energy when converted back to source energy, and likewise, you offset your natural gas by a similar factor. (Natural gas has its own site-to-source ratio, but it is only 1.047 or 1.092, depending on reference used.) The EPA recommends using source energy, and provides all the factors to convert between site and source (see second reference above).

I don't know that the term "true net-zero" is well defined, but net-zero source energy and net-zero site energy are well defined. I would stick to those terms.

I would stay below the point where the utility won't offset your cost. Ie, avoid any chance of hitting the "no carry over to the next fiscal year" issue that some net metering plans have.

More generally, I'd look at all the options, not just home design, in terms of what provides the best ROI for the environment, energy depletion, etc. Eg, at some point you are better off spending less on the house and buying a natural gas or electric car.

OK, I know I'm not answering your question - but I hate to see well intentioned dollars sub optimally spent achieving some term like "true Net-Zero".

So to be sure I understand the article well enough and your post, Lee, if I generate 1 kWh on the site that's actually worth 3.365 kWh from the source of the electric provider?

I didn't mean to get into confusing terms of "true net-zero" I am just trying to find a way to make up for gas consumption as well. (If gas were to be used of course.) Any ideas in that respect?

Thanks

HVAC-Engineer-

So let's take your case of 578 kWh of electricity per month and 56.65 therms (1660 kWh using the 29.3 conversion factor) of natural gas per month. So in terms of source energy, use the factors of 3.365 and 1.092 for electricity and gas respectively, and you used 1945 kWh of energy for electricity, and 1813 kWh of energy for natural gas, for a total of 3758 kWh of source energy. For that particular month, if you generated 1117 kWh of electricity using solar PV, you would directly offset you 578 kWh of electricity used at your house, and have an excess of 539 kWh. Count this as a use at your house of -539 kWh, use the same equations described above, and you would get a net source energy of -1 kWh, so essentially net-zero source energy.

However, you need to do this analysis over a year rather than over a month since usage likely changes dramatically month-to-month. So, you should set up a spreadsheet with the equations above to compute the cumulative source energy over a one year period, and use that to size your solar PV system. That is what is shown at: http://www.residentialenergylaboratory.com/net_energy_use.html.

Once you get the kWh of generation needed, you can use PVWatts (free software from the National Renewable Energy Laboratory) to determine the size of the PV system needed to generate that amount of power annually. You can see a comparison of PVWatts predictions and measured energy output at: http://www.residentialenergylaboratory.com/rel_energy_use_pv.html and at:
http://www.residentialenergylaboratory.com/comparison_of_pv_systems.html.

Wiithout knowing the local grid sources you have ABSOLUTELY NO IDEA what the proper multiplier is for source-fuel BTUs.

In the BPA area of the Pacific Northwest the vast majority of all kwh going onto the grid is from hydro power, some nukes some gas peakers and combined cycle gas, but very little of that sub-30% thermal coal.

In the ISO -New England regional grid roughly half of all kwh going onto the grid is from combined-cycle gas running better than 50% efficiency, with 15-20% slice from nukes, a growing slice from wind and PV solar, with maybe 20% from 30% gas & oil peakers.

No WAY is either of those broad scenarios rightly modeled as a 3.4ish source-fuel multiplier!

And the very local grids often have a different mix.

The crudely granulated state kwh pie-charts by fuel can be found here:

http://www1.eere.energy.gov/vehiclesandfuels/facts/2012_fotw753.html

But they don't break it down by combined cycle (50%+ efficiency) vs peaker or thermal-gas (30-35% efficiency)

Bottom line, you have to get local (VERY LOCAL) to get to something that looks like "true" source fuel BTUs of electricity. (If you really want to refine it, time of day of power use counts too.)

Got a zip code, and the name of your electric utility?

Dana1-

It is important to grasp the concept of source energy versus site energy first. That is what the OP was asking. Then we can argue about the details of what site-to-source factors to use.

Since electrical grids are connected over large areas, many people would argue that you do not need to get very local to determine site-to-source conversion factors. It is hard to identity if the electrons coming to your house are coming from a coal plant or a wind generator.

Xcel Energy sent a flyer along with my bill this week that states that 57.7% of grid power here comes from coal, and I can only guess that those coal plants have a thermal efficiency around 30% to 35%. Line losses might be 10% or so. My zip code is 81201, so if you have a better site-to-source ratio, I would be interested in hearing about it.

Dana1-

According to the link that you provided, http://www1.eere.energy.gov/vehiclesandfuels/facts/2012_fotw753.html, the coal fraction in Colorado is 68.1%, so as you get more local, it looks like the site-to-source factor gets worse here. However, since it is an interconnected grid, I don't know if that is a valid approach.

Dana,

Our zip is 73064, electric service is provided by OG&E, and natural gas is ONG (Oklahoma Natural Gas)

Some of the US grids are inter connected, others less so. Texas has only tenuous links to other grids. But all grid operators measure where their power is coming from and going to. A powerplant operator in Colorado has literally no way of selling power to a willing customer in Maine without paying multiple transmission system operators for access to the transmission line capacity. There simply isn't enough transmission capacity to power up (relatively small) Maine from western power sources- the links are way too tenuous. Colorado is on the western grid, Maine is on the NPCC grid, which has reasonable tie-ins with the grid systems adjacent to it, but the eastern grid have little to no interconnect with Texas or the western grid, and only limited interconnection between the sub-regions.

See:

http://theenergyfix.com/wp-content/uploads/2011/11/NERC_Interconnections_color-map1.jpg

Most power is used 4 transformers or fewer away from the generating source. Some local municipal operators in New England buy wholesale only via ISO New England. Other utilities have both their own generation sources or power-purchase agreements with nearby (and only sometimes more remote) powerplants. The regional grid operators are responsible for making sure loads are matched with sources, but they DO know where those sources & loads are, and the kwh moved over third party transmission sources is measured, and the transmission service is paid for.

Bottom line, the regional mix IS a reasonable model for modeling the source fuel BTUs for most situations, but not where the local utility is practically islanded, and only sips from the regional grid when their own generating resources are not sufficient for meeting their customers' load. The model DOES vary at the extremely local grid level at times. But just because Colorado's grid resources are connected to the WECC doesn't mean that power is ever moving from CO to WA. Almost all power used in WA is from local resources. But there are times when both CO & WA export power to CA when CA hits critical peak air conditioning loads (the extreme exception, not the rule.)

Posted By HVAC-Engineer on 09 Oct 2013 05:16 PM
Dana,

Our zip is 73064, electric service is provided by OG&E, and natural gas is ONG (Oklahoma Natural Gas)

As of 2010 the Oklahoma grid source were about 44% thermal coal, 47% natural gas, and the rest a mix of renewables, mostly wind.  Since that time I suspect some or maybe even most of that coal fired base-load generation has been displaced by combined-cycle gas and wind, just as it has in neighboring Texas.  (In Texas the net total fossil fired base generation has shrunk even in the face of increasing base load, due to the impact of massive wind deployment. But the per kwh cost of combined cycle gas is shuttering coal plants where ever there is adequate gas-grid resource to do it, and  both OK & TX are fairly well hooked up from large-scale gas-grid point of view.) Wind power accounted for about 7% of all kwh moved on the OK grid as of 2011, and it's probably nearly doubled since then.

OG & E has been doing quite a bit with smart-grid resources to reduce the amount of low efficiency peaker plant operation via demand  response (paying customers to shed load during peak periods), which increases the average local-grid efficiency too.

So, its a moving target and a fast moving target at that.  In 2010 a multiplier of 3 might have been a reasonable WAG for Oklahoma City but I doubt it's over 2.5 today, and in 10 years I expect it may be down to the 1.5x range with the massive deployment of cheap distributed photovoltaics and the continuing build-out of cheaper-than-natural-gas wind.  The deployment of PV is doubling roughly every 2.5 years, so even if PV is only 1% of your local grid total today, it's likely to be into double-digits by 2023, cutting ever deeper into low-efficiency fossil fired peaker use.  By then I'd expect wind to be in the 25-35% of all grid power in OK, given the amount of the local resource.  (It's already hit that range in IA & SD.)  So if by 2023 the local grid source is 30% wind, 15% PV, and 45% combined cycle gas with only 10% from low efficiency peaker, your multiplier would be well under 2, maybe under 1.5.

Perhaps I don't understand exactly what OP is asking. He wants to know how much electricity to generate to offset gas usage. He is in a moderate location, OK, as I read it. His gas usage is likely for heating and domestic water. Both needs are best met with heat pump technology, with a COP over 1. I won't even try to guess what the overall COP is, but to provide same results with electricity, he's not talking 1600 KWH, but probably closer to 600-700. At that point you can start the above discussion of where the electricity is generated and grid efficiency.

Posted By McFish on 11 Oct 2013 01:34 AM
Perhaps I don't understand exactly what OP is asking. He wants to know how much electricity to generate to offset gas usage. He is in a moderate location, OK, as I read it. His gas usage is likely for heating and domestic water. Both needs are best met with heat pump technology, with a COP over 1. I won't even try to guess what the overall COP is, but to provide same results with electricity, he's not talking 1600 KWH, but probably closer to 600-700. At that point you can start the above discussion of where the electricity is generated and grid efficiency.

He had two questions, or really three:

"Now for the questions:

1. Is this the correct method in calculating the amount of electricity I would need to generate to make my home true Net-Zero?

2. Let's say I were on electric heat for the house, does this also mean for that same month I would use simply an extra 1660 kWh for that electric heat? Or is it not that simple?"

Much of the discussion has been taking a stab at the not-so-simple estimation of the  source-fuel energy of heating with electricity, which is highly variable.

Clearly heat pumps can leverage a lot for space & water heating in an Oklahoma climate.  In that climate ductless technology COP annual averages in excess of 3.5 are likely, or about 12,000 BTU/kwh.  A heat pump water heater would have an EF of about 2.2, meaning it delivers ~ 7500BTU/kwh during the cooling season (and peeling a bit off the AC load), but during the heating season about 4000 BTU of that are from the heating system, which takes another (4000 / 12000=) 0.33 kwh, thus during the heating season if heating with pretty-good ductless the heat pump hot water heater is delivering only (7500 / 1.33=) ~5600 BTU/kwh, a net-EF of about 1.6 with the imposed heat loading factored in.

Assuming it was a minimal space heating month (probably a mix  of heating & cooling though, eh?) and the ~57 therms that month was burned at an average of 80% combustion efficiency the total heat used was about (0.8 x 57 x 100,000=) 4,560,000 BTU.  If 2/3 of the that was hot water, and the other 1.3 was space heating the kwh use would be:

Space heating:  (4,560,000 / 3 ) / 12,000= 127 kwh

Hot water:( 2 x 4,560,000 / 3 ) / 5600= 542 kwh

Total: 669 kwh

(Which is a heluva lot less than 1660, eh? )

The opening statement for the OP was, "For those of us that are considering solar panels for a potential Net-Zero home, I have a question regarding usage/utility conversion. For a dual fuel home, (gas and electric for me) I believe in generating enough electricity to make up for the gas usage as well in order for a home to be true Net-Zero."

He already had records of his usage in kWh for electricity and therms fo natural gas, and the conversion factor between them. He used that to calculate the electriical energy required from PV panels to meet net-zero site energy, and asked, "is that all that is required?" Indeed that is all that is required for net-zero site energy. However, most people in the field prefer net-zero source energy rather than net-zero site energy ("Numerous building programs, like Building America, EPA Energy Star, Architecture 2030, and the German Passiv Haus, all use source energy metrics." from Building Science Corp.'s http://www.buildingscience.com/documents/digests/bsd151-understanding-primary-source-site-energy). Therefore, I gave him the references that explain the differences between site and source energy, as well as the procedures to calculate it.

You guys have answered the question that he did NOT ask, which was, "If I throw out my gas appliances and switch over to electrical appliances, then how could I do that efficiently?" His question about heating electrically appeared to me to be again asking about relating the different forms of energy, not how could it be done efficiently.

I think both Lee and Dana answered my questions. Lee answered #1 in that I would have to generate that much electricity to make up for the gas usage to become a net-zero site. (BTW, my apologies for not being aware of site and source energy and its differences. I seemed to have caused a stir.) However, Dana answered #2 (I believe) in that if my heat were by electric means then I would not necessarily use the full 1660 kWh, depending on efficiency of course. (I actually should have known that myself, my mistake.)

Thanks to you all for educating me about source vs. site energy. I hope I've made myself a little more clear.

Posted By Lee Dodge on 11 Oct 2013 11:30 AM


You guys have answered the question that he did NOT ask, which was, "If I throw out my gas appliances and switch over to electrical appliances, then how could I do that efficiently?" His question about heating electrically appeared to me to be again asking about relating the different forms of energy, not how could it be done efficiently.

I dunno, my English is my primary language, but I can't figure out another way to parse  "Let's say I were on electric heat for the house..."  other than to suggest exactly a question about what it would mean if, say he were to  "... switch over to electrical appliances...".  Multiple part questions deserve more than the narrowest of interpretations.

Even assuming it was a "what if" question, it has a complex set of answers, and there's no reason to avoid the  "Or is it not that simple?" part, even if the efficiency question wasn't asked directly.  There are several ways to be  "...on electric heat for the house..."  with dramatically differing numerical answers.

NO energy-equivalences can accurately inferred by the raw kwh/therm equivalence without the efficiency factors (of the grid, of the equipment) etc, and how you approach it depends on exactly which aspects are meaningful in the contexts you care about.  The power grid efficiency is somewhat irrelevant if you're only interested in how much PV it takes to net-meter out both your gas usage, but the gas-grid efficiency IS, and went unaddressed in this thread. Only looking at the energy content of gas that came through the meter doesn't account for the exlporation/extraction/pipelining energy those metered therms represent.

Solar PV seems to be the least expensive method to heat a house providing you have the right house and geographic location.
Problem is that is a long time investment so is like you will pay in advance for the next 20 to 25 years of heating.
I recently moved off-grid and as soon as I can get the money I will use solar PV to heat the house.
I will give you just a few numbers from memory on different fuels that I investigated.
a) 1 liter of gasoline or diesel about 1.2$ with a total energy of about 10kWh/liter will give you a rate of 12 cent/kWh but since most heaters with this fuel are about 85% efficient that rate will be more in the 14 cent range not including the cost amortization of the heater.
b) 1kg of propane stores about 12.9kWh and the price that I pay here for a 20pund standard barbecue propane tank is about 21$ without tax the content of that tanks is 7.5 to 8kg or about 100kWh of thermal energy. Cost will be about 21cent/kWh again not including heater efficiency typical 85 to 95%
c) Solar PV cost now about 1$/Watt since you can find 240 or 250W solar panels at about 250$ At my location somewhere in the middle of Canada I get an average of about 0.12kWh/month from each watt of installed solar PV They should last for at least 25 years and at the end of this period should still have more than 80% of the initial capacity. So 25 years x 12 month x 0.12kWh= 36kWh maybe a bit less during they lifetime that will be less than 3 cent/kWh
There are a few small problem with my calculation on solar PV.
One is that I assume I use all the energy they produce if I don't then is lost and cost/kWh will be higher at 25 years amortization.
There are two aspects of the power not utilized in this case. One is when thermal accumulators are full and the other is that you maybe have no use for this amount of power in the summer but even so if you just use 1/3 of the available power is still 9 cent/kWh probably the least expensive source of heat and by far the most convenient and clean.
Second is that you need some form of storage for at least 24h with some backup or larger storage with no backup. As an example my small 65sqm (700sqft) house has an insulated concrete floor slab of 14 cubic meters that can store about 10kWh for each degree Celsius of heat. I can probably go to a max delta of at least 10 degree Celsius so at least 100kWh storage capacity that will last me for at least 3 days of really cold weather since the estimated monthly heat requirement in the coldest month is under 1000kWh.
I will be curios what is the real cost for heating with natural gas? Is not an option for me but just curios.
The pellets are also expensive I do not remember the numbers exactly but they are.
Also to answerer more exactly to the op.

1.Yes close enough.
2.Not exactly that simple the current gas furnace has a certain efficiency so you probably need 15 to 25% less electricity but there is also the problem of storage since the sun is not 24h/day available and some times is cloudy so almost no power.

> estimated monthly heat requirement in the coldest month is under 1000kWh

That would last me about 4 days of cold weather, not a month.

If I were generating heat from PV, I'd use a heat pump and then store heated water in a tank. Batteries are not cost effective.

> estimated monthly heat requirement in the coldest month is under 1000kWh

That would last me about 4 days of cold weather, not a month.

If I were generating heat from PV, I'd use a heat pump and then store heated water in a tank.

That must be a huge house and, or low level of insulation but no matter if 1000kWh are good for 1 month or 4 days of heating you still need to heat the house and PV in the long term will probably be the least expensive option.
What sort of fuel do you use currently to heat the house and what is the cost of that fuel in your location?

1000kWh are good for 1 month

If this were true, the biggest monthly bill for electric heat would be $90-$160 and you could use a hair dryer for heat. Possible, but not likely in central Canada.

I use natural gas and it's cheaper than PV + heat pump.