Electric vs Gas Heat and PhotoVoltaic Panels

jhwuJune 29, 2008

Hi, I've been doing some thinking about energy production and use for my home in San Francisco - I live on the west side so its fairly foggy and cool.

Anyways, I've determined that I'd like to install radiant heat - hydronic.

The question then is: how do I heat the water? Its a given that I'll put a ~5KW PV system on the roof. For mechanical complexity and simple roof space issues, I cannot put a solar water heater on the roof.

My choices for heating the water for hydronic heat are then:

1) Closed system (doesn't mix with potable water) electric water heater

2) Closed system (doesn't mix with potable water) gas water heater

Conventional wisdom says that gas is cheaper than electricity ... but I was doing some simple math on the latest energy costs (June 2008) and it doesn't seem to be so....

PG&E charges me:


12 cents per KWh in the summer and 7.5 cents per KWh in the winter -> Average is 9.75 cents per KWh


$1.91 per therm

Given that there is 100,000 BTU in a therm of gas and 3413 BTU in a KWh of electricty, I can say that:

1 therm gas = 29.3 KWh electricity

So, to receive 100,000 BTU energry to power my water heater, I would have to buy $1.91 of gas or $2.86 of electricity.

However, we can consider that Gas water heaters are about 82% efficient at converting energy in natural gas to water heat, whereas electric water heaters are about 98% efficient at converting electrical energy to water heat.

Subsequently, we can say that 1 therm gas = 24.5 KWh electricity.... so taking this efficiency into account, to receive 82,000 BTU of energy in the form of heated water(gas water heater being 82% efficient) I would need to spend:

$1.91 of gas or $2.39 of electricity

Electricity is still about 20% more expensive, but that's where PV panels on the roof come in. Has anyone considered this, or are my assumptions way off?


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A couple thoughts:

The best non-solar heat source for a radiant floor is a condensing boiler. These are compact (suitcase size) boilers that are 95% efficient.
Electric water heaters are also 95% efficient, so for your rates, the electricity would be 1.5 times as expensive.
The electric solution will also result in more CO2 emissions (see the tankless heater discussion).

But, the thing I really don't understand is why you can't use solar water heating collectors to heat the water? They are 5 times more efficient than PV and less expensive.

Just comparing the PV panels to the water heating collectors (not the full systems, but just the collectors):

5KW of PV panels will cost about $23000, and take up about 500 sqft of roof.

Solar water heating collectors that provide the same amount of heat will cost about $1500 and take up about 50 sqft of roof (leaving you lots of room for a PV system).

The heat output from either of these is going to be modest, so you might want to have a look at your heat bills, and just see how much of your heat bill either would displace. You do live in a mild climate, but I am guessing you will probably need more collector area to make a big dent in your heat bills.
PVWatts (see link) says a 5KW PV array in SF puts out about 420 KWH per month in mid winter -- equivalent to about 15 therms of gas.

Also, bear in mind that even if you run and efficient household, your household electricity use (lights, ...) are going to use up most or all of that 420 KWH.


Here is a link that might be useful: PVWatts Calculator

    Bookmark   June 30, 2008 at 11:07AM
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Hello solargary,

Thanks for that counterpoint. The reason why I'm not 100% sure about solar water heating to heat the water used for a radiant heat system is because of the weather. (I want to like solar water heating but I'm not sure...)

Generally speaking, the west side of San Francisco needs heat the most in Nov, Dec, Jan, and Feb. During these months, I see the least amount of sunshine (as verified by the PVWatts calculator). Often times, we see very little direct sunshine during these months as it is raining.

During these conditions, I don't think I'd be generating any heat with solar heat collectors (I'm guessing - I have no direct experience with solar heat collectors - I'd love to be proven wrong). Consequently, I might be in a position where I need hot solar water to use in my heating system the most during the time that I don't have it.

However, given that there is *light*, I could certainly be making electricity, hence my query on a electric tankless water heaters.

While the PVwatts link shows that a DC 5KW PV system generates 420 KWh AC electricity a month in mid winter, it also shows the over the course of the year, it could generate about 600 KWh per month. The beauty of a grid-tie system is then (I'm assuming) is that the utility company calculates your balance on a yearly basis - so the energy deficit that I'm have in the winter (since lights, computers, etc would eat up most if not all of that 420 KWh) would be balanced by an energy surplus in the warmer months (where I could be generating around 710 KWh per month)

The issue with gas tankless water heat for the radiant heat for me is venting. I need to figure out a way to vent the water heater - I live in a rowhouse so its not easy to find a hole on the exterior surface. I'd probably have to vent the thing on the roof .... which would make it difficult for me to place the unit 2 or 3 stories below the roof.

An issue with solar water heat for the radiant heat system for me is finding the room to store all that heat - I would be extremely hard pressed to find space for a 200 gallon tank or even a 100 gallon tank for that matter. Actually, I *could* find the space - but that would be 3 stories down from the roof - which would then make it difficult to vent a gas fired heater....

So many constraints....


    Bookmark   June 30, 2008 at 12:29PM
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I'll think about your last message, and post a note if I have any further thoughts, but I wanted to correct an error in my first note:

The solar heating panels that would produce equivalent heat to the 5KW PV I said were 50 sqft and $1500 should have been 100 sqft and $3000 -- solar heating panels are good, but not quite that good :)

Just one quick note on the venting -- the venting on condensing boilers is pretty simple. The vent gas is pretty cool -- cool enough that steam condenses into water. You can sidewall vent them, and I would not be surprised if some of them just use PVC pipe.


    Bookmark   June 30, 2008 at 12:49PM
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