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:

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

GAS
$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?

Thanks

I am building a new house and your math, which seems fine, is very relevant to what I have been considering.

Isn't the other key variable the actual price of electricity given your PV
plans? For my SoCal location my electricity costs are projected to be
zero so that seems to be make the electric water heater a no brainer.

Plus it invests in equipment where the power source -- PV provided
electricity -- is predictable in terms of supply and cost, while the
gas choice ties me to less predictable source, particularly in terms of
cost.

In your case, in foggy SF, you would need to get
comfortable with your PV projections. One tool that may help is this
calculator at

http://www.sunpowercorp.com/For-Homes/How-To-Buy/Solar-Calculator.aspx

 

If you switch to a time-of-use rate, you can get paid a high rate during some of the time the PV is producing (especially 12-6pm M-F, summer) and pay a low rate off-peak.  Since heat is more needed at night and winter (off-peak) you can effectively reduce the cost per kwh produced.

Also, there is a system that combines PV and thermal (taking heat off the PV modules) for a modest increase in cost - see http://www.pvtsolar.com

It at least doubles the energy captured - but the additional is as thermal energy.  We've installed 3 of these so far, and about to install our 4th.  All well proven components, just used in a novel way.

Being on the west side, you will see 10-15% less energy produced on an annual basis than on the east side.  The calculators probably won't show that difference as they don't have that level of discrimination.  The impact comes in particular during the fog times - June, July, Aug, and sometimes Sep.