Negawatts for EV's

Apr 07, 2019

How the potential savings from a more efficient heat pump water heater could power an electric car for several thousand miles.

We have been studying water heaters recently so we can be prepared for the time when our current propane water heater needs replacing. We want to switch over to an electric water heater at that time, so we can reduce our carbon footprint, because we have grid tie solar panels. While this is doable, we will have to hire an electrician to wire up a 240 volt supply line in our water heater closet. It pained me however to learn that an electric water heater would cost us about an extra $120 a year or $10 a month to operate over propane.

looking around You Tube for videos on water heater choices I ran across the Heat Pump Water Heater which I had not been that aware of before. Heat Pumps grab heat out of the air and pump it to a new location which is to water in a tank in this instance. The output from this process is refrigerated air. Heat pump water heaters like this are amazingly efficient and can save you up to 3.5 to 1 as compared to a standard resistance electric water heater.

Note here, the Heat Pump Water Heater only requires 240 volts for the backup heating elements which you can turn off. A heat pump water heater has slower recovery times, which is not a real concern given our use patterns, so we could leave the backup elements on if we liked. The fact that this unit is a 50 gallon model will help with this recovery issue.

Our current propane water heater has a 40 gallon tank and uses approximately 100 gallons of propane a year costing us around $243. An electric resistance style tank water heater could as much as $322 a year, which would increase our monthly cash flow out for energy by six and one half dollars. While this extra energy consumption isn’t bad, it could use up our valuable Photovoltaic Panel Array electrical production which we would like to keep for other uses.

In about 20 months we will be switching from a Plug in Hybrid Electric Vehicle (PHEV) to a Battery Electric Vehicle (BEV) and we would like to conserve as much electrical production from our solar panels as we can to charge our new electric vehicle. A new water heater that uses 2,666 kWh’s a year is far from ideal in this instance, even if it were reducing our carbon footprint.

I was excited to learn about Heat Pump Water Heaters as the potential is there to provide our water heating needs for as little as $92 a year using a 50 gallon model. They do cost more to purchase up front, about $800 more. From day one we would have more spare solar power to recharge our electric car, so it would be really good investment for us. I ran the numbers so I will present them here in a truncated form.

We shall call the Propane model a PWH, while a Standard Electric model shall be labeled as a SEWH. The Heat Pump model we shall label as an HPWH. The cost to install a 240 volt power supply for either of the new water heaters we shall assume to be $300. All my calculations are based on our use patterns and not the energy sticker that comes with the units. I just used an 80% factor to adjust these energy sticker numbers.

Cost to buy a PWH $400 plus a $100 installation cost, a SEWH is $500 plus a $300 installation cost, and a HPWH is $1,300 plus a $300 installation cost. The cost total is for a PWH $500, a EWH $800, and a HPWH $1,600. As you can see we would be starting out $800 in the hole over my second new option of a standard electric water heater.

The cost to operate is where the return on investment (ROI) resides. The $320 per year of electrical operating cost (at 12 cents a kWh) for a SEWH adds up to a total cost of $2,088 by year four. The HPWH cost of $92 a year brings its total to $1,968 so just after year three the lines cross.

After six years I will have saved enough to surpass the cost to buy, install, and operate a propane water heater replacement of the unit I have now. The figures for year seven are for a PWH $2,250 versus $2,244 for a HPWH so the ROI is close to seven years in this instance. The continuing choice of a Propane WH doesn’t reduce my carbon footprint however.

Better still the yearly electric consumption of only 766 kWh’s per year saves us a staggering 2,666 kWh’s – 766 kWh’s = 1,900 kWh over the more costly SEWH alternative. The question is just how many miles can you run an electric car like the Tesla Model 3 that we are going to buying in the year 2020? The answer is quite a lot, more than you would think.

A Tesla Model 3 has a rated efficiency of 243 watt hours per mile. With a 90% charging efficiency 1,900 x .9 = 1,710 kWh’s a year is left over for getting the power into the Tesla Model 3 battery pack. Hence 1,710 kWh/.243 kWh = 7,037 miles, while we drive on average 15,000 miles per year in our current PHEV. This amounts to 15,000/7,037 = 46.9%, so let’s say 47% of our driving can be from electrical savings a Heat Pump Water Heater.

This is the very definition of a “Negawatt” (that is electric savings acquired through an applied efficiency that can be used for other purposes). But wait, there is more, we get cold air from the HPWH we can vent into our living spaces in the summer when we are running the central air conditioning. The rest of the year we can just vent the cold air to the crawl space under the insulated floor of our house.

Our current propane fired water heater has not given out yet, but we now have a plan. Most of the time people end up scrambling for a replacement water heater when suddenly they do not have any hot water, because their water heater has stopped working, or it is leaking. This was our situation the last time this happened.

We are looking towards buying a new (or used?) Tesla Model 3 in the late fall of 2020 when we have paid off our Ford F-150 XLT 4x4 truck. We mostly drive our PHEV now, and just drive the truck enough to keep it in good operating shape. We can sell our Ford C-Max Energi on at that time also.

When our current water heater will fail is anyone’s guess. A Tesla Model 3 will eliminate the gasoline we currently use for driving the PHEV roughly half the time. The new water heater when we need to install it will eliminate the propane we currently use to heat water.

We will be reducing our gasoline consumption by 209 gallons a year or 209 x 19.6 = 4,107 pounds of CO2 a year. The propane reduction of 100 gallon per year is equal to roughly 12.6 pounds of CO2 per gallon or 1,260 pounds.

Together they add up to 2.68 tons of CO2. This will be quite a coup, as both of these duties (transportation and water heating) will become solar powered for at least for eight months of the year, without the installation of additional solar panels to our 5,100 watt grid tie system.

We looked into an eight panel addition to Photovoltaic Array about three years ago and it would have cost us $12,500. This proposed addition of an energy saving heat pump water heat looks like our best bang for the buck until such a time as we can afford to expand our solar array. We will want to add more power generating capability to better cover the air conditioning season which is the only time our electric bill goes negative.

In the summer we currently do not receive a stipend from the electric company because our array doesn’t generate enough power to cover the increased consumption at that time. This is why installing Negawatts is such an important concept, and is of primary importance until such a time as we can expand the size of the array.

Having solar power has reduced our carbon footprint for most of the year. This will continue to be true even with the addition of full Battery Electric Vehicle and switching our water heating to electricity to eliminate the current fossil fuel powered model. This scenario as I have outlined it will be a win, win, win in several ways and as Martha Stewart might say (“And that is a good thing”).

Addendum a Heat Pump Water such as I am describing requires 700 cubic feet of operating space so we will have to provide supply are from the crawl space. We also will have to make provisions for redirecting the ducted supply of cold air from the unit away from our living spaces when it isn’t desired. This all is doable in our situation.

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