Kanab House
Info Blog

Fridge, Water Heater, and Solar Panels

Yesterday, we went shopping for appliances. Found some we liked, all with good Energy Star ratings, so we thought they would be fine. Today, I looked at the energy use of the fridge, and now everything is in question, as we re-evaluate water heater and solar panels.

Let me back up a bit.

Solar panels

Solar panels are great, and we had planned to add them. However, the electricity in Kanab is too cheap, and the cost of solar is still dropping, so when we ran the numbers, it makes more sense to add those in a few years.

Here are the numbers:

  • To install enough solar panels to produce all the energy we need: about $10k. (We don’t have a quote yet, but close enough.)
  • Cost of electricity in Kanab:
    • No solar: $30/month + ¢7/kWh
    • Solar: $40/month + ¢7/kWh, or we get ¢3/kWh back if we produce extra
  • Electricity use is already low, so we could save at most about $500/year.

So the savings might barely cover the increased mortgage. Good for the planet, not a great return on investment. Since we’re already paying a lot to make the house very energy efficient, we decided solar panels could wait a few years.

Batteries

Batteries make the house (and the whole grid) more resilient, but since the energy price doesn’t vary with time of day, they don’t reduce our energy cost. Battery prices are coming down, so again, something to add in a few years.

Electricity use

One of the requirements for PHI certification is that the Renewable Primary Energy Demand (PER) must be at most 60 kWh/m2a. (Never mind how PER is calculated, I’ll go into details of the PHPP spreadsheet in a future post.) Since our house packs of lot of functionality into a small space, we were already close to the limit on that metric.

Looking at rough numbers, the projected electricity usage adds up to 7700 kWh/year, distributed at follows:

  • Heating: 25%
  • Cooling: 10%
  • Ventilation: 5%
  • Residential (fridge, cooking, dishwasher, lights, etc.): 30%
  • Domestic hot water: 30%

With heating and cooling already optimized due to the thermal envelope, hot showers are responsible for 30% of the energy budget!

Hot water

Instead of heating the hot water directly, it’s much more efficient to use a heat pump. Hybrid water heaters like the Rheem ProTerra work that way, at a cost of $2k-$3k instead of less than $1k for a good electric water heater.

Replacing a regular electric water heater by a heat pump water heater sounds like a great idea, but it’s not a straightforward win:

  • They work well when they can suck heat out of room-temperature air. They don’t work as well in a possibly cold or hot garage.
  • They need to draw the heat from a sufficient volume of air. Our planned utility room is too small, so we would need vents in the door.
  • They make noise – the vents in the door would carry that noise into the living room.
  • They produce cold air, which is helpful in the summer, but not in the winter, so in the winter you’d switch off the heat pump mode, and you only gain half the benefit.
  • So maybe put the water heater in the garage, even though it won’t work as well, but then it’s further away from the showers, and people will waste water waiting for the shower to get hot. (There are re-circulating loops to avoid this, but those waste energy, and are not a good idea in a passive house.)

However, there’s a much better solution: the Sanden SanCo2 water heater. You can put the heat pump in the garage (or even outdoors), and keep the water tank indoors where it can be close to the showers. Their heat pump works efficiently at a wide range of temperatures, and cuts the energy needed to heat water by about a factor of 5. The SanCo2 also produces significantly more hot water, great if multiple people take showers at the same time.

The only problem: it costs about $6k. It would save us about $150 per year. Makes great sense for the planet, does not make sense financially.

Solar & batteries

Looking at the bigger picture, the SanCo2 significantly reduces electricity demand for the whole house, by at least 20%. So if we’re planning to spend $20k on solar and battery, we can downsize that whole installation by 20%, and save $4k. Then the SanCo2 is making more sense: the more you spend, the more you save.

Building a house is expensive enough, and it’s always easy to spend more. To keep costs under control, we had decided to use a regular water heater, and install solar when it becomes cheaper.

The fridge

So far, so good. Now, the fridge. The PHPP spreadsheet that we had been working with had the energy use of the fridge at 230 kWh/year. The fridge we looked at is estimated to use 630 kWh/year, and that moved our PER value above 60 kWh/m2a. We can choose a somewhat smaller fridge using 570 kWh/year, but that’s not enough. To get to a much lower number, we’d have to drastically reduce the size of the fridge and sacrifice the ice maker, not tradeoffs we’d be happy about.

So that’s where we’re at. At this point, we have a number of options:

  • Give up on PHI certification: would be a pity, since we’re doing well in other criteria, and are so close on the electricity use.
  • Add solar panels: PHI accepts houses with PER slightly over 60 kWh/m2a if they generate enough of their own electricity. Invest $10k, gain $500/year.
  • Use a SanCo2 water heater to drastically reduce our electricity use; the fridge will no longer even get us close to the limit. Invest $6k, gain $150/year, plus future savings when installing solar and battery.
  • Find some other way to reduce electricity usage. But there’s not much fluff, so not clear where to gain enough.

Stay tuned to figure out which path we’ll take. Any advice, please let me know.