Number crunching

her system quote claims 135% of usage, so that would help with the heat pump electrical usage.
she's on natural gas, which is skyrocketing. so a price comparison would be interesting.
either way, i will advise her to leave the furnace in place if possible.

 

What utility company?

To address this problem, make sure your battery can not be charged from the grid (an option) nor discharge into the grid (an option). In effect, the battery is only connected to the house load and never can load or return power to the grid. This means the battery can never be a load or source to the grid. But this assumes you are not "selling" your excess solar to the grid.

Another work around is to have two EVs. One is used for daylight errands and the other is charged during the sunlight hours. The next day, swap the cars so the fully charged one runs errands and the 'drained' one takes the solar charge.

Another option becoming more available is V-to-G converters. In effect, the EV (the utility doesn't have to know about) buffers the excess solar. However, this has to be 'engineered' just it may defer the forced transformer upgrade.

My Huntsville utility had to be dragged to a permit to operate. So I lost about three months of solar because the "permit wasn't posted!" Part of the TVA system, they are NOT friends to home solar. Regardless, here are my specs:

• 4.8 kW - peak solar production
• 13 kWh battery - solar ONLY!
  • 5 kW max rate
  • no grid charging to battery
  • no grid discharging from battery
  • measured round-trip efficiency 86%
• Significant loads
  • 7.5 kW - Tesla (44 kWh), derated to 3.7 kW but online adjustable.
  • 7.3 kW - BMW i3-REx (28 kWh), derated to 3.4 kW
• Emergency power generator
  • 16 kW - only when grid is down and battery drained to 5% SOC
  • Does not charge solar battery

In 2016, I upgraded from 100 A (25 kW) to 200 A (50 kW) service when transitioned from Prius to EVs. That was also when I added the 16 kW, emergency generator that is natural gas fueled.

My November 1 to 19 metrics:

• 339 kWh - solar production, saved $0.14/kWh (cost avoidance of high usage rate)
• 217 kWh - grid production, $0.12/kWh

Bob Wilson

 

Are you doing this purely for lowest cost, or also to reduce carbon emissions?

I'm often willing to pay a bit extra for the later.

 

There are "all electric" heat strips, there are electric heat pumps with heat strip backup and there are dual fuel heat pump/ hydrocarbon solutions.

In many parts of the country oil is not an option, metro areas have natural gas while propane is the only hydrocarbon for most of us.

Dual fuel is an excellent option for the rest of us since heat pumps are at least 1/3 of the operating costs of heat strips alone and propane costs as much as heat strips to operate standalone.

Dual fuel means the system can switch automatically and can be set to do so based on an economic or performance balance point.

You get ac by default with a heat pump.

In my mind the downside of heat pumps, with inverter based low ambient compressors or standard one or two stage psc compressors, is they are complex and don't have the same kind of truck stock serviceability as hydrocarbon based heat solutions. These days, parts can be a week out if available at all for most inverter based systems. Inverters are now shipping in traditional one or two stage units in the form of variable speed blower and condenser fan motors.

In addition heat pumps are lucky to maintain rated btus at 32f and most lose btus as it gets colder. Meanwhile the heat load is increasing.

Hydrocarbon heat can easily supply rated btus at any temperature and are usually oversized to begin with.

The other advantage of hydrocarbon solutions is the relative ease of generator backup which is nearly impossible for a week of ice storms when depending exclusively on a heat pump.

Overall I like dual fuel which costs a little more upfront but provides reliable comfort in winter and nice ac in the summer.