Thursday, March 14, 2019

Power use under the microscope (+ Day #5 off-grid)

Here are the electric appliances we've used this week off-grid, and their instantaneous power consumption.  Obviously, the longer an appliance runs the more power it consumes.
Multiply kilowatts (kW) by hours to get kilowatt hours (kWh).

Power demand by appliance:

6.7 kW - Level 2 EV charger
6.0 kW - Clothes dryer
2.6-3.1 kW - Central air conditioner (more on that below)
2.3 kW - Microwave oven
1.8 kW - Dishwasher (startup only, drops to 0.1 kW during wash)
1.0 kW - Level 1 EV charger
0.8 kW - Toaster
0.7 kW - Freezer compressor for refrigerator (Energy Star). Intermittent.
0.6 kW - Clothes washer on spin cycle
0.2 kW - Lights in the evening (LEDs of course)
0.2 kW - TV
0-2.0 kW - demand water heater. Power draw depends on temperature of the water in the solar water heater tank.  Zero if the water’s already hot.

We have a 1953 Chambers stove that runs on propane.

Our batteries

The two Powerwall batteries together provide about 27 kWh of use capacity. 
Even with intermittent light clouds, our solar system generated 27.2 kWh (see below).

The two Powerwalls together can provide 10 kW continuous output, or 14 kW peak.  That's enough that we don't really have to worry about what the family turns on, provided we get some sunshine to recharge the batteries.  One of those 3-day gloomy periods might put the kibosh on heavy use of the clothes dryer.

Level 2 EV charging works!

Today I discovered, that despite my earlier impression to the contrary, we can indeed run Level 2 charging of our Nissan Leaf EV from the house batteries.  Here are screen shots from the phone app during Level 2 car-charging on batteries alone, and on 90% solar power:

Batteries: whole house or just critical load?

We went with whole house, in part to prove it could be done, but ultimately because two Powerwall 2 battery units is the minimum Tesla will install for retrofits.  Also, two Powerwalls are necessary to start up our A/C.  Our house has a 3 ton, 16 SEER, 2-stage, central A/C to cool 1550 s.f. under air.  We keep the house at 79°F, mostly to limit the humidity.  Tesla installed a “Sure-Start” soft-starter device in our A/C compressor cabinet to reduce power demand when the A/C starts up.  If you are remodeling, consider installing mini-split A/C units to reduce the power requirement - people love them and they're very efficient.

One could make do with less battery capacity if one restricted battery use to supply only “critical loads”.  First decide what you need to use during an outage; for instance a mini-split A/C in the bedroom, the fridge, medical equipment, telecom equipment, and maybe the EV trickle-charger and microwave.  For sure not the clothes dryer or a Level 2 EV charger.  Batteries can be configured to operate a separate critical load panel that powers just the critical devices.  It's less expensive than backing up the whole house.

Day #5 off-grid

Again the house ran fine with no power from the grid.  Despite some light clouds, it was a productive solar day.  I restored the charge in the battery that was depleted by the crazy-heavy use on laundry day.  I could have charged the car, but it had enough power for the next day so I decided to hold off in lieu of filling the house batteries.

No comments:

Post a Comment

Note: Only a member of this blog may post a comment.