One thing I learned many years ago following the energy usage of buildings I designed was that as the energy needs of the building are reduced, and the fraction of those needs supplied by solar energy increases, the variation in back-up energy from year to year increases. Let's look at House 5 this winter and last winter.
First, the variation in heating load - this winter (Nov-Feb) is 20% colder:
We've used 23% more energy over these months, and here's how it breaks down between the heat pump and all other uses:
Heat pump energy used is up 32%:
As it gets colder outside, the heat pump efficiency drops, so it makes sense that heat pump energy goes up faster than heating demand. However, another difference in what we use is that Jill now works 3/4 time and is home Mondays and Fridays, so I'm guessing the average thermostat setting has gone up a tad, and there are more kWh going to lights, computer, music, etc. All other uses are up 15%, with the biggest bump in January:
Meanwhile...it's been noticeably cloudier this winter, so PV production over the four month period is down 22% over last year:
Consequently, our net imported energy last year over this period was 18 kWh, and this year is 45 times higher at 804 kWh!
If you design or build zero net energy buildings, it's important to communicate to the owners that these year to year variations are significant. Manage expectations! How much they use for plug loads/appliances/lighting is under their control beyond a certain baseline. How much they use for hot water is similar. At a given thermostat setting, how much they use for heating and cooling is dependent on the weather and the amount of sun available, and how much a solar electric or solar thermal system generates depends on solar availability and how much of the time the collectors are covered with snow. If you want to be pretty certain that a building is net zero every year, the solar electric system probably needs to be oversized by close to 30% (a SWAG). Driving the building's heating load down will make this variation smaller, as it takes one weather-dependent factor and makes it smaller.