One of the most common questions that people ask us about our house is what we mean by zero-energy. Let me begin by explaining what we don’t mean by zero-energy. Our house is not off the grid, like a house you might find out in the country, away from public utilities. And while we produce our own solar electricity, we’re still connected to the city’s electrical grid. We don’t have a bunch of batteries in the basement. But more on that later.
Now, let me explain what we are building: It’s called a net-zero energy house. Quite simply, this means that we expect our solar panels to produce as much (or more) electricity over the course of a year than we use for heating and household use.
Like ours, all net-zero houses are connected to city power lines, but not all achieve the stringent standards we’re shooting for. In fact, the definition of net-zero can get a little muddy. For instance, some claim net-zero status for houses that produce enough of their own renewable energy to offset only that portion of their power that comes from dirty sources. In Seattle, where 91% of our electricity is hydroelectric, this is a relatively easy standard to achieve if you have electric heat. Other people define net-zero as zero energy bills over the course of a year, even if they are using more city power than they are actually producing. And in what seems to me like the most egregious misuse of the term, some so-called zero-energy homes simply produce enough solar electricity to power their sockets but depend on natural gas for heat.
Another question people ask us is whether all our power comes from the sun. It usually goes something like this: “If you’re connected to the grid, you’re still using dirty power at night and on cloudy days, right?” The answer is–sort of, but not really.
As I mentioned above, we don’t have a bunch of batteries in the basement (actually, we don’t even have a basement). Why? Because storing electricity is inefficient, not to mention expensive. When you charge a battery bank, you don’t get as much electricity back as you put in. Some of it is lost as heat. That’s why most houses that have solar panels hooked up to a battery bank rely on some other source for heat, like a wood stove.
It’s far more efficient and cost-effective to hook our solar panels up to the city’s electrical grid. That way we can put power into the grid during sunny days, when we’re producing more than we can use, and take it out at night and when it’s cloudy. It’s like we’ve set up our own little clean solar power plant, which means less natural gas needs to be extracted and burned and less electricity needs to be generated, avoiding all the associated negative environmental consequences.
Alex and I like to compare it to a bank. When you have an excess of cash, you make a deposit. For all practical purposes, it’s still your money when you go to withdraw it from the ATM later, even if it was mixing with everyone else’s money in the vault.
What do you think? Make sense, or do we need to keep searching for a better metaphor?