With no load shifting and no generator, the author tackles the economics of taking her almost all-electric on-grid home off the grid.
When people visit our rural house for the first time and see the roof-mounted PV array—52 modules covering most of the south-facing rooftop—they often assume that we’re off-grid. Nothing could be further from reality—in fact, all but one of our household loads are supplied by electricity, which would be highly unusual for an off-grid house. The exception is space heating, which is normally provided by direct solar gain and a small, efficient wood heater.
I often find myself explaining why we chose a grid-tied PV system—how, even though we have a multitude of electrical loads, our system capitalizes on the utility’s net metering program and offsets nearly 100% of our electricity use annually. Then I explain how much an off-grid system would cost to meet our home’s loads—especially in winter, when it may be necessary to supplement our passive solar gain with electric heat, but sun-hours dwindle. Without any load shifting (using a non-electric energy source for a task, such as propane refrigeration) or generator use, an off-grid PV system would be ridiculously large, complex, and come with a similarly ridiculous price tag.
Just how big? My son and I took an afternoon to record, measure, and estimate every wintertime household load—down to the LED nightlight in the master bath. Even though we rarely use it, we also included the estimated energy use of our backup hydronic heating system, which is served by a 30-gallon electric water heater. (We almost exclusively use our wood heater for space heating, but I wanted this exercise to reflect taking an all-electric, “efficient” home off the grid.) We calculated an average daily winter electrical load of approximately 48 kilowatt-hours (kWh).
Generating that detailed list was time-consuming enough, so I turned to AltE Store’s online off-grid system calculator (bit.ly/altEcalculator) to estimate how much the system might cost. I didn’t get into the complexity of sizing for surge loads of various appliances, which was beyond the scope of this exercise, but those need to be considered to size the inverters.
I sized the system for only two days of autonomy (i.e., relying strictly on the battery bank), which would mean extreme load shedding measures or dependence on a backup generator during longer periods of cloudy weather. According to their calculator, we’d need:
If we used 315-watt SolarWorld modules, that means we’d need about 105 of them. At 39 by 78 inches each, we’d need 2,218 square feet to accommodate them. The south-facing roof on our house is about half that size, so we’d also need to consider a ground-mounted system or build a very big barn with a very big roof. It’s important to note that location plays a prime role in the system’s size. For example, if we lived in Boulder, Colorado, we could get by with an array that is 54% smaller—48 modules instead of 105.
We’d need to build a power shed to house this equipment or dedicate a room in the new barn we’d need to build—adding more cost to the project.
To fund just the basic equipment cost, we’d need very deep pockets indeed. Keep in mind that this is the bare-bones price; I haven’t included mounts, combiner boxes, circuit breakers, wiring, mains panel, nor labor costs, which would add thousands more.
Unless we win the lottery, an off-grid system would definitely be out of reach for us without some serious load shifting (see “Shifting Loads Renewably” table). Some loads are obvious and easier to shift in a more sustainable way (i.e., without resorting to fossil fuels). For instance, on sunny winter days, we can use our portable solar oven to bake instead of an electric oven, and hang clothes on a line to dry outside. When the rain hits, though, we’d be out of luck on both counts, and would need to shift loads away from electric. For example, we could use a wood cookstove for baking and line-dry our clothes indoors.
The most difficult load for us to meet and still be “sustainable” would be water heating, since cloud cover can be long-lasting here during the winter and solar water heating collectors will work only moderately well. Marrying the solar collector piping to a hydronic loop(s) in a wood heater could be a renewable solution, albeit somewhat complicated, as we’d have to retrofit our existing wood heater or replace it.
The consequences of this load shifting would be substantial: a reduction in our electrical loads to about 14 kWh, which would be a daily savings of about 34 kWh.
The lesson: When you’re off-grid, it’s usually far less expensive to load-shift than to try to serve heating loads with PV-made electricity. An additional investment in appliances and systems for load-shifting ($14,700) reduces the off-grid system equipment costs to about $45,000—together, only about one-third of the cost of the originally designed system (9.58 kW: 31 modules vs. 105; 24 instead of 72 batteries).