Before we designed the off-grid system, we set a rough budget for the house, with two major considerations: We needed to wire the house so that we could use whatever electricity our system would make and we needed a septic system so that we could have indoor plumbing.
Early in the process, I decided on a solar-only system. There was no hydropower source on the property, and while wind electricity is widely used in the area, I just didn’t have the funds to do a hybrid system, nor did I want to go with just a wind system. Once I focused on a solar-only system, I began the critical process of sizing it.
We used about 1,000 kWh per month at our grid-tied home. That works out to be about 33 kWh of electricity a day used (or, mostly, wasted). I had never examined our electricity usage before. All I saw and complained about was the monthly bill.
The first step in system design is to figure out how many kWh a day are needed. That seems simple enough, but it is very difficult to arrive at an accurate number. I bought a Kill A Watt meter and measured the energy use of the toaster, computer, TV, dryer, washing machine, and everything else in our existing house that plugged into a 120 VAC outlet. I recorded all of the watts and watt-hours used for everything in the house on a spreadsheet.
Some of our appliances were very energy-efficient and other older ones were energy hogs. But knowing how much energy everything used was crucial. Next, I needed to estimate how many kWh we would need per day in our new house. That sounds easy, but it is also quite difficult. How many lights would we use during the evenings and how long would they be on? The same question applied to everything electrical in the house. Going off-grid forces you to be very aware of how you use your energy.
The trick to off-grid living is conservation and efficiency. After looking at many different scenarios, I came up with an estimated minimum of 6 kWh per day. After getting real data from our appliances and deciding that we would be able to change some of our electrical usage habits, I targeted a 3 to 4 kW system. (Since the system was installed, we’ve loosened our conservation belts a little this summer and have used as much as 6 to 7 kWh per day on a couple of occasions.)
Once your average daily electricity use is estimated, the size of the battery bank can be calculated, based on a specific depth of discharge and the number of days of autonomy (no sun) you expect.
Since the solar energy is stored in a battery bank, its capacity has to be enough to provide the energy you need without overdischarging your batteries. The battery bank feeds the inverter that converts the DC to AC electricity used by standard household appliances.
Determining the size of your array and battery bank is a bit of a balancing act. An array that’s too small will struggle to keep your batteries charged. An array that’s too large will cost more up-front. I gathered all my data, calculations, and best guesses and came up with a target system: a 48-volt, 400- to 600-amp-hour battery bank and a 3 kW solar-electric array.
We knew that heating anything significant with electricity was going to be too big a burden on our PV system. We kept the two Hitzer gravity-fed coal stoves, and decided to use propane for backup space heating, water heating, cooking, drying clothes, and to run the backup generator. But we would be reducing our electricity usage by nearly 80%—going from 33 kWh a day to relying on a system that in the depths of winter would produce only 6 to 7 kWh. Our grid-tied house used natural gas for space heating, water heating, and clothes drying. We had a very large chest freezer, electric convection oven, two refrigerators, five TVs, three DVR/cable boxes, a large gaming computer, two printers (one inkjet; one laser), incandescent lights, and a bevy of other little electronics. A pellet stove ran 24/7 all winter. Everything was plugged in and powered most all times. Every night, there were fans and other electronics running. I made some drastic assumptions to reduce our electricity usage. That worried me. What if I had made a wrong calculation? And what if I could not install the system in time for our first winter? With those fears, I decided to hire a professional to help design and install our system.