Off-grid solar is a commitment to energy independence, of not being reliant on the grid, and not paying bills for electricity. It is also a lifestyle enjoyed by those who are mindful of their energy use and enjoy being in control of their home energy system.
In this post I address the essential topics for understanding off-grid living and choosing wisely.
Also see - Hybrid Power Systems.
The most common applications for off-grid power systems include energy efficient homes and cabins. Large family homes, commercial operations, and even communities can be powered off-grid as well, as the scaleability of systems is very large. In all cases, the essentials are the same and combining energy efficiency is a good economical choice.
Economically, avoiding the high costs of a grid connection (especially for new construction), and the absence of an electric power bill, can also mean significant upfront and long term savings. But in the case of an already grid-tied home, a regular grid-tied or hybrid system may be a better choice.
How it works
Energy storage (battery power) is central to the system. High-capacity batteries supply energy to a power-inverter that produces mains AC power (120V/240V) for an electrical panel.
As the sun shines, solar powers loads through the inverter and keeps the battery charged automatically.
When extra power is needed (esp. winter time), a back-up generator is used to help charge the battery and keep the loads running. Combined, solar plus a generator offers a continuous, reliable power supply.
Sizing a system
Sizing a solar power system begins with estimating the average and maximum electric use needed throughout the year. The number residents/users, types of appliances (loads), and square footage all help to estimate the energy and power requirements to size for.
A typical family-sized home in South Dakota uses about 11,000 kWh per year (kWh/Year). On average, about 30 kWh to 40 kWh per day (kWh/Day). However, every home is different.
As much as possible, energy efficiency and conscientious energy use is beneficial. Smaller and more efficient homes can reduce energy consumption significantly below average.
To size a system, the maximum power and total energy needs must first be determined. Power - for the maximum amount of load at any one time, and energy - for the cumulative amount of power used over time (learn more about this).
A typical home can be powered by an 8 kW inverter (with surge capability), to run several appliances at one time. The battery capacity must be high enough to deliver the power of the inverter, and the solar array must be have enough capacity to keep the battery recharged on a daily basis.
Equipment and appliances that generate heat (including refrigeration and air-conditioners) account for >50% of energy consumption in a home. For example, a water heater (4500 watts) requires 450 times more power than a 10 watt LED light bulb. Be mindful when heating and cooling to save electricity. Turn off lights and electronics so they don't operate continuously, and most home devices will remain a low percentage of overall energy use. A little planning and effort goes a long way to reducing energy use and the capacity of a system needed.
Pro tip: An appliance that runs on lower power for more time is preferable to one that uses a lot of power to run for a short time. Don't oversize appliances like heat pumps, as a large draw can overload an inverter. Have heating elements installed in your water heater reduced from 4500 watts to 3000 watts, so you can run other appliances simultaneously.
Tips for appliances/equipment selection:
Space heating needs don't line-up well with solar, since heating is needed most when there is least sunshine and at night. Electric heating is good on a sunny day, but for heating on a cold winter night it is essential to have an on-demand energy source like propane and a high efficiency furnace. Biomass heat with a log or pellet stove is a great choice to supplement the furnace and reduce propane use.
Air-conditioning is easily accommodated by solar because it is needed most when the sun shines. Since off-grid systems are sized for December, there is usually an abundance of energy available in the summer. A split-system heat pump with a soft starter is a good choice for air conditioning.
Heat pumps are efficient for air conditioning, but despite their efficiency they may not be sufficient for heating during the colder winter months.
A soft-starter is a device used with motors and compressors (i.e. 240-volt well pumps and air conditioners), to reduce surge power at startup. They also conserve energy and are reduce wear-and-tear of the motor/compressor. Soft starters are recommended because they reduce stress on the inverter and battery as well.
Clothes washers can easily be accommodated in an off-grid power system, as many modern washing machines are very high efficiency, including integrated water heaters.
Clothes dryers are major energy users, so propane is a good option. Even a smaller electric dryer can be a heavy load, but it can be offset by a generator if solar power is low.
Water heating with and electric tank is suitable for sinks, showers and laundry, and can used effectively by using a large tank like a battery. Water heaters can be automatically switched on when there is extra solar power, reducing battery use. On-demand water heaters can be used, but only with propane. Electric on-demand heaters are incompatible with battery inverters due to their very high load. Hybrid electric and propane water heaters are great if a lot of hot water use is expected.
Geothermal systems (a.k.a. ground-source heat pumps) are efficient but may not provide the results needed. The actual effectiveness of a geothermal system depends on the location, and the costs to install may be very high compared to alternatives.
Propane heat and backup power
Propane is recommended with a standby generator (10 kW to 14 kW) and a furnace. A high-efficiency propane furnace is a reliable on-demand heating source, but biomass stoves are great too. A propane generator for backup can ensure uninterrupted power, especially during the winter when days are shortest and cloudiest.
For other uses, propane is also a good choice for clothes drying and water heating, but optional for cooking (induction cook tops are very efficient).
Ensure you optimize for efficiency when designing or updating your home. Reducing waste of heat is the best first step for saving energy. If you're building new, this is your best opportunity to get a properly sealed and air-tight structure (with the necessary mechanical ventilation). Attic insulation and good windows are especially important, but so is addressing air leaks trough unsuspected places such as electrical outlets, door frames and crawl spaces.