The showcase has one off-grid solar system installed and extensively tested since June 2013. It consists of a 945 watt solar array, Outback Flexmax 80 MPPT solar charge controller, Outback 2800W inverter, and battery bank, along with both AC and DC circuits wired into the house. The system components ran about $6500, plus installation. Once set up and correctly configured, the system has been exceptionally reliable.
DC circuits run all lights (LED), water pumps, a Novakool 12V refrigerator/freezer, laptop computer, cell phone and other 12V accessories. With the inverter on, the solar system powers a microwave oven, KitchenAid mixer, blender, grain grinder, printer/copier, wireless router, sewing machine, keyboard, sound system and LED projector. With sun shining, the system can also power the a/c during the day.
The limiting factor on the system is the size of the battery bank (315 Amp-Hr), not the panel array. On a sunny summer day the battery bank will be topped off by noon. The bank was kept quite small due to expense (about $750 for 3, 105 Amp-Hr Deka-Solar batteries).
Grid electricity is used for 3 appliances: a/c when needed, electric panel wall heaters (when propane system is off), and the incinerator toilet (a needed solution for sanitary waste disposal in an urban area, absent water/sewer hookups). The house could be entirely off grid with a larger battery bank (for a/c), use of all propane for heat, and a composting toilet.
Off-grid electric system design
- In general it is very expensive to accommodate any electric heating/cooling appliances, as these take a large amount of energy, and consequently more solar panels and batteries. So it is best to eliminate these wherever possible (i.e. use propane for heat, cooktop).*
- It is also always more cost effective to invest in efficiency vs additional solar capacity. For example, for lighting, only use LED bulbs vs CFL or incandescent.
- It is also always more efficient to choose DC appliances in an off-grid system. DC (12 volt) loads run directly off the battery bank. AC loads require you to have an inverter on, which has some inherent inefficiencies (typically 5%) when converting DC to AC. So in general it is best to keep the inverter off unless you need it to run AC appliances.
- There are many off-grid solar system providers who can help you get through system design. I suggest working with a local one. If none are available, you might start with the Alt-E store calculator.
Battery bank sizing. With your list of electric loads, size your system accordingly based on expected usage and wattage. There are many calculators you can use such. You will then have a final watt-hours per day total. Also think through how long you would wish to go without sunlight, and estimate the lowest temperature your battery system will be exposed to (lower temperatures=less capacity). Remember that to preserve battery life a lead acid/deep cycle battery bank should never be cycled to less than 50% of capacity.
There is a little discussed battery sizing issue I call ‘effective capacity’. At some point your solar array will fill up your battery bank- the smaller your bank, the faster. Once filled, the charge controller basically shuts the system down. However, as long as the sun is shining you can run heavy load appliances essentially direct from the sun- effectively getting more power out of your system than the battery bank alone could provide each day. I do this all day on sunny summer days with the a/c (which generally draws too much power to be run off the batteries for long), as well as electric water heater. So it is possible to get more watt-hours per day out of your solar system than the battery bank alone would suggest, with some babysitting.
May 2015 update: Outback is now offering a new integrated, prewired, and very compact off grid system- ‘FlexPower One‘ -that seems ideal for micro houses. This should make installation of an off grid system considerably easier.
*One notable exception I’ve found is water heating- a small electric water heater (4-6 gallons) requires just 10 min or so of power (typically 1400 watts) to heat up. This is a lot of power, but for a short time. This system replaced the tankless (propane) system in the Minim successfully (and avoided additional gas hookup, and a ceiling penetration for venting).