Basic off-grid system

The components of a basic off-grid system are as follows:

• Solar panels convert sunlight to electricity and output DC power.
• Charge controller takes DC power from panels, and regulates output.
• Battery bank takes regulated DC output from controller, charges batteries, and outputs DC. Best Solar Charge Controllers
• Backup generator (e.g. gasoline), also ouputs DC to the inverter. Honda’s generators are well-regarded. The Ultra-Quiet 2200i weighs 46lbs, runs 8.1 hours on a single 3.6 litre tank, which costs $4.60 at Nov 2019 Vancouver gas prices of $1.30, and outputs max 2,200 watts (1,800 continuous) at 120 VAC.
• Transfer switch switches two sources of power. In our case: the battery bank and generator. Transfer switches can be manual or automatic (e.g. switch to generator when batteries are depleted).
• Inverter takes 12 volt DC from battery bank, and outputs 120 volts AC, which is what home appliances run on.
• AC load center helps organize load across different outlets and devices.
• Home appliances plug into the outlets.

Misc details

• For top-rated panels, see: Best selling panels in California for 2018.
• A south facing roof is best (assuming you’re in the northern hemisphere). East and west facing are good. North facing is just OK. Optimal angle is 45°. Per Energyhub.org.
• Clouds are fine.
  • “Ironically, solar panel kits work best under cold and cloudy conditions with full sun. This is because temperature af fects the efficiency of a solar panel; a 100-watt panel at room temperature will be become an 83-watt panel in 110 degrees.” - Best Solar Charge Controllers
• Doubling your batteries gives redundancy for a critical-component.
• Lithium Ion batteries: Lighter and cheaper than lead acid. “Because of the better efficiency and deeper discharge depth, Lithium battery banks tend to be only 50-60% of the size of a comparable lead acid bank!” Off-Grid Battery Bank Sizing
• Solar-optimized charge controller: Cheap charge controllers only prevent batteries from overcharging. A charge controller designed for solar (e.g. a Maximum Power Point Tracking (MPPT) controller) provides more efficiency by better resolving the delta between the voltage of the solar panels and the batteries. See: Best Solar Charge Controllers.
• Pure sine-wave inverter: is better for your electronics.
• Motors draw significantly more power during the first few seconds of operation: Can easily be 3x. So buy inverters that have more wattage than you think you need. Some also have two ratings: “surge” and “continuous”.

How much panel capacity?

The simple formula for estimating a solar system is as follows:

(Yearly kWh Usage ÷ 365 days ÷ average sun hours) x 1.15 efficiency factor = DC solar array size required.

To date, my studio apartment’s average kWh consumption has been: 110 kWh/month. That includes a fridge, dishwasher, washer/dryer, electronics, lights, etc. Times 12 months, that’s 1320 kWh/year.

I’m assuming 3.5 average hours sunlight for Vancouver. That’s the same value as Seattle. ClimaTemps claims an average of 5:01 hours sunlight per day, taking into account historical weather and variations in length of day by season. But I’ll go with the more conservative 3.5 estimate.

With those figures, I get the following:

(1320 ÷ 365 ÷ 3.5) x 1.15 = 1.18 kW

If I used SunPower SPR-X21-335-BLK-D-AC panels (335 W, 21% efficiency), I could use 4 panels x 335W = 1340 watts.

Those panels look to go for about $1 USD per watt, so that’s $1340 USD for the panels. Then there are the costs of other system components and installation.

Units

• 1 watt, flowing for 1 hour = 1 watt hour (Wh).
  • 40W lightbulb on for 8 hours = 320Wh
• 1000Wh = 1 kilowatt hour (kWh). This is what our utilities are billed in.
  • 1000W microwave on for 8 hours = 8000Wh = 8kWh
• Amps x Volts = Watt.
  • Fridge runnning at 127V and 2.4A = 305W.
  • Using water pipe analogy, volts are water pressure, amps are flow through the pipe, adn watts are what we can do with that energy (e.g. turn a water wheel)
• Batteries are rated in watt hours
  • Yeti 400 battery is 396 Wh. It’s 12 volts x 33 Amp hours = 396Wh.
  • A 100W lightbulb would last four hours.
• Solar panels are measured in watts.

Consumption examples

The power draw of portable Macs can be found in System Information > Power > AC Charger Information.

In 2019, my average monthly kWh consumption has been 110 kWh.

Example builds

• Gambier Island home
  • 3.12 KW panels, 40KW batteries, 8.5KW inverter
• DIY home system
  • 123 watt panels x 3, 2 x 232 amp hour batteries.
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