Residential Solar Power

Josh Carpenter
  • Climate

Notes on the basic science, typical components and sample configurations of residential solar installations.

Basic off-grid system

An off-grid solar installation

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
  • 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.


  • 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.

Device Watts
LED light bulb. E.g. G9 dimmable 4.5
55" LED TV. E.g. Vizio P55-F1 172 (0.5 standby)
MacBook Pro 86
Freezer. E.g. 115 VAC, 5A draw (2x at peak). 575

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

Device kWh / year
Gaming PC (with display) 1400
Gaming console 134
Average household PC 246
Fridge. E.g. Kitchen Aid KBRS22KVSS4 459

Example builds

Josh Carpenter

Josh Carpenter

I'm a user experience designer based in Vancouver, Canada. Most recently I led design teams at Google and Mozilla, working to bring virtual and augmented reality to the open web. My experience is a mix of product leadership, interaction design, visual design, motion graphics, programming and community engagement. A fox—not a hedgehog. Since 2019 I've been traveling and working on a series of personal projects, including an Electron-based Markdown editor and a deep dive into climate change.