Sunny Watts' office

Sunny Watts has the brightest office on campus: a glass wall onto a roof covered in panels, a wind turbine turning, plants everywhere. There is also a lamp shaped like a raincloud, because someone has to ask about the cloudy days. The Solar & Renewables calculators are on the install board, the shelves, the desk and the floor.

Sizing a solar system starts with energy, not power: how many kilowatt-hours a day the loads use, and how many hours of full sun the site gets in its worst month. From there come the size of the array, an inverter big enough for the largest load running at once, the charge controller between panels and batteries and, off the grid, a battery bank that can carry the loads through a run of dark days. The calculators here walk through each of those, plus panel tilt, wind turbine output, net metering and payback.

The cloud lamp asks the right question of every number in this room: what happens in the worst week, not the average one.

“Size an off-grid battery for the darkest week, not the average one: it has to carry you through it.” — Sunny Watts

In this office

• Solar Array Sizing: How many solar panels it takes to cover a daily energy need
• Solar Payback Period: How many years it takes a solar installation to pay for itself
• Battery Bank Sizing: Battery capacity needed to cover a daily energy need for a target number of backup days
• Inverter Sizing: Continuous and surge power ratings an inverter needs to cover a set of loads
• Charge Controller Sizing: Required current rating for a solar charge controller
• Off-Grid Autonomy: How many days a battery bank can cover a daily energy need with no sun
• Net Metering Savings: Monthly bill savings and banked credit from a net-metered solar system
• Wind Turbine Output: Power a small wind turbine captures at a given wind speed, and the annual energy if that speed never changed
• Panel Tilt Angle: A latitude-based starting point for fixed solar panel tilt angle

Project guides

• Go Off-Grid: Size a Solar Power System: Size the panels, batteries, charge controller and inverter for an off-grid solar setup, in the order a real system gets designed.

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