Colorado cabin

The site and the design

At ~7,500 ft the sky is clear most of the year, but winter days are short and the sun rides low. The fix is geometric: the array is tilted 50°, steeper than the site's latitude, trading summer surplus for winter harvest. The bundled PVWatts TMY data shows how well that works; at this tilt the monthly plane-of-array irradiance only ranges from 4.87 to 5.97 kWh/m²/day across the whole year, so December looks almost like June from the panels' point of view.

• Solar array: 1.2 kW, tilt 50°, facing due south
• Battery bank: 200 Ah LiFePO4 at 24 V (4,800 Wh)
• Charge controller: MPPT; inverter: pure sine wave, 92%
• Loads: chest freezer, LED lighting, well pump average, laptop and router, a 10-minute microwave run; about 1,900 Wh/day total

Things to try

Switch the charge controller from MPPT to PWM to see how a poor controller choice costs about a quarter of the harvest. Drop the Solar Access fraction for December through February to model snow sitting on the panels. Or flatten the tilt to 20° and watch the winter rows weaken while summer overflows.

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Frequently asked questions

How much solar do I need for an off-grid cabin?

Size for your worst month, not your average one. This cabin near Golden, Colorado draws about 1,900 Wh per day (chest freezer, well pump, lights, laptop, a short microwave run). A 1.2 kW array covers that with margin even in February, the worst-case month, because the panels are tilted 50 degrees to favor low winter sun. Sizing for the July average instead would leave the battery flat in midwinter.

What tilt should solar panels have for winter?

A common rule for winter-weighted systems is latitude plus 10 to 15 degrees. This cabin sits at 39.8° north and uses a 50° tilt. The steep angle sacrifices some summer harvest, when there is surplus anyway, and boosts the weak months; the bundled TMY data for this site shows December and January nearly matching June because of it. Steep panels also shed snow faster.

Does an MPPT controller really matter compared to PWM?

Yes, and this example is built to show it. Switch the charge controller field from MPPT (about 97 percent effective) to PWM (about 75 percent) and watch the daily solar column drop by roughly a quarter. PWM drags the panel operating voltage down to the battery voltage and throws the difference away; MPPT converts it. In a high-altitude winter where every watt-hour matters, the controller choice can decide whether February balances.

More worked examples

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