Worst Month

Size your off-grid system for the worst month of the year

Most calculators average the whole year and leave you dark in December. Enter your electricity use, enter your coordinates, and choose your technologies - we size your solar, wind, and battery system to survive the hardest month, and show you the math.

Your electricity use

≈ 10 kWh/day assumed for a medium home - a typical off-grid figure from survey data, not a measurement. Enter your exact usage if you know it.

To find your coordinates: in Google Maps, right-click (or long-press) your spot and copy the numbers that appear, e.g. 48.14, 11.58. In Apple Maps, press and hold to drop a pin.

Currently using: Boulder, CO

Technologies
Solar arrayDrag to size your own array. Each panel is 400 W. The chart and callouts show how your choice holds up month by month.12 × 400 W(4.8 kW)
060 panels

Tip: about 12 panels would cover all 12 months here.

Battery reserveHow many days your battery should power your home with zero generation - no sun, no wind. More days = bigger bank. Industry rule of thumb is 3 or more.3.0 days
1 day10 days

Tip: that needs about 42.4 kWh of batteries (38.2 kWh usable) at your usage and climate.

Cold-charge warning: standard LiFePO4 batteries refuse to accept a charge below 0 °C - your location's design minimum is -9 °C. Options: a self-heating battery, a heated insulated enclosure, or keeping the bank where it stays above freezing.
Covers all 12 months. This system generates enough electricity in every month of the year at Boulder, CO. Your tightest month is Dec with about 2% headroom.
Solar array
12 × 400 W panels
4.8 kW total - your choiceYou sized this array with the slider. The chart below shows how it holds up in every month.
Battery bank
42.4 kWh battery bank
9 × 48 V / 100 Ah (4.8 kWh) modules - sized for your 3-day reserveThe bank size is computed from your chosen days of reserve, your usage, the chemistry's usable share, and your climate's cold factor.
≈ 3 days of power with zero generationIf nothing is generating - no sun, no wind - this is how long the battery alone powers your normal usage, including the cold-weather factor at your location.
Month by monthOrange is solar, teal is wind. The red line is what your home needs. Red months fall short.~6,586 kWh produced per year vs 3,647 kWh needed
How we calculated this - every assumption
  • Sizing basis: we check your system against all 12 months, never a yearly average. Your tightest month is Dec - generation comes closest to your load there (2% headroom).
  • Usage: estimated from a medium home (3-5 people) ≈ 10 kWh/day - a typical off-grid figure from survey data, not a measurement = 10 kWh/day (months counted as 30.44 days, years as 365).
  • Seasonal shape: off - a gentle seasonal curve: about 1.15× your average in January, 0.85× in July.
  • Solar: you choose the array with the slider. Monthly production = array × that month's sun hours × 0.8 (20% is lost to inverter, wiring, dust, and charging losses). Standard 400 W panels. The tip under the slider shows the smallest array that would cover all 12 months.
  • Battery: you choose the days of reserve with the slider (3 days). The bank then needs ≈ 42.4 kWh: your daily usage × days ÷ (90% usable for LiFePO4 (lithium) × 0.79 cold factor for a -9 °C design minimum at Boulder). Usable portion: 38.2 kWh, shown as 9 × 48 V / 100 Ah (4.8 kWh) modules. Industry rule of thumb is 3+ days.
  • Not included: costs, charge controller / inverter sizing, and appliance-by-appliance audits. Those come in the full tool.
Demo data: all location numbers are synthetic per-climate-archetype values from the nearest of 31 US reference sites (currently Boulder, CO). Enter your coordinates above to pull location-specific climate data (NLR PVWatts + NASA POWER).