Off-grid solar sizing calculator

How this is calculated

The array is sized to replace one day's load during the design month. In words: daily load, divided by the energy one watt of panel delivers in a day, divided again by the fraction of that energy that survives the trip to the load.

Array W_DC = daily load Wh ÷ (peak sun hours × system derate)

A peak sun hour is 1 kWh/m² of irradiation on the panel face, equal to one hour at the 1,000 W/m² test condition at which panels are rated. Monthly mean values for a given tilt come from a solar model (PVWatts, PVGIS, NSRDB). The system derate of 0.75 is the conventional off-grid figure. It stacks inverter loss (5–10%), wiring (2–5%), charge controller (2–3%), battery round-trip loss (10–15%), soiling and heat, which the Sulfur Springs Land Management guide puts at 75–85% overall. Grid-tied models use a smaller loss because there is no battery in the path; the PVGIS run cited below assumed 14% losses.

The battery bank is sized so the load can come back out of it for the autonomy period. Energy leaves through the inverter, so the stored amount is the load divided by inverter efficiency. The bank can only give up its depth-of-discharge fraction, so nominal capacity is usable capacity divided by DoD. Amp-hours are nominal watt-hours divided by bank voltage.

Usable kWh = (daily load ÷ inverter efficiency) × autonomy days ÷ 1,000
Nominal kWh = usable kWh ÷ DoD
Bank Ah = nominal Wh ÷ bank voltage

Inputs are clamped: load 1–10,000,000 Wh, peak sun hours 1–9, autonomy 0.5–14 days, DoD 0.10–1.00, voltage 12–96 V, derate 0.50–0.95, inverter efficiency 0.70–1.00. Nothing here is weather-dependent or site-specific beyond the numbers you type.

Cochise County defaults

Peak sun hours: 4. Size to December, the weakest month. Cochise Land Management modelled a fixed array at Sunsites (31.94° N, 109.84° W, 1,323 m) at 32° tilt, due south, with PVGIS using ERA5 data for 2005–2023: December 5.66 kWh/m²/day, April 7.91 (the best month), July 6.16 (monsoon cloud), annual mean 6.78, and about 1,912 kWh per year per kW_p at 14% losses. A 19-year monthly mean is the middle of the distribution, so half of Decembers are worse. The default of 4 is about 70% of that mean and is the "conservative winter estimate" both county guides use. If a generator is wired in, 5 to 5.5 is a reasonable design value and the array shrinks by a fifth.

Latitude 31.3°–32.3° N (Douglas 31.35°, Sierra Vista 31.55°, Sunsites 31.94°, Willcox 32.24°). A fixed tilt near 32° gives the annual figures above; a steeper winter tilt of 45–47° raises December and lowers July. See the tilt calculator.

Depth of discharge 0.80 for LiFePO4, 0.50 for lead-acid. Autonomy 2 days; 3 if the site is unattended and has no generator, and more than 5 is rarely worth the battery cost. 48 V for anything above a shed. Winter nights at 4,000–5,000 ft routinely go below freezing; LiFePO4 will discharge in the cold but most cells will not accept charge below 0 °C without a heater, so a bank in an unconditioned enclosure needs either heating or a 10–15% capacity margin.

Arizona's 2026 solar law. Senate Bill 1419 became Laws 2026, Chapter 175, amending A.R.S. 44-1761 through 44-1763. When a licensed contractor sells or installs a solar energy device, 44-1762 requires a written warranty statement with performance data on the Registrar of Contractors' form, a certificate that the device complies with the section, at least a two-year warranty on collectors, storage units and their installation, and a licensed solar contractor doing the work to local fire, safety and building codes. Subsection F exempts devices designed or installed by the final owner, so a self-install is outside it. 44-1763 adds signed, dated disclosures and a three-business-day right to rescind on sale and lease agreements. The chaptered law states no effective date; the general effective date for 2026 legislation is 12 September 2026. Details in the Cochise Land Management summary.

Cochise County adopted the 2024 IRC and the 2023 NEC effective 1 September 2026. The county guides report that a ground array on your own parcel does not itself need a county permit; confirm with Development Services at 520-432-9300 before you pour footings.

A worked example

A 600 ft² cabin near Pearce: full-size fridge, well pump on a pressure tank, LED lights, laptop, phone chargers, a small chest freezer. Measured load 6,000 Wh/day. Design month December at 4 peak sun hours, 2 days of autonomy, LiFePO4 at 0.80 DoD, 48 V bank, 0.75 derate, 0.90 inverter efficiency.

Array: 6,000 ÷ (4 × 0.75) = 2,000 W DC. Usable storage: (6,000 ÷ 0.90) × 2 ÷ 1,000 = 13.33 kWh. Nominal: 13.33 ÷ 0.80 = 16.67 kWh. Amp-hours: 16,667 ÷ 48 = 347 Ah at 48 V. In hardware that is five 400 W panels and four 48 V 100 Ah server-rack batteries (20.48 kWh nominal), or three 5 kWh wall units (15 kWh, slightly short). Run the same load through the autonomy calculator to see what a given bank actually buys you.

What this does not cover

Load estimation. The result is only as good as the watt-hour figure; a plug-in meter on the fridge and pump for a week beats any table. Panel count, string voltage and charge-controller input limits: cold January mornings raise panel voltage, and the controller's maximum must be checked against the string's open-circuit voltage at the site's design low temperature. Wire gauge, fusing, grounding and the 2023 NEC Article 690 and 706 requirements. Shading, which the derate does not include. Generator sizing and charger current. Seasonal tilt adjustment. Air conditioning, electric heat and electric water heating, which change the load by an order of magnitude and usually need a different design approach.

This page is arithmetic on numbers you supply. It is not an electrical design, a permit document or a bid. Before buying equipment, have the design checked by the installer who will warrant it, or by a licensed electrical contractor if you are building under the owner-builder exemption.

FAQ

How many peak sun hours should I use for an off-grid system in Arizona?

Use the worst month you need to get through, not the annual average. At Sunsites in Cochise County a fixed array tilted 32° models at about 5.66 kWh/m²/day in December and 7.91 in April (PVGIS, ERA5 2005–2023). A design value of 4 peak sun hours is roughly 70% of the December mean and leaves room for a run of cloudy days. If you have a generator, 5 to 5.5 is defensible.

What depth of discharge should I enter?

0.80 for LiFePO4 (do not routinely run below 20% state of charge) and 0.50 for flooded or AGM lead-acid. The calculator clamps the value to 0.10–1.00. Entering 1.0 for lithium sizes a bank you will cycle to empty, which shortens its life.

Why is the nominal battery bank larger than daily load times autonomy days?

Two divisions. The load leaves the bank through an inverter at about 90% efficiency, so stored energy must be load ÷ 0.90. Then the bank can only give up its depth-of-discharge fraction, so nominal capacity is usable ÷ DoD. For 6,000 Wh/day and 2 days: 13.33 kWh usable, 16.67 kWh nominal at 0.80 DoD.

Sources

• Cochise Land Management, "Grid Power or Solar? What an SSVEC Line Extension Actually Costs" — PVGIS (ERA5 2005–2023) monthly output at Sunsites, 32° tilt: Dec 5.66, Apr 7.91, Jul 6.16 kWh/m²/day; 1,912 kWh/kW_p/yr at 14% losses.
• Cochise Land Management, "Off-Grid Solar System Sizing Guide for Cochise County" — 4 peak sun hours as the winter design value; 2–4 days autonomy; LiFePO4 0.80 DoD.
• Sulfur Springs Land Management, "Off-Grid Solar Sizing for Cochise County Properties" — loss breakdown (inverter 5–10%, wiring 2–5%, controller 2–3%, battery 10–15%; 75–85% overall).
• A.R.S. 44-1762, Solar energy device warranties; installation standards; inspections
• A.R.S. 44-1763, Distributed energy generation system agreements; disclosures
• Cochise Land Management, "Arizona's 2026 Solar Law: What an Off-Grid Buyer Must Now Be Handed" — SB1419, Laws 2026 Ch. 175.
• Cochise County Development Services, Owner Builder Amendment

Related guides

• Off-Grid Solar System Sizing Guide for Cochise County
• Off-Grid Solar Sizing for Cochise County Properties: A Practical Guide
• Arizona's 2026 Solar Law: What an Off-Grid Buyer Must Now Be Handed