Loads → watt-hours
Each device’s draw is converted to the system bus voltage (with converter losses when voltages differ), then multiplied by its duty hours. The daily total is what your generation has to beat.
Tool
Solar Power Calculator
Size an off-grid solar + battery system for a remote site — radio towers, repeaters, sensors. Enter your equipment loads, battery bank, and panels; get days of autonomy, recharge time, and NEC breaker sizing.
Inputs
System
DC bus voltage the batteries supply.
Applied when a device’s voltage differs from the bus (step-up or step-down).
Real-world panel output vs. nameplate — dust, temperature, wiring. 0.72 is a conservative default.
Usable fraction of rated capacity — depth-of-discharge limit, cold weather, age.
Equipment loads
Battery bank
Wired in series to reach system voltage.
Strings wired in parallel for capacity.
Solar array
Average full-sun-equivalent hours for your worst month (usually December). Look it up below, or at pvwatts.nlr.gov.
Wind turbine (optional)
Worst-month average for the site. Informational.
From the turbine’s power curve. A 0.5 safety factor is applied automatically.
Results
Days of uptime on batteries
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Total load
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Battery bank (derated)
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| Solar only | Solar + wind |
|---|
Sizing & protection
Array output current
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Charge controllers needed
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Min. battery breaker
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| Panel model | Isc | Min. breaker (NEC 690.8) |
|---|
Per-string solar breaker = Isc × 1.56, rounded up. Battery breaker = total array Impp × 1.25, rounded up. Verify against your AHJ and current NEC edition.
Battery voltage → runtime (when dark)
Read the bank’s resting voltage and estimate how much runtime remains. Linear approximation for lead-acid chemistry.
| Charge | Volts | Runtime left |
|---|
24 hours remaining at
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12 hours remaining at
—
Useful as dispatch alert thresholds — when telemetry reads these voltages, that’s how long you have to get someone on site.
Notes
How it works
All math runs in your browser. Solar radiation lookups use public solar resource data from the National Laboratory of the Rockies (formerly NREL), cached server-side.
Autonomy & recovery
Days of uptime = usable battery watt-hours ÷ daily load. Recovery = usable watt-hours ÷ daily surplus after the load is fed — how long the array needs to refill a drained bank.
Design for the worst month
Size against December sun and winter loads. A system that survives the darkest month coasts through the rest of the year — and batteries that stay topped up last years longer.