Off-Grid Solar System Sizing Calculator
Estimate the solar array, battery bank, charge controller, and inverter size for an off-grid setup.
How to size an off-grid solar system correctly
An off-grid solar setup is only as good as the math behind it. If the system is undersized, you run out of power and cycle your batteries too deeply. If it is oversized, you spend more than necessary. A practical off-grid solar calculator helps you find the balance between reliability and cost.
This calculator estimates four core components:
- Solar array size (W) — how much PV power you need to generate enough daily energy.
- Battery bank capacity (Ah and kWh) — how much energy storage you need for autonomy days.
- Charge controller current (A) — approximate controller amp rating for safe charging.
- Inverter size (continuous and surge) — supports your peak simultaneous loads.
What inputs matter most
1) Daily energy consumption
Start with your daily load in kWh. This is the most important number. Build it from appliance-by-appliance estimates:
- LED lighting: watts × hours
- Fridge/freezer: daily kWh from label or meter
- Laptop, internet, TV, fans, tools, well pump, etc.
If you can, measure with a plug-in power meter for 3 to 7 days. Real usage beats assumptions.
2) Peak sun hours
Peak sun hours are not daylight hours. They represent equivalent full-intensity solar hours. Local weather and season matter, so use conservative averages—especially for winter operation.
3) Autonomy days
Autonomy is how many days your battery bank can power loads with little or no charging. Higher autonomy improves reliability but significantly increases battery cost.
4) Battery depth of discharge (DoD)
DoD defines how much of your battery you actually use. A lower DoD means a larger battery bank for the same usable energy, but longer battery life.
- Lead-acid: often designed around 50% DoD
- LiFePO4: often 80% to 90% DoD
Core formulas used in this calculator
These equations are simplified and intended for planning. Final equipment selection should follow manufacturer specs, wiring limits, and local electrical code.
Example off-grid sizing scenario
Suppose your cabin uses 3 kWh/day, your site averages 5 sun hours, and you want 2 days of autonomy on a 24V system.
- Derating: 75%
- DoD: 50%
- Battery efficiency: 90%
- Safety margin: 20%
You would land near:
- ~960W solar array
- ~13.3 kWh nominal battery storage
- ~556Ah at 24V battery bank
- ~50A charge controller minimum (typically round up)
If using 400W modules, that suggests 3 panels (1,200W installed), which gives useful headroom during imperfect weather.
Common design mistakes to avoid
Ignoring seasonal differences
A system that works in summer may fail in winter. If year-round reliability matters, size from winter solar conditions.
Undersized inverter
Running power tools, pumps, or refrigerators can trigger startup surges 2–3x normal draw. Plan for surge power, not just continuous load.
Battery bank too small
Frequent deep cycling shortens battery life. A slightly larger battery bank can reduce stress and improve long-term economics.
No room for expansion
Loads almost always increase. Leave controller and inverter headroom if you plan to add appliances later.
Recommended workflow before buying equipment
- Audit loads carefully and separate critical vs. optional loads.
- Run this off-grid solar calculator with conservative assumptions.
- Round component sizes up to standard product ratings.
- Verify voltage compatibility across panels, controller, battery, and inverter.
- Confirm wire gauge, overcurrent protection, and disconnects.
- Review final design with a qualified installer if needed.
Final note
This tool is ideal for planning an off-grid cabin, tiny home, workshop, van conversion, or backup microgrid. Treat the result as a design baseline, then refine with local irradiance data, panel orientation, temperature effects, and real-world load monitoring.