Practical tools for measured work
Solar Array & Battery Bank Sizing Calculator
Size a solar array and battery bank from your daily energy use, peak sun hours, system losses, depth of discharge and days of autonomy.
These are energy-budget estimates for planning, not a system design. Panel output varies with weather, tilt, shading and season; battery capacity fades with age and temperature.
How it works
How to use it: enter your total daily energy use in watt-hours (add up each device's watts times hours used per day), your system voltage, the average peak sun hours for your location and season, an allowance for system losses, your battery's usable depth of discharge, and how many days you want to run without sun (autonomy).
Formula. Required array wattage accounts for losses from wiring, temperature, dirt and inverter/charge-controller inefficiency: array W = daily Wh ÷ (peak sun hours × (1 − system loss %)). Required battery energy scales daily use by days of autonomy and divides by how much of the battery you can actually use: battery Wh = (daily Wh × autonomy days) ÷ (depth of discharge % ÷ 100), then battery Ah = battery Wh ÷ system voltage. The rough charge-controller current estimate adds a 25% safety margin over the array's current at system voltage: controller A = (array W ÷ system voltage) × 1.25.
Worked example. 1000 Wh/day, 12V system, 4 peak sun hours, 25% system losses, 50% usable depth of discharge, 2 days of autonomy: array = 1000 ÷ (4 × 0.75) ≈ 333W; battery = (1000 × 2) ÷ 0.5 = 4000 Wh ÷ 12V ≈ 333 Ah; charge controller ≈ (333 ÷ 12) × 1.25 ≈ 35A.
Peak sun hours is not daylight length - it is the number of hours of standard 1000 W/m² sunlight that would deliver the same total energy as your actual day. It typically runs 2-3 in winter at higher latitudes and 5-7 in sunny, low-latitude locations; check a solar-irradiance map or your utility's solar data for your location and the season you are designing for.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- Daily energy use
- 1000 Wh
- System voltage
- 12 V
- Peak sun hours
- 4 h
- System losses
- 25 %
- Usable depth of discharge
- 50 %
- Days of autonomy
- 2 days
Primary result: Required solar array: 333 W.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
This is a daily-average energy budget, not a system design. It does not account for consecutive cloudy days beyond your chosen autonomy, panel temperature derating, shading, tilt and orientation losses, MPPT versus PWM charge-controller efficiency differences, or battery capacity fade with age, temperature and charge cycles.
Depth of discharge varies by battery chemistry - commonly around 50% for lead-acid and flooded batteries and 80-100% for lithium (LiFePO4), but always check your specific battery's datasheet.
The charge-controller estimate is a rough sizing starting point only. Confirm your actual controller, wiring and fusing against your equipment's specifications and applicable electrical requirements.
Frequently asked questions
How do I find my daily watt-hour use?
List each device, its running watts (or amps × volts) and hours used per day, multiply each, and add them up. A 10W light run 5 hours is 50 Wh; a 60W fridge compressor cycling roughly a third of the day is about 480 Wh. Add every device to get the daily total to enter here.
What peak sun hours figure should I use?
Use the average for your location in the worst season you want the system to cover, not the annual average - sizing for summer sun hours alone often leaves a system undersized in winter. Solar irradiance maps and PV-sizing tools published by government energy agencies list monthly averages by location.
Why does a lower depth of discharge need a bigger battery?
Depth of discharge is how much of the battery's rated capacity you can actually use before recharging. If you can only safely use half of a lead-acid battery's capacity, you need twice the rated Wh to deliver the same usable energy as a battery you can discharge more deeply.
Does this size my wiring too?
No - use the DC Wire Gauge & Voltage Drop Calculator with your array or battery current and cable run length to size the wiring between panels, charge controller, battery and loads.