Inputs
Example Data Table
| Load | Type | Watts | Hours/day | Qty | Wh/day |
|---|---|---|---|---|---|
| LED Lights | DC | 40 | 5 | 1 | 200 |
| Ceiling Fan | AC | 60 | 8 | 2 | 960 |
| Laptop | AC | 70 | 6 | 1 | 420 |
| Phone Charging | DC | 15 | 2 | 2 | 60 |
| Total | — | 1,640 | |||
Formula Used
- Wh/day per load = Watts × Hours/day × Quantity
- Total daily battery energy = DC Wh + (AC Wh ÷ Inverter Efficiency)
- Autonomy energy = Daily battery Wh × Autonomy Days
- Nominal bank Wh = Autonomy Wh ÷ (Battery Eff × Temp Factor × DoD) × Reserve Factor
- Required Ah = Nominal bank Wh ÷ System Voltage
- Series count = System Voltage ÷ Battery Unit Voltage
- Parallel strings = ceil(Required Ah ÷ Battery Unit Ah)
How to Use
- List each device, its watts, runtime, quantity, and AC/DC type.
- Choose system voltage and autonomy days for your location.
- Set DoD and efficiencies based on your battery chemistry.
- Adjust temperature and reserve factors for realistic conditions.
- Click calculate, then download CSV or PDF for planning.
Load inventory and daily energy profile
Build a device list that matches real habits. Compute watts × hours × quantity for each item, then total daily watt‑hours. Many small cabins stay near 1.5–3.0 kWh/day, but refrigeration, pumping, and cooking can push higher. A 60 W fan for 8 hours uses 0.48 kWh/day; two fans use 0.96. For big loads, verify with a plug meter.
Autonomy days and reserve margin
Autonomy is how many days you want power without meaningful charging. Two days suits stable weather; three to five days helps during winter cloud or monsoon. Reserve factor adds headroom for aging, wiring losses, and new devices. A 1.15 reserve increases required storage by 15%, which often prevents low‑voltage shutdowns at night.
Efficiency and conversion losses
AC loads pass through the inverter, so their energy is divided by inverter efficiency. At 92%, 1.84 kWh/day of AC demand becomes 2.00 kWh/day drawn from the battery. Battery efficiency then raises storage again; at 90%, 4.00 kWh over two days becomes 4.44 kWh stored. These losses can change series or parallel counts.
Depth of discharge and temperature derating
Depth of discharge defines how much of the bank you routinely use. Lead‑acid designs often target 50% DoD for cycle life, while lithium banks may use 80%. Temperature factor covers reduced capacity in cold conditions; 0.90 represents a 10% derate. Together, 0.50 DoD and 0.90 temperature require about 2.22× nominal capacity versus usable energy.
Budget signals for system planning
Cost outputs combine battery subtotal, other components, contingency, and tax, then estimate cost per usable kWh. This supports apples‑to‑apples comparisons across battery sizes and chemistries. If a bank costs 1,200 and yields 3.6 usable kWh, that is 333 per usable kWh. Use surge guidance to size the inverter for motors and pumps. Track seasonal usage, then rerun sizing before purchasing any long‑lead equipment.
FAQs
What is the difference between nominal and usable battery capacity?
Nominal capacity is the bank’s rated energy at nameplate voltage. Usable capacity applies your depth of discharge, temperature factor, and efficiency, so the delivered energy is lower. Design to the usable number, not the sticker rating.
Why do AC loads increase required battery energy more than DC loads?
AC loads pass through the inverter. If the inverter is 92% efficient, the battery must supply about 1/0.92 times the AC energy. DC loads bypass that conversion, so they avoid inverter losses.
How should I choose autonomy days for my site?
Use 1–2 days for reliable sunshine or generator backup. Use 3–5 days where cloudy stretches are common, travel limits refueling, or critical loads must stay on. Higher autonomy increases bank size and cost.
What depth of discharge should I enter for different chemistries?
For lead‑acid, 0.50 is a common planning limit to protect cycle life. For lithium iron phosphate, 0.70–0.85 is typical depending on warranty and temperature. Lower DoD means more batteries but longer life.
How does temperature affect sizing?
Cold batteries deliver less capacity, especially lead‑acid. A temperature factor like 0.90 assumes a 10% derate, so the calculator increases required bank size. If you have an insulated, conditioned battery room, use a higher factor.
Do I need to size for motor starting surge?
Yes for pumps, compressors, and tools. Enter your expected peak load and a surge factor. The calculator provides surge guidance so the inverter can start motors without tripping, while the battery bank supports short bursts of higher current.