Calculator Input
Use the responsive input grid below. Large screens show 3 columns, smaller screens show 2, and mobile shows 1.
Example Data Table
This sample table shows how loads can be added before entering totals into the calculator.
| Appliance | Running Load (W) | Daily Backup Hours | Energy Need (Wh) | Notes |
|---|---|---|---|---|
| LED lighting | 120 | 6 | 720 | Efficient lighting circuit |
| Fans | 250 | 6 | 1500 | Two medium ceiling fans |
| Refrigerator | 250 | 6 | 1500 | Use surge load in calculator |
| Wi-Fi and router | 35 | 6 | 210 | Low but essential load |
| Television | 140 | 4 | 560 | Optional entertainment circuit |
| Laptop charging | 100 | 5 | 500 | One charger and accessories |
| Total example | 895 | — | 4990 | Use a safety margin for design |
Formula Used
- Adjusted continuous load = Running load × (1 + safety margin).
- Recommended inverter size = Greater of adjusted load or surge load, then increased by 15% and rounded up.
- Required AC energy = Running load × backup hours.
- Required battery energy = Required AC energy ÷ (battery efficiency × inverter efficiency ÷ 10000) ÷ depth of discharge fraction.
- Required bank capacity in Ah = Required battery energy ÷ actual bank voltage.
- Batteries in series = Ceiling of target system voltage ÷ single battery voltage.
- Parallel strings = Ceiling of required bank Ah ÷ single battery Ah.
- Total batteries = Batteries in series × parallel strings.
- Estimated project cost = Battery cost + inverter cost + accessory percentage add-on.
- Simple payback = Estimated project cost ÷ annual protected energy value.
How to Use This Calculator
- List the appliances or circuits you want during outages.
- Add their normal running watts to get total running load.
- Identify the highest startup surge among motors or compressors.
- Enter the backup time you want the system to deliver.
- Choose your target bank voltage and battery unit size.
- Set realistic efficiency, depth of discharge, and safety margin values.
- Add battery prices, inverter prices, and accessory percentage.
- Press Calculate System to view sizing, cost, and runtime.
- Download the result as CSV or PDF for planning or quotations.
FAQs
1) What does this calculator estimate?
It estimates inverter size, battery bank capacity, battery count, expected runtime, and a simple project cost based on your load, backup time, and component pricing.
2) Why is surge load important?
Many appliances, especially refrigerators, pumps, and motors, briefly draw more power at startup than during normal operation. Ignoring surge can cause inverter trips even when average running load looks safe.
3) What is depth of discharge?
Depth of discharge is the usable share of battery capacity. An 80% value means the design allows 80% of stored energy to be used before recharging.
4) Why do efficiency values change the result?
Energy is lost while storing power in batteries and converting DC to AC. Lower efficiency means you need a larger battery bank to deliver the same usable output.
5) Why is the bank voltage sometimes rounded up?
Real battery banks use whole batteries in series. If the chosen battery voltage does not divide evenly into the target bank voltage, the tool rounds to the nearest practical series arrangement.
6) Does this calculator replace an electrician or solar designer?
No. It is a planning tool. Final sizing should also consider cable size, charging current, breaker coordination, battery temperature, appliance diversity, and local electrical rules.
7) Can I use it for off-grid and backup systems?
Yes. It works for backup planning and many off-grid estimates. For full off-grid design, also include charging source size, solar production, and seasonal energy variation.
8) How should I choose a safety margin?
Many users start with 15% to 25%. Higher margins help when loads may grow later, when wiring losses are uncertain, or when startup behavior is difficult to predict.