Calculator Inputs
The page stays single-column, while the calculator fields switch between three, two, and one column layouts by screen size.
Formula Used
Expanded Load = Running Load × (1 + Future Expansion %)
Real Output = Expanded Load × (1 + Safety Margin %)
Apparent Rating = (Expanded Load ÷ Power Factor) × (1 + Safety Margin %)
Surge Rating = [(Expanded Load − Largest Motor Load) + (Largest Motor Load × Surge Multiplier)] × (1 + Safety Margin %)
Battery Current = (Real Output ÷ Inverter Efficiency) ÷ Battery Voltage
Battery Bank kWh = [(Expanded Load × Backup Hours) ÷ 1000 ÷ Inverter Efficiency] ÷ Depth of Discharge
Battery Ah = (Battery Bank kWh × 1000) ÷ Battery Voltage
PV Array kW = Daily Energy ÷ [Peak Sun Hours × (1 − System Losses %)]
The calculator rounds the inverter recommendation up to a common market size for easier equipment selection.
How to Use This Calculator
- Choose the system type and waveform style that match your project.
- Enter the total running load that may operate at the same time.
- Enter the largest motor or compressor load that creates startup surge.
- Add daily energy use, planned backup time, and future expansion allowance.
- Enter inverter efficiency, battery voltage, usable depth of discharge, and system losses.
- Enter peak sun hours and your preferred DC/AC ratio.
- Press the calculate button to show the result above the form.
- Review the inverter class, surge demand, battery storage, PV target, graph, and design notes before choosing hardware.
Example Data Table
| Scenario | Running Load | Largest Motor | Daily Energy | Battery Voltage | Recommended Inverter | Battery Bank | Suggested PV Target |
|---|---|---|---|---|---|---|---|
| Cabin backup example | 2,200 W | 900 W | 9.6 kWh/day | 48 V | 4,000 W | 13.60 kWh | 2.95 kW |
| Small workshop example | 3,100 W | 1,500 W | 13.2 kWh/day | 48 V | 6,000 W | 18.51 kWh | 3.97 kW |
| Hybrid home backup | 4,500 W | 1,200 W | 18.0 kWh/day | 48 V | 8,000 W | 23.38 kWh | 5.11 kW |
FAQs
1. Why does the calculator use both watts and VA?
Watts describe real output power. VA includes the effect of power factor. Some loads, especially motors and electronics, can demand more apparent power than their watt rating suggests.
2. Why is surge rating important?
Motors, pumps, compressors, and some tools draw extra current for a short time at startup. If the inverter surge rating is too low, the system may trip or fail to start the load.
3. What safety margin should I use?
A margin of 15% to 30% is common for most projects. Higher margins may help where loads vary, future expansion is likely, or operating temperatures are high.
4. Why does battery voltage matter so much?
Higher battery voltage lowers current for the same power level. Lower current reduces cable size, voltage drop, heating, and stress on terminals, fuses, and busbars.
5. Should I always choose pure sine wave output?
Usually yes. Pure sine wave output is safer for appliances, chargers, motors, audio gear, and sensitive electronics. Modified sine wave can be cheaper, but compatibility is more limited.
6. What does the DC/AC ratio tell me?
It compares PV array size to inverter AC size. A moderate oversize can improve energy harvest, but too much oversizing may increase clipping during strong sunlight periods.
7. Does this replace a full electrical design?
No. It is a sizing tool for planning. Final system selection should still check cable limits, protection devices, charger ratings, battery chemistry, temperature effects, and local code rules.
8. Can I use this for off-grid and hybrid systems?
Yes. The calculator supports both styles. Off-grid users should focus on storage autonomy, while hybrid users should also verify transfer behavior, charging limits, and essential-load panel sizing.