Enter your inputs
Example data table
| Appliance | Qty | Running (W) | Surge (W) | Hours/day |
|---|---|---|---|---|
| LED Lights | 10 | 12 | 12 | 6 |
| Ceiling Fan | 4 | 70 | 140 | 10 |
| Refrigerator | 1 | 180 | 900 | 10 |
| LED TV | 1 | 110 | 110 | 5 |
| Router | 1 | 15 | 15 | 24 |
| Water Pump | 1 | 750 | 1500 | 1 |
Formula used
- Total running watts = Σ (Qty × Running W).
- Startup extra watts = Σ (Qty × Surge W − Qty × Running W), floored at 0.
- Adjusted surge needed = Running + Concurrency × Startup Extra.
- Inverter continuous = Running × (1 + Headroom) ÷ Inverter Efficiency.
- Inverter kVA = Inverter W ÷ Power Factor.
- Battery Wh (nominal) = (Running × Backup Hours) ÷ (Inverter Eff × Battery Eff × DoD).
- Battery Ah = Battery Wh ÷ System Voltage.
- PV watts = Daily Wh ÷ (Peak Sun Hours × PV Derate).
How to use this calculator
- List appliances with quantity, running watts, and surge watts.
- Add realistic “Hours/day” to estimate daily energy.
- Set backup hours for the outage duration you want.
- Choose battery type and system voltage for your inverter.
- Enter peak sun hours and derate for your location.
- Review results, then download CSV or PDF for sharing.
Load inventory quality
Accurate appliance lists reduce oversizing and nuisance trips. A household with 900 W running load and 2.2 kW adjusted surge can often use a 1.5–2.0 kW class inverter, depending on efficiency and headroom. Enter each device’s steady watts, then verify with a plug meter when possible. Small errors compound when you scale quantity, especially for lighting and fans.
Surge behavior and motor starts
Motors draw higher startup power for fractions of a second. This calculator separates running watts from “startup extra” watts and applies a concurrency factor. If only one compressor starts at a time, 40–60% concurrency is realistic; if multiple pumps can start together, move toward 80–100%. For refrigerators, 3× to 6× surge is common, while resistive loads stay near 1×.
Autonomy and battery sizing
Backup hours translate directly into battery energy. For example, 1.2 kW for 4 hours equals 4.8 kWh of load energy. After inverter and battery losses, and after limiting depth of discharge, the nominal bank must be larger. A 48 V system reduces current for the same power and can simplify cable sizing and losses for medium loads.
Solar production assumptions
Daily watt‑hours drive PV sizing using peak sun hours and a derate factor. With 6.0 kWh/day demand, 5.0 peak sun hours, and 0.75 derate, the PV target is about 1.6 kW. Derate captures temperature, dust, wiring, and inverter MPPT efficiency. If shading exists, derate may drop below 0.70 and panel count should increase accordingly.
Cost, savings, and payback
The finance section estimates inverter, battery, and PV costs, then adds fixed balance‑of‑system and installation markup. Savings use tariff, daily kWh, and an offset percentage that reflects how much energy solar replaces. Payback is a simple ratio, so it improves when tariffs rise, when solar offset increases, or when battery replacement cycles are optimized. In practice, compare two configurations: a smaller inverter with higher efficiency and a larger inverter with lower utilization. Track battery kWh per cycle and set replacement reserves. If annual savings are PKR 200,000 on a PKR 1,000,000 system, the simple payback is 5.0 years, quarterly. for planning only.
FAQs
1) Should I size the inverter by running watts or surge?
Use both. Running watts set the continuous rating, while surge covers motor starts. This calculator models surge with a concurrency factor, then adds headroom and efficiency to reduce overload risk.
2) Why does battery capacity increase after I add efficiency and DoD?
Your load energy must pass through conversion losses and must stay within usable depth of discharge. Lower efficiency or lower DoD means more nominal kWh is required to deliver the same backup hours.
3) What system voltage should I choose?
Higher voltage reduces current for the same power, which helps cable size and losses. For small setups, 12–24 V may work. For medium loads, 48 V is typically more practical.
4) How do I estimate surge watts if I do not know them?
Start with 2.5× running watts for mixed motor loads, then refine using nameplate LRA/starting current or a clamp meter. Refrigerators and pumps can be higher; resistive heaters are usually near 1×.
5) What does PV derate represent?
Derate combines real‑world losses: temperature, dust, wiring, module mismatch, and MPPT conversion. If your site is hot and dusty or shaded, use a lower derate to avoid under‑sizing panels.
6) Is the payback result a guaranteed return?
No. It is a simple estimate based on your tariff and assumed solar offset. Actual results depend on usage patterns, battery life, maintenance, grid availability, and system performance over time.