Continuous Rating · Surge Capacity · DC Current Draw · LiFePO4 Voltage Window
An undersized inverter trips under motor starting loads. An oversized inverter runs cold but wastes money on standby losses. This calculator finds the correct minimum continuous rating and peak surge capacity for your load profile, calculates DC current draw from your battery bank, and generates the full LiFePO4 voltage window with setpoints for your MPPT, inverter, and BMS. Built for real off-grid installations in Nigeria and West Africa.
How to use this calculator
Step 01
Build your load list
Add every appliance you plan to run simultaneously. Assign the correct load type — resistive, motor, compressor, or air conditioning. Surge multipliers are applied automatically based on load type.
Step 02
Set system parameters
Choose your battery bank voltage (12V, 24V, or 48V), inverter efficiency, safety margin, and ambient temperature. The calculator applies temperature derating if the installation location exceeds 40°C.
Step 03
Select your inverter model
Choose from the Victron Multiplus-II and generic model list, or enter custom continuous and surge ratings. The calculator cross-checks your selection against both the continuous and surge requirements.
Step 04
Check the voltage window
Switch to the Voltage Window tab and select your battery chemistry and system voltage. Get the full setpoint hierarchy — MPPT absorption, inverter cutoffs, and BMS protection levels — in one table.
Off-Grid Inverter Sizing Calculator
Continuous rating · Surge capacity · DC current draw · Voltage window
Eneronix
Your Energy, Our Precision
1 Load & Inverter Sizing
2 Voltage Window
Step 1 — Appliance load list
Appliance
Running W
Load type
Surge W (auto)
Qty
Hrs/day
Wh/day
Total simultaneous load
Sum of running watts × qty
0 W
0 Wh/day
Step 2 — System parameters
93%
25%
35°C
Inverter Sizing Result
Load breakdown
Appliance
Type
Run W
Surge W
Wh/day
% load
Surge analysis
Engineering summary
LiFePO4 battery voltage setpoints must be correctly coordinated between the MPPT, inverter, and BMS. Incorrect setpoints cause premature low-voltage cutoffs, failed charging cycles, or BMS protection latching that requires manual recovery. Enter your battery configuration to get the full recommended voltage window.
The continuous rating is the maximum power the inverter can deliver indefinitely without overheating. The surge rating is a brief peak — typically for 3–10 seconds — to start motor loads. A 5000VA inverter with a 10000W surge rating cannot sustain 10000W continuously.
Surge multipliers by load type
Resistive and electronic loads (lights, TVs, computers) draw minimal or no surge. Induction motors (fans, pumps) draw 2.5× running watts at start. Compressors and refrigerators draw 3×. Air conditioning units are the hardest — 3.5× or more. A soft starter on your AC reduces inrush by 40–60%.
Temperature derating
Most inverters are rated at 25°C. Above 40°C — common inside enclosures in Nigeria — the inverter dereates linearly. An inverter mounted in a sealed metal cabinet in direct sun can experience 55–60°C internally, reducing usable capacity to 65–70% of nameplate. Forced ventilation is not optional.
LiFePO4 voltage setpoint hierarchy
The critical rule: MPPT absorption voltage must sit below the inverter high-voltage cutoff, which must sit below the BMS high-voltage protection threshold. If the inverter trips the AC output at the same voltage the BMS disconnects, the BMS latches and requires manual reset. Get the hierarchy right at commissioning.
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