Off-Grid Inverter Sizing — How Much Power Do You Actually Need
10 min read
Sizing an off-grid inverter incorrectly is one of the most common and costly mistakes in off-grid system design. Oversize and you waste money on a device that operates permanently in its low-efficiency zone. Undersize and your inverter trips every time a motor starts, or simply cannot sustain your loads. Correct sizing requires understanding three distinct figures — your continuous load, your peak surge demand, and your daily energy consumption — and sizing each component of the system accordingly.
The three numbers you need
Off-grid sizing starts with three distinct load figures. Each one sizes a different component of the system:
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Continuous load (W)
The total wattage of all devices that may run simultaneously. Sizes the inverter continuous rating. Add up running watts of all devices that could be on at the same time.
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Peak / surge demand (W)
The highest instantaneous demand including motor startup surges. Sizes the inverter peak rating. Typically 3–7× the running watts of your largest motor.
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Daily energy (kWh/day)
Total energy consumed per day. Sizes the battery bank and solar array. Watts × hours of use per day for each device, summed.
The most common mistake is conflating continuous power with daily energy. A 200 W refrigerator running 24 hours/day consumes 4.8 kWh/day — far more than a 2000 W kettle used for 10 minutes (0.33 kWh). The kettle requires a bigger inverter; the refrigerator requires a bigger battery.
Off-grid system architecture
Complete off-grid system architecture. Each component is sized independently: solar panels for daily energy production, battery for storage and autonomy, MPPT controller for charge current, inverter for peak AC power demand.
Complete off-grid sizing calculator
Build your device list, set your system parameters, and get full sizing recommendations for inverter, battery and solar array:
⚡ Off-Grid System Sizing Calculator
Step 1 — load preset or build custom list
Step 2 — devices (name / watts / qty / hours/day)
DeviceWattsQtyHrs/day
Step 3 — system parameters
Battery sizing — step by step
Battery sizing is often the most complex part of off-grid design. Click each step to see the calculation method and a worked example:
Solar panel sizing
The solar array must produce enough energy on an average day to supply the loads and recharge the battery. The key variables are daily energy consumption, peak sun hours, and system efficiency losses.
Solar array sizing formula:
Required array (Wp) = Daily consumption (Wh) ÷ ( Peak sun hours × System efficiency )
Example: 5000 Wh/day load, 4 peak sun hours, 80% system efficiency:
5000 ÷ (4 × 0.80) = 5000 ÷ 3.2 = 1562 Wp → round up to 1600 Wp (e.g. 4 × 400 W panels)
What counts as a system loss?
System losses include: battery charging and discharging losses (typically 5–10% for lithium, 15–20% for lead-acid), inverter conversion losses (2–5%), wiring and connection losses (1–3%), and soiling or shading losses on panels (2–5%). A conservative total of 20% (0.80 efficiency factor) is reasonable for a well-designed lithium system; 25% for lead-acid.
Sizing for the worst month, not the average
Solar systems should be sized for the worst solar month of the year in your location — usually December in the Northern Hemisphere. If you size for the annual average, your system will be under-powered in winter when days are shortest. Resources like PVGIS (for Europe and Africa) and PVWatts (for North America) provide monthly peak sun hour data for any location.
Panel orientation and tilt: A solar panel facing true south (in the Northern Hemisphere) at a tilt angle equal to your latitude produces the best annual average yield. Deviations reduce yield — a west-facing panel produces approximately 15–20% less than a south-facing equivalent. For off-grid systems, maximise tilt angle in winter to improve low-season performance at the cost of some summer production.
Rules of thumb and sanity checks
These rules provide quick sanity checks for any off-grid sizing calculation:
Inverter continuous rating ≥ load × 1.25
25% headroom ensures the inverter operates below its maximum rating for extended periods, improving reliability and efficiency. Do not size the inverter to exactly your running load.
Inverter peak rating ≥ largest motor surge × 1.5
Motor startup surges are not always precisely predictable — a 50% margin above the expected surge prevents nuisance tripping on cold starts or worn motor bearings.
Battery capacity (kWh) ≥ daily consumption × autonomy days ÷ DoD
This is the fundamental battery sizing equation. Divide by DoD because you should never discharge the battery to zero — partial discharge protects battery lifespan.
Solar array (Wp) ≥ daily consumption ÷ (PSH × 0.8)
Using 0.8 (80% efficiency) accounts for typical system losses. Replace 0.8 with your actual estimated efficiency for more precise sizing.
Use 48 V for all systems above 1.5 kW
At 48 V, a 3 kW system draws 62.5 A. At 12 V, the same system draws 250 A — requiring much heavier cables, larger fuses, and causing significantly higher I²R losses.
Converts the energy-based calculation into the Ah capacity figure you will see on battery datasheets. Use the nominal voltage of your battery bank for this calculation.
Revisit your sizing after installation: Actual consumption almost always differs from the design estimate. After the first month of operation, review your energy monitor logs and compare actual daily consumption against your design figure. Most systems need one adjustment cycle — usually increasing battery capacity or solar array — before the design is fully optimised.
Next in this series
Single-phase vs three-phase inverter — which do you need?
