Solar Inverter Sizing Calculator
Continuous VA rating, motor-starting surge rating, and DC:AC array ratio — all in one place.
Load-Based Sizing
DC:AC Array Ratio
Continuous VA = (Load/PF)×(1+Margin) • Surge VA = (Other Loads + Motor×Surge Factor)/PF
1200W general load only, PF=0.8, 20% margin
500W other + 150W fridge (3× surge)
Inverter AC Rating = Array Wp / DC:AC Ratio
1980Wp array, ratio 1.10-1.30
3000Wp array, ratio 1.15-1.25
Three Different Inverter Sizing Questions
"What size inverter do I need?" is really three separate questions, and skipping any one of them is a common cause of an inverter that trips, shuts down, or underperforms:
- Continuous VA: can the inverter run all your simultaneous loads indefinitely? VA (not just watts) matters because real loads have a power factor below 1, so the inverter's apparent-power (VA) demand is higher than the simple wattage sum.
- Surge (starting) VA: can the inverter survive the brief but large inrush current when a motor-driven load (fridge compressor, water pump, air conditioner) starts? This spike is often 2–6× the appliance's running wattage for a fraction of a second, and undersizing here causes the classic "inverter trips the moment the fridge kicks in" failure.
- DC:AC array ratio (grid-tie/hybrid systems): is the inverter's AC output rating sensibly matched to the DC array feeding it? Oversizing the array relative to the inverter (DC:AC > 1) is standard practice, since the array rarely produces its full rated wattage simultaneously across all conditions — but too large a mismatch either clips available energy or wastes inverter capacity.
| Quantity | Formula |
|---|
| Continuous VA rating | (Total Load W / Power Factor) × (1 + Safety Margin) |
| Surge VA rating | (Other Running Loads + Motor Running W × Surge Factor) / Power Factor |
| Typical motor surge factor | 2–3× for fridges/pumps, up to 4–6× for some AC compressors |
| Typical DC:AC ratio | 1.10–1.30 (array oversized relative to inverter AC rating) |
Once you know your required inverter rating, cross-check it against your battery bank's discharge current capability (for off-grid systems) and your array's total wattage to make sure every part of the system is consistently matched.
Real-World Applications & Fully-Explained Examples
- Off-grid home inverters: sizing for a household's full simultaneous load.
- Refrigerator/freezer circuits: the classic motor-surge sizing problem.
- Water pump systems: another common high-surge-factor motor load.
- Grid-tie residential solar: matching inverter AC rating to a home's rooftop array.
- Commercial/hybrid inverter design: balancing DC array cost against AC inverter cost.
- RV/mobile inverters: sizing for a mix of general loads and one larger motor appliance.
Worked examples — explained in full
1. 1200 W general (non-motor) load, PF=0.8, 20% margin. Continuous VA=(1200/0.8)×1.20=1500×1.20=1800 VA — with no motor loads, this is simply the continuous rating needed, since there is no separate surge to check.
2. 500 W other loads + a 150 W fridge with a 3× starting surge, PF=0.8. Surge VA=(500+150×3)/0.8=(500+450)/0.8=950/0.8=1187.5 VA — the brief starting spike alone demands nearly 1.2 kVA, even though total running wattage is only 650 W.
3. The same combined running load (650 W) checked as a plain continuous rating, PF=0.8, 20% margin. Continuous VA=(650/0.8)×1.20=812.5×1.20=975 VA — noticeably lower than example 2's 1187.5 VA surge figure. In this case the surge rating governs the final inverter choice, since the inverter must survive the fridge's startup moment, not just its steady running state.
4. Sizing an inverter for a 1980 Wp array with a 1.10–1.30 DC:AC ratio. Minimum inverter rating=1980/1.30≈1523 W, maximum=1980/1.10≈1800 W — a reasonable inverter choice falls somewhere in this 1523–1800 W AC range for this array size.
5. A larger 3000 Wp array with a tighter 1.15–1.25 ratio range. Minimum=3000/1.25=2400 W, maximum=3000/1.15≈2609 W — a narrower, more centered inverter selection window than example 4's wider ratio range.
6. Why the surge rating can matter even when continuous load looks modest. Comparing examples 2 and 3 directly: the identical 650 W of running load needs only 975 VA continuously, but a 3× motor-starting surge pushes the true requirement to 1187.5 VA — about 22% higher than the continuous-only estimate would suggest, exactly the kind of gap that causes an underspecified inverter to trip in real use.
Frequently Asked Questions
How do I calculate the inverter size I need for my solar system?
Calculate both the continuous VA rating ((Total Load/Power Factor)×(1+Margin)) and the surge VA rating ((Other Loads + Motor Watts×Surge Factor)/Power Factor), then choose an inverter rated at or above the larger of the two numbers.
Why does the inverter rating use VA instead of watts?
VA (volt-amps, apparent power) accounts for a load's power factor, while watts is real power alone. Since Power Factor = Watts/VA, and PF is typically below 1 for motor-driven and reactive loads, the actual apparent power the inverter must supply (VA) is higher than the simple wattage sum.
Why does my inverter trip when the fridge starts, even though it handles my other loads fine?
Motor-driven appliances draw a brief but large inrush ("starting surge") current when they start, often 2-6× their normal running wattage. If the inverter's surge/peak rating is not high enough to cover this spike, it can trip or shut down at that exact moment, even if its continuous rating easily covers the appliance's steady-state running wattage.
What surge factor should I use for common appliances?
Refrigerators and small pumps commonly need 2-3× their running wattage at startup; some larger pumps, air conditioner compressors, and power tools can need 4-6× or more — always check the specific appliance's datasheet or manufacturer specification for its actual starting current if precise sizing matters.
What is the DC:AC ratio and why is it usually above 1?
It is the ratio of a solar array's total DC wattage rating to the inverter's AC output rating. Because an array rarely produces its full rated wattage simultaneously (due to temperature, angle, partial cloud, and time-of-day effects), a modestly oversized array (DC:AC of 1.10-1.30) typically produces more usable annual energy per dollar of inverter cost than a 1:1 matched system.
What happens if my DC:AC ratio is too high?
On the sunniest, coolest days when the array approaches its full rated output, the inverter may "clip" — limiting output to its own maximum AC rating and wasting the excess available DC power — though for most real-world conditions and moderate ratios (up to about 1.3), the annual energy lost to clipping is small relative to the benefit of the extra array capacity.
What power factor should I use if I don't know my exact loads?
0.8 is a common conservative assumption covering a typical mix of resistive and motor-driven household loads; purely resistive loads (heaters, incandescent bulbs) are close to PF=1.0, while motor-heavy loads can be lower, so check individual appliance nameplates for precise power factor if available.
Should I add margin on top of my calculated continuous VA rating?
Yes, a 15-25% safety margin above the bare calculated minimum is standard practice, covering load growth, voltage sag under load, and general safety headroom — the calculator above already includes an adjustable margin input for this.
Do grid-tied inverters need surge sizing too?
Less so in the traditional sense, since grid-tied inverters typically do not directly power household motor loads the way an off-grid inverter does (the grid itself supplies any surge current) — surge sizing matters most for off-grid and hybrid inverters that directly serve the home's AC loads.
How do continuous and surge inverter ratings relate to my battery bank?
The inverter draws its DC input current from the battery bank at both continuous and surge power levels, so the battery bank and its wiring/fusing must also be able to safely deliver the corresponding DC current — check your battery and cabling are rated for at least the inverter's maximum surge current, not just its continuous rating.
Can one inverter serve both continuous appliances and a motor load simultaneously?
Yes, that is the normal case, and this calculator's surge calculation already accounts for it by adding the motor's surge demand on top of the other simultaneously-running loads, since both draw from the inverter at the same moment the motor starts.