Inverter Sizing Calculator — Free Online Calculator
Calculate the inverter wattage needed for your loads. Accounts for surge requirements and continuous power.
How to Use This Calculator
Enter your total continuous load and the largest motor/compressor wattage for surge calculation.
The Formula Explained
Continuous rating = Total load × 1.25 safety factor. Surge rating = Continuous load + (Largest motor × 3 for startup surge). Select an inverter that meets both requirements.
Inverter Sizing: The Heart of a Solar System
The inverter is the most critical component in a grid-tied solar system. It converts DC power from the panels into AC power compatible with the grid, and its sizing directly determines both the cost of the system and how much energy you actually capture from sunlight. Too small an inverter clips peak production on sunny days; too large wastes money on unused capacity and operates less efficiently at partial loads. The optimal sizing depends on the panel count, their tilt and orientation, local climate, and the intended use of the system.
Modern string inverters come in standard sizes: 3, 5, 6, 7.6, 10, 11.4, and 15 kW for residential. Commercial three-phase inverters scale to 100+ kW. Microinverters are typically 240-400 watts per unit, matching one panel each. The sizing decision starts with the panel array you want to install, then selects an inverter that handles the peak DC output with an appropriate DC-to-AC ratio. Inverter datasheets specify maximum DC input voltage, maximum DC input current, maximum PV input power, and continuous AC output.
Worked Example: 10 kW Residential System
A homeowner installs 25 panels at 400W each = 10,000W DC nameplate. What size inverter?
Target DC-to-AC ratio: 1.20. Inverter size = 10,000 / 1.20 = 8,333W. The closest standard inverter is 7.6 kW or 10 kW. Use the 7.6 kW inverter for optimal ratio (1.32) or the 10 kW for zero clipping (ratio 1.0 — undersized on DC side).
Clipping analysis for the 7.6 kW inverter: panels produce above 7.6 kW only on cold, clear days near solar noon. Average annual clipping is about 2.5% for a ratio of 1.32 in sunny climates. Total annual production: 10 kW × 1,600 sun-hours × 0.85 system efficiency × 0.975 (clipping loss) = 13,260 kWh, versus 13,600 kWh unclipped. The 340 kWh per year loss equals about 55 USD at 16 cents per kWh. The 7.6 kW inverter costs 1,500 USD less than the 10 kW, so payback on the smaller inverter is immediate with minimal ongoing cost.
Worked Example: Multi-Orientation Array
A homeowner has a complex roof: 12 panels on the south face (ideal orientation), 8 panels on the east face (lower production), and 5 panels on a west-facing dormer (some afternoon shade).
Option A: single string inverter. All 25 panels on one inverter, string sized for the worst-case panel (shaded west dormer). The mismatched orientations cause 8-12% annual loss because the string inverter optimizes for the average orientation.
Option B: three string inverters (one per roof orientation). Each inverter optimizes its own string. Cost is about 50% higher than option A but captures 95% of theoretical maximum.
Option C: microinverters (one per panel). Each panel operates independently at its own maximum power point. Captures 97% of theoretical maximum. Cost is about 2x option A but simplifies permitting and allows future panel-level monitoring.
For this roof, option C (microinverters) is the typical recommendation because the per-panel flexibility pays back over 25 years through higher production and easier maintenance.
Common Inverter Sizing Mistakes
1. Undersizing without considering clipping. A 1.4 ratio sounds efficient but loses 4-6% annual production to clipping. Economics depend on the cost difference between inverter sizes.
2. Oversizing the inverter for "future expansion". Running a 10 kW inverter with 5 kW of panels operates it at very low load where efficiency is poor. Buy the right size now; add a second inverter when you expand.
3. Mixing string inverters and microinverters on the same array. They cannot share monitoring, permitting is more complex, and aesthetics suffer. Pick one architecture and stick with it.
4. Ignoring maximum DC voltage. Cold weather increases panel Voc. A string of 12 panels at Voc 45V nominal is 540V, but at -20°C the Voc increases about 20% to 650V. If your inverter max input is 600V, the cold-weather Voc exceeds the limit and damages the inverter. Calculate cold-weather Voc using NEC temperature coefficients before finalizing string length.
5. Forgetting rapid shutdown requirements. NEC 690.12 requires rapid shutdown on rooftop PV systems. Ensure your inverter plus combiner or optimizer solution complies. Most modern inverters include rapid shutdown compliance but verify before purchase.
Inverter Types Quick Comparison
String Inverters — One central inverter per array. Lowest cost, highest efficiency (97-98%), best for simple south-facing arrays. Drawbacks: whole string affected by shading of any panel, no panel-level monitoring without added optimizers. Typical brands: SMA, Fronius, SolarEdge (with optimizers), Growatt.
Microinverters — One small inverter per panel. Higher cost (about 30-50% premium), slightly lower efficiency (95-96%), best for complex roofs or shaded installs. Each panel operates independently. Warranty often 25 years vs 10-12 for string. Typical brands: Enphase, APsystems.
Power Optimizers — DC-DC optimizers at each panel feeding a string inverter. Middle-ground cost and efficiency. Panel-level monitoring without the full microinverter cost. Typical brands: SolarEdge, Tigo.
Hybrid Inverters — String inverters with integrated battery charge control. Ideal for homes planning to add batteries. Tesla Powerwall, Enphase IQ Battery, and LG Chem work with specific inverter partners.
NEC and Standards for Solar Inverters
NEC 690.8 covers circuit sizing for PV systems including the inverter DC input calculations. NEC 690.9 covers overcurrent protection at the inverter output. NEC 690.12 requires rapid shutdown capability for rooftop PV systems — the inverter solution must include approved rapid shutdown initiator.
UL 1741 is the safety standard for grid-tied inverters. UL 1741 SA adds advanced grid support functions. IEEE 1547 defines the interconnection and interoperability requirements. Any inverter sold in the US must carry UL 1741 listing. For utility interconnection, the inverter must also be on the utility approved equipment list, which typically requires both UL 1741 and IEEE 1547 compliance.
Inverter sizing: continuous, surge, and the dual-rating problem
Inverter sizing has two numbers: continuous wattage (sustained load) and surge wattage (peak for motor starts). A 3000 W continuous inverter typically surges to 6000 W for a few seconds. Sizing has to cover both: the steady-state load AND the worst-case motor start.
The formula and what it does
Add up all simultaneous loads in watts. Identify the largest motor (well pump, AC compressor, refrigerator) and check its locked-rotor amps. Surge requirement is continuous load plus motor LRA in VA. Inverter surge rating must exceed this.
Worked example
Scenario: Off-grid cabin: fridge (700 W start, 150 W run), microwave (1200 W), well pump (3500 W start, 800 W run), lights and TV (200 W).
Steady-state worst-case: fridge run + microwave + lights = 1550 W. Surge worst-case: above plus well-pump LRA = 1550 - 800 (no run while starting) + 3500 = 4250 W needed at surge. Pick inverter: Victron MultiPlus 3000 (3000 W cont, 6000 W surge) handles both easily. Smaller 2400 W cont / 4500 W surge inverter would also work but with no headroom.
Code references and standards
NEC 705 grid-interactive inverter requirements: anti-islanding, disconnects, system labels.
NEC 706 ESS-paired inverters: rapid shutdown, working space, conductor sizing at inverter rating.
Common mistakes to avoid
undefinedFrequently asked questions
What is the difference between pure sine and modified sine inverters?
Pure sine outputs a clean 60 Hz waveform like the grid. Modified sine (square-wave or stepped) is cheaper but can damage some electronics, motors, and HVAC. Use pure sine for anything except simple resistive loads.
Do I size inverter to peak or average?
Continuous rating must cover sustained simultaneous load. Surge rating must cover the worst startup transient. Most inverters surge 2x continuous for 3-5 seconds.
Can I parallel inverters?
Yes, most modern inverters (Victron MultiPlus II, Outback Radian, Sol-Ark, EG4) support parallel operation up to 3 or 6 units in parallel for larger systems. Some support 3-phase output from 3 single-phase inverters.
What is the efficiency of a typical inverter?
Modern high-frequency inverters: 90-95 percent at moderate loads. Low-frequency transformer inverters: 92-96 percent, with better surge capacity and slightly lower idle draw at low loads.
How does idle draw affect off-grid systems?
A 3 kW inverter typically idles at 20-50 W. Over 24 hours that is 0.5-1.2 kWh, significant for a small off-grid system. Some inverters have search mode that wakes up only when a load is detected.
Hybrid inverter or separate solar/battery inverter?
Hybrid (Sol-Ark, EG4, Schneider XW) integrates solar, battery, and grid management. Simpler install, single point of failure. Separate inverters (Enphase + ESS, SMA + Tesla) more flexible but more equipment.