Find the right inverter size for your off-grid, RV, or battery backup system. Calculates surge loads, power factor, DC current draw, and battery runtime — built for solar professionals and serious DIYers.
An inverter load calculator determines whether your inverter can safely and reliably power your connected AC loads — including the critical startup surge that motors and compressors draw when they first kick on. Under-sized inverters trip during motor startups, causing equipment damage, system downtime, and potentially dangerous failures.
Most online tools only ask for total watts and multiply by 1.25 — leaving out the nuances that cause real systems to fail. This calculator goes further: it calculates per-appliance surge loads, applies power factor correction for inductive equipment, computes DC current draw at your battery voltage, and estimates runtime from your battery bank. It's the depth solar professionals need, in a tool anyone can use.
Whether you're designing an off-grid cabin, sizing a van solar system, planning a home backup installation, or checking whether an existing inverter can handle new loads — this tool gives you the numbers your design depends on.
Every load type carries a specific surge multiplier. Refrigerator compressors surge to 4×, well pumps to 5×, and air compressors to 6× their running wattage. This calculator applies the correct multiplier to every appliance and identifies your worst-case simultaneous startup scenario.
Calculates the DC amps your battery bank must supply at 12V, 24V, or 48V — critical for wire gauge sizing. If you enter your battery capacity, it calculates exact runtime using your battery type's depth of discharge and inverter efficiency.
Already have an inverter? Enter its continuous and surge ratings to get a Pass / Warning / Fail assessment. The tool tells you exactly which load will cause problems and why — not just a generic "too small" message.
This tool is essential in every project that converts DC battery power to AC power. Use it at the design stage — not after equipment arrives on site.
Off-grid systems often run every major appliance through a single inverter. This is where surge load sizing is most critical. An off-grid inverter that trips on startup is a system that doesn't work. Use this calculator before specifying any inverter in an off-grid design.
Space and battery capacity are severely limited in mobile solar systems. The battery runtime calculation is critical here — running out of power miles from anywhere isn't an option. Use this tool to verify your 12V or 24V inverter can handle your compressor fridge, CPAP, and other loads.
During grid outages, homeowners rely on battery backup to keep critical loads running. The "Check My Inverter" feature is designed exactly for this: enter the inverter already in your system and get an immediate verdict on whether it handles your critical loads including all startup surges.
Select appliances from the dropdown — grouped by Kitchen, HVAC, Laundry, Electronics, Pumps, Tools, EV, and RV/Marine. Each preset auto-fills running watts, load type, and surge multiplier from industry-standard reference data.
All table fields are editable inline. Change watts if your specific appliance differs from the preset, increase quantity for multiple units, and set hours per day for accurate daily energy consumption figures.
Confirm that each appliance's load type is correct — this drives the surge multiplier. If you have the manufacturer's LRA (Locked Rotor Amps) data, you can override the surge multiplier directly in the table for a more precise calculation.
Enter your inverter efficiency (typically 92–96% for quality pure-sine inverters), battery bank voltage, and power factor for your load mix. For battery runtime, enter your bank's Ah capacity and select the battery chemistry.
If you already own or have spec'd an inverter, enter its continuous watt rating and surge rating. The tool immediately shows whether it passes, warns if you're near the limit, or flags exactly which condition causes a failure.
The results panel updates in real-time. Use the Recommended Inverter Size to spec your purchase, the DC Current figure to size your battery-to-inverter cables, and the daily kWh output to feed into battery and solar panel sizing.
The calculator produces seven distinct output values — each answers a different design question. Here's what each one means and how to use it.
Continuous load × 1.25 safety factor, rounded up to the next standard commercial inverter size. This is the minimum inverter wattage you should purchase. Never spec an inverter at exactly the calculated load — you need headroom for temperature derating and future loads.
The sum of all running watts × quantity for every appliance. This is the steady-state power demand the inverter must sustain indefinitely without overheating. It must not exceed the inverter's rated continuous output wattage.
The worst-case simultaneous demand: the highest-surge appliance starting while all other loads are already running. Your inverter's surge (peak) rating must equal or exceed this number. This is the most commonly missed sizing constraint.
Apparent power in volt-amperes, calculated as (continuous watts / power factor) × 1.25. For inductive loads, the inverter must supply more VA than the actual watt demand. Check both the Watts AND VA ratings on the inverter spec sheet.
The amps the inverter draws from the battery bank at your selected voltage. Use this number with the Wire Size Calculator to size the DC cables between your battery and inverter. Reducing battery voltage dramatically increases this current.
Hours your battery bank can power the current load at full continuous draw. Based on usable capacity (Ah × V × DoD), divided by power demand accounting for inverter efficiency. In real use, loads cycle on and off, so actual runtime is typically longer.
All calculations follow industry-standard electrical engineering principles used by professional solar system designers.
Continuous Load (W) = Σ (Running Watts × Quantity)This value must not exceed the inverter's rated continuous output wattage.Peak Surge (W) = (Largest Single Appliance Surge Watts) + (All Other Appliances' Running Watts)Surge multipliers: Resistive 1.0×. Capacitive 1.5×. Inductive (small motors) 2.5×. Inductive (motors, pumps) 4.0–5.0×. Inductive (compressors) 5.0–6.0×.Required VA = (Continuous Watts / Power Factor) × 1.25 safety factorDefault power factor: 0.85 for mixed loads. Use 0.75 for motor-heavy systems. Resistive loads: PF = 1.0.Minimum Inverter (W) = Continuous Load × 1.25Recommended Size = Round up to next standard sizeStandard sizes: 300, 500, 1000, 1500, 2000, 3000, 4000, 5000, 6000, 8000, 10000W. The 1.25× safety factor is NEC and industry standard.DC Current (A) = AC Load (W) / Battery Voltage (V) / Inverter EfficiencyExample: 2,000W load, 48V battery, 95% efficiency → 2000 / 48 / 0.95 = 43.9ARuntime (h) = (Battery Ah × Battery Voltage × DoD) / (AC Load W / Inverter Efficiency)DoD defaults: LFP/NMC = 80%, AGM/FLA = 50%. Example: 200Ah @ 48V LFP, 2000W load, 95% eff → (200 × 48 × 0.80) / (2000/0.95) = 3.65 hours.
Worked example: A homeowner wants to run: refrigerator (150W), 4 LED lights (40W), laptop (65W), TV (80W). Total continuous load: 335W. Refrigerator startup surge: 600W. Minimum inverter: 600W surge-rated, 500W continuous (add 50% safety margin). Recommended: 1,000W pure sine wave inverter. At 24V battery bank: running load draws 335W / 24V = 14A DC. Battery required for 8 hours: 14A × 8h / 0.80 DoD = 140 Ah.
Calculations sourced from SurgePV’s Inverter Load Calculator — surgepv.com/tools/inverter-load-calculator/
Reference data for common appliances in off-grid and backup solar systems. Motors and compressors produce the highest startup surges — these are the loads that most often cause inverter sizing failures.
| Appliance | Running W | Load Type | Surge × | Surge W | Hrs/Day |
|---|---|---|---|---|---|
| LED Light (10W) | 10 | Resistive | 1.0× | 10 | 6 |
| Ceiling Fan | 60 | Inductive (Small) | 2.5× | 150 | 8 |
| Laptop | 65 | Capacitive | 1.5× | 98 | 8 |
| Refrigerator (Std) | 150 | Inductive (Compressor) | 4.0× | 600 | 24 |
| Microwave (1000W) | 1,100 | Resistive | 1.2× | 1,320 | 0.5 |
| Washing Machine | 500 | Inductive (Motor) | 3.5× | 1,750 | 1 |
| Window AC (5,000 BTU) | 500 | Inductive (Compressor) | 5.0× | 2,500 | 8 |
| Window AC (10,000 BTU) | 1,000 | Inductive (Compressor) | 5.0× | 5,000 | 8 |
| Well Pump (1/2 HP) | 750 | Inductive (Motor) | 5.0× | 3,750 | 2 |
| Well Pump (1 HP) | 1,500 | Inductive (Motor) | 5.0× | 7,500 | 2 |
| Air Compressor (1 HP) | 1,500 | Inductive (Compressor) | 6.0× | 9,000 | 0.5 |
| Central AC (3 ton) | 3,500 | Inductive (Compressor) | 4.0× | 14,000 | 6 |
| Electric Water Heater | 4,000 | Resistive | 1.0× | 4,000 | 3 |
* Surge watts shown for a single unit. Multiply by quantity. Actual surge values vary by manufacturer — use LRA data from nameplate when available.
The #1 cause of inverter failure in off-grid systems. A refrigerator compressor starting while a well pump is running can produce a surge load 3–4× the continuous load. Always calculate peak surge, not just running watts.
A 2,000W load at 12V draws 175A — requiring #1/0 AWG or larger cable. The same load at 48V draws only 44A. For any inverter system over 1,500W, use 24V minimum. For residential systems, 48V is the industry standard.
Inverters derate in hot environments — a unit rated at 3,000W at 25°C may deliver only 2,400W at 45°C. Always size to 75–80% of rated continuous output. The calculator's 1.25× factor accounts for this exactly.
Modified sine wave inverters cause compressors to run hot, refrigerators to fail to start, and digital controls to malfunction. For any system running motors or appliances with electronic controls, always specify a pure sine wave inverter.
Continuous watts is the power an inverter can supply indefinitely. Surge watts (peak watts) is a higher level the inverter can sustain for a short burst — typically 5–20 seconds. Motors and compressors draw 3–7× their running wattage at startup. A refrigerator rated at 150W running might need 600–700W for 1–2 seconds when the compressor kicks on. Your inverter must handle both ratings, or it will trip.
The standard recommendation is to size your inverter at 125% (1.25×) of your total continuous load as a safety margin for unexpected additions and efficiency losses. Additionally, the inverter’s surge capacity must handle the highest simultaneous surge demand — typically when your largest motor starts while other loads are already running. Never size at exactly the calculated load with no margin.
A standard household refrigerator draws 100–200W running but needs 400–700W when the compressor starts. A 1,000W continuous inverter might have a 2,000W surge rating — enough for a modern efficient refrigerator. To be certain: find your refrigerator’s LRA (Locked Rotor Amps) on its label, multiply by AC voltage (120V), and compare to your inverter’s surge rating. Pure sine wave inverters handle refrigerators better than modified sine wave.
Watts measures real power — the work actually done. VA (volt-amperes) measures apparent power. For resistive loads like heaters and lights, watts and VA are equal (power factor = 1.0). For inductive loads like motors and air conditioners, the power factor drops below 1.0, meaning the inverter must supply more VA than the load’s watt rating suggests. A 1,000W motor with a power factor of 0.8 requires 1,250 VA from the inverter.
Battery voltage determines DC current on the battery side. A 1,500W AC load through a 95% efficient inverter draws 131A from a 12V battery, 65A from 24V, and only 33A from 48V. High current requires expensive, heavy-gauge wire and causes more heat and losses. For any system over 1,500W, 24V is the minimum. For systems over 3,000W, 48V is standard.
Battery runtime formula: Runtime (hours) = (Battery Ah × Voltage × Depth of Discharge) / (AC Load Watts / Inverter Efficiency). For LFP batteries use 80% DoD; for lead-acid use 50%. Example: 200Ah @ 48V LFP (80% DoD), 2,000W load, 95% efficiency: Runtime = (200 × 48 × 0.80) / (2000 / 0.95) = 3.65 hours.
Pure sine wave inverters produce AC power identical to grid power. Modified sine wave can cause problems with refrigerators, compressors, digital controls, dimmer switches, and medical equipment. For any off-grid solar system that runs standard household appliances, always use a pure sine wave inverter. Modified sine wave is only acceptable for very simple resistive loads.
Yes, but it requires careful sizing. A 10,000 BTU window AC draws about 1,000W running but may surge to 4,000–6,000W on startup. A mini-split is more efficient — 800–1,200W running with a 3,000–5,000W surge. To run AC reliably: use a dedicated 3,000W+ inverter, ensure your battery bank is large enough, and consider a soft-starter device (like the MicroAir EasyStart) which can reduce AC startup surge by 50–70%.
Inverter sizing is step one of a four-step design process. Use these tools to complete the full system — each accepts outputs from this calculator directly.
Size your battery bank for off-grid or backup solar storage.
Determine the right solar system size for your energy needs.
Calculate the right solar system size based on energy usage.
Design optimal PV string configurations for any inverter.
Size AC breakers and conductors for solar inverter output.
Find the correct AWG gauge for solar AC and DC circuits.
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