Solar Inverter Calculator

Enter your appliances and get the right inverter size — continuous watts, surge rating, and DC current requirements.

Appliances

Appliance	Running W	Surge ×	Qty

Simultaneous usage?

System voltage?

Inverter sizing recommendation

500W continuous

Total running load430 W

Simultaneous load323 W

Surge rating needed700 W

Max DC current20.8 A @ 24V

Choose an inverter rated at least 500W continuous with a peak/surge rating of at least 700W. Size up to the next standard size (e.g., 500W) for headroom and longevity.

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How to Use This Calculator

Enter your appliances

Add every appliance you plan to run through your inverter. Use the table to enter the running watts (normal operating power), surge multiplier (how many times running watts the motor draws at startup), and quantity. The running wattage is on the appliance nameplate or in its manual. If you don't know it, use our appliance wattage reference below.

Set simultaneous usage %

The simultaneous usage field controls what fraction of your total load runs at the same time. In a home, you rarely run everything at once — 60-75% is realistic. For a workshop with one machine running at a time, try 40-50%. For an RV where most things run together, use 80-90%.

Choose system voltage

Match the system voltage to your battery bank: 12V for small RV and van builds, 24V for mid-size off-grid systems, 48V for home backup and larger off-grid installations. Higher voltage means lower current draw at the same power, which allows thinner, cheaper wiring.

Read the results

The calculator outputs your minimum continuous inverter rating, recommended size with 25% headroom, and the surge/peak rating needed to handle motor start-up spikes. Always buy an inverter with a surge rating above the calculated surge requirement.

The Formula

Total running load = Sum of (appliance watts × qty) Simultaneous load = Total running load × simultaneous usage % Recommended continuous = Simultaneous load × 1.25 (25% headroom) Surge required = Largest single surge + remaining simultaneous load Max DC current = Recommended watts ÷ system voltage (V)

The 25% headroom on the continuous rating protects the inverter from running at 100% capacity continuously, which shortens its lifespan and generates excessive heat. Most inverter manufacturers derate their units at high temperatures — a 3,000W inverter at 40°C ambient may only deliver 2,400W safely.

The surge rating is separate from continuous rating. A motor (refrigerator, AC, pump, power tool) draws 3-7× its running watts for 0.5-2 seconds at startup. Your inverter's peak/surge specification must exceed this spike or it will trip under load.

Common Appliance Wattages

Refrigerator (standard)100–200W running, 3× surge

Window AC (5,000 BTU)500W running, 3–4× surge

Window AC (10,000 BTU)1,000W running, 3–4× surge

Microwave900–1,500W running, 1.5× surge

Washing machine400–600W running, 3× surge

Well pump (1/2 HP)750W running, 5× surge

Air compressor (1 HP)1,500W running, 5–7× surge

Table saw (10")1,500–2,000W running, 3× surge

TV (55")100–130W running, 1.5× surge

LED lights (per fixture)10–20W running, 1× surge

Laptop45–90W running, 1× surge

Phone charger10–25W running, 1× surge

Example

Home essentials backup system — Portland, OR

The Garcias want to power their refrigerator (150W), 8 LED lights (80W), TV (120W), and laptop (60W) during outages. They're building a 24V lithium battery system.

Total running load410 W

Simultaneous usage75%

Simultaneous load308 W

Recommended continuous385 W (round up to 500W)

Largest surge (fridge 3×)450 W peak

Surge rating needed~700 W peak

System voltage24V

A 1,000W inverter with 2,000W surge rating is the right choice — it's the next standard size above the 500W minimum and easily handles the surge. The Garcias have comfortable headroom for adding small appliances later. At 24V, peak DC current is only 41A, requiring modest wire sizing.

FAQ

What is the difference between continuous and surge (peak) watts? ▼

Continuous watts is the power an inverter can deliver indefinitely. Surge or peak watts is the higher power level it can handle for a short burst — typically 1-2 seconds — to start motors. A refrigerator compressor, AC unit, pump, or power tool motor draws 3-7× its running watts at startup. If your inverter's surge rating is too low, it will shut off or blow a fuse when the motor starts. Always size your inverter so its surge rating exceeds the largest motor startup load in your system.

Should I get a pure sine wave or modified sine wave inverter? ▼

Pure sine wave inverters produce AC power identical to the grid and work with all appliances including sensitive electronics, variable speed motors, and medical devices. Modified sine wave inverters are cheaper but can cause problems with some motors (run hotter and louder), LED dimmers, some battery chargers, and audio equipment. For any serious solar system, choose pure sine wave. Modified sine wave only makes sense for very basic loads (simple power tools, incandescent lights) on an extremely tight budget.

How do I size an inverter for an air conditioner? ▼

Air conditioners have high surge demands — a 1,500W window unit can draw 4,500-6,000W at startup. For a 1,500W AC running load, you need an inverter rated at least 2,000W continuous with a surge rating of 6,000W or more. Many standard inverters cannot handle this. Look specifically for inverters marketed for AC units or with high surge ratings (2× to 3× continuous). Mini-split inverter AC units have much lower surge requirements and are more inverter-friendly.

What voltage system should I use — 12V, 24V, or 48V? ▼

12V is standard for small RV and van builds under 1,000W — most 12V accessories work directly. 24V suits systems from 1,000-3,000W and halves the current compared to 12V, making wiring easier and cheaper. 48V is best for home backup and off-grid systems above 3,000W — current is quartered, wire costs drop significantly, and efficiency improves. As a rule: under 1,000W use 12V, 1,000-3,000W use 24V, above 3,000W use 48V.

Can I run my inverter at 100% capacity all the time? ▼

Technically yes, but it shortens inverter life and reduces efficiency. Most inverter manufacturers rate continuous output at 25°C (77°F). At higher ambient temperatures, the inverter derates — a 3,000W unit might only deliver 2,400W at 40°C. Running at 80% of rated capacity (the 25% headroom our calculator adds) keeps the inverter cooler, more efficient, and extends its operating life. Inverters running at 100% capacity also have higher no-load losses and generate significant heat requiring adequate ventilation.