Will a 1,000 watt inverter start your refrigerator, or will it shut down at the first motor kick? Many inverter problems come from ignoring surge or from tiny battery banks. The Inverter Size Calculator checks your running load and your surge load together.
It then suggests a standard inverter rating, the DC current it draws, and a rough battery bank size. RV owners, van builders, and anyone planning backup power can use it as a first sizing pass.
Table of Contents
- What the Calculator Does
- How to Use It, Step by Step
- The Inverter Sizing Formulas
- Worked Example
- How to Read the Results
- Choosing an Inverter and Battery Voltage
- Limitations and Assumptions
- FAQs About the Inverter Size Calculator
What the Calculator Does
The tool updates as you type or change the voltage menu. Four inputs produce five results.
- Recommended Inverter Continuous Power: a standard size that covers your buffered load and half your surge.
- Continuous Load w/ Buffer: your running watts plus the safety buffer.
- Max Peak Surge Required: the surge watts you entered, rounded.
- DC Current at Running Load: battery-side amps for your real load at the chosen voltage.
- Suggested Battery Bank Size: amp-hours for about two hours of your running load.
- Assumptions note: a short line under the results that lists the fixed values the math uses.
- Copy Inverter Specs and Reset buttons.
How to Use It, Step by Step
- Add the running watts of everything used at once in Continuous Running Load (Watts).
- Enter the highest startup draw in Peak Motor Surge Load (Watts). Motors in pumps and fridges cause most surges.
- Set extra headroom in Safety Capacity Buffer (%). The default is 25%.
- Pick 12, 24, or 48 volts in Battery System Voltage.
- Read the inverter rating first, then the four result cards.
- Press Copy Inverter Specs for a summary, or Reset to restore 1,200 watts, 2,400 surge watts, 25%, and 12 volts.
The Inverter Sizing Formulas
The tool compares your buffered load with half your surge. It assumes an inverter can briefly surge to about twice its continuous rating.
Load × (1 + Buffer %)vsSurge ÷ 2→Larger Value=Next Standard Size
- Buffered load = running watts × (1 + buffer % ÷ 100).
- Required watts = the larger of buffered load or surge ÷ 2.
- Rating = the first of 300, 600, 1,000, 1,500, 2,000, or 3,000 watts that fits. Above 3,000, it rounds up to the next 1,000.
- DC current = running load ÷ 0.85 ÷ system volts.
- Battery amp-hours = running load × 2 hours ÷ 0.85 ÷ 0.5 ÷ system volts.
The 0.85 stands for 85% inverter efficiency. The 0.5 means you only use half the battery, a common rule for lead-acid banks. The buffer only adds inverter headroom, so it does not raise the amps or battery size.
Worked Example
Keiko wants to run a fridge, lights, and a laptop in her camper. They draw 800 watts together, and the fridge spikes to 1,800 watts at startup. She keeps the 25% buffer and compares three system voltages.
| Result | 12 V | 24 V | 48 V |
|---|
| Continuous Load w/ Buffer | 1,000 Watts | 1,000 Watts | 1,000 Watts |
| Recommended Inverter Power | 1,000 Watts | 1,000 Watts | 1,000 Watts |
| DC Current at Running Load | 78 Amps | 39 Amps | 20 Amps |
| Suggested Battery Bank Size | 314 Ah | 157 Ah | 78 Ah |
Buffered load is 800 × 1.25 = 1,000 watts. Half the surge is 900 watts, so 1,000 watts decides the rating. At 12 volts, DC current is 800 ÷ 0.85 ÷ 12, about 78 amps. Battery size is 800 × 2 ÷ 0.85 ÷ 0.5 ÷ 12, about 314 Ah.
If the fridge surged to 3,000 watts instead, half would be 1,500 watts. The rating would jump to 1,500 watts.
How to Read the Results
Recommended Inverter Continuous Power is the continuous rating to shop for, shown in plain watts. Max Peak Surge Required repeats your surge entry, which the inverter must briefly cover.
DC Current at Running Load guides battery cable and fuse sizing. Suggested Battery Bank Size is a rough two-hour figure at the selected voltage.
Choosing an Inverter and Battery Voltage
Higher system voltage cuts DC current for the same power. Lower current allows thinner cables and smaller fuses, which is why larger systems often use 24 or 48 volts.
List every device with an appliance wattage calculator before you start. Daily energy use comes from the power consumption calculator, which helps size a bank for longer runtimes.
Protect the battery cables with a fuse sized by the fuse size calculator. Plan cable runs with the cable length calculator. To compare grid costs, try the appliance running cost calculator. Related tools live in the home and construction hub.
Follow local electrical codes, and use a licensed electrician for any connection to home wiring.
Limitations and Assumptions
- Efficiency is fixed at 85%, and runtime is fixed at two hours.
- The battery figure assumes 50% usable capacity. Lithium banks often allow deeper use, so they may need fewer amp-hours.
- The tool assumes surge capacity of twice the continuous rating. Check the inverter's real surge spec.
- Running and surge loads must be 50 watts or more, and the buffer must be 10% to 50%.
- A blank, negative, or out-of-range entry shows a message, and every result reads N/A.
FAQs About the Inverter Size Calculator
Why does the tool divide surge watts by two?
It assumes an inverter can briefly deliver about twice its continuous rating. Many inverters list a short surge rating near that level.
What safety buffer should I use?
The default 25% keeps the inverter from running at full output for long periods. Larger buffers leave room for devices you add later.
Does the battery result cover a full day?
No. It covers about two hours of your running load at 50% depth of discharge. Longer runtimes need a proportionally larger bank.
Should I choose 12, 24, or 48 volts?
Small systems often use 12 volts. Larger systems benefit from 24 or 48 volts because current, and cable size, drop sharply.
Does the tool check whether I need a pure sine wave inverter?
No. The assumption note recommends pure sine for motors and sensitive electronics, but the tool does not test your devices. Some simple resistive loads can run on cheaper modified sine units.
This calculator gives planning estimates only and is not professional engineering, electrical, or building code advice. Reviewed for general accuracy by Prof. Dr. Khalil Mudassar, PhD.