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Your inverter’s continuous rating must cover every appliance you will run at the same time, plus about 25% headroom, and its surge rating must handle the start-up spike of your biggest motor. For a typical off-grid cabin running a fridge, lights, a TV and occasionally a microwave, that usually means a 2,000 to 3,000 W pure sine wave inverter.
Inverter size calculator
Estimates use the formulas explained in this guide. Plan a complete system with the all-in-one calculator →
Step 1: List the loads that run at the same time
Inverter size is about power at one moment (watts), not energy over a day (watt-hours). Write down everything that might be on together during your busiest moment.
Example evening in a cabin:
| Appliance | Running watts |
|---|---|
| Refrigerator | 150 W |
| LED lights | 60 W |
| TV + streaming box | 100 W |
| Laptop | 60 W |
| Microwave | 1,100 W |
| Total | 1,470 W |
Step 2: Add a 25% margin
1,470 W × 1.25 = 1,838 W, so a 2,000 W continuous inverter is the right class.
The margin keeps the inverter out of its hot, inefficient top end and leaves room for a load you forgot.
Step 3: Check surge
Motors and compressors draw a burst of current for a fraction of a second when they start, typically 3 to 7 times their running watts. A 150 W refrigerator can briefly pull 600 to 1,000 W.
Required surge = (biggest motor’s running watts × 3 to 7) + everything else already running
For the cabin: fridge start-up (about 900 W) + microwave (1,100 W) + other loads (220 W) = about 2,200 W. Most 2,000 W pure sine inverters are rated for 4,000 W surge for a few seconds, so this passes comfortably. Read more in inverter surge vs continuous rating.
Typical inverter sizes
| Use | Typical loads | Inverter size |
|---|---|---|
| Van or small camper | Laptop, lights, fan, phone charging | 300 – 1,000 W |
| RV with microwave | Above + microwave or coffee maker | 2,000 W |
| RV running roof AC | Above + 13,500 BTU AC (with soft start) | 3,000 W |
| Off-grid cabin | Fridge, lights, TV, small pump, microwave | 2,000 – 3,000 W |
| Off-grid home | Well pump, washer, fridge, freezer, mini-split | 5,000 – 8,000 W |
| Whole home with central AC | All of the above + 3–4 ton AC | 10,000 W+ or two inverters |
Match the inverter to the battery voltage
Higher power means more battery current. Keep currents manageable:
- up to about 1,500 W: 12 V is fine
- 1,500 to 3,000 W: 24 V is better
- above 3,000 W: use 48 V
At 3,000 W a 12 V inverter draws about 280 A from the battery, which needs very thick cables. See 12V vs 24V vs 48V.
Pure sine wave only
Modern electronics, motors, CPAP machines and anything with a variable-speed compressor should run on a pure sine wave inverter. Modified sine units are cheaper but cause buzzing, heat and sometimes damage. See pure sine vs modified sine wave.
Do not oversize too much
A huge inverter is not “safer”. Bigger inverters waste more power when idling: a 6,000 W low-frequency inverter can draw 40 to 60 W around the clock, which adds 1 to 1.4 kWh per day to your battery and solar requirements. See inverter idle power draw.
Grid-tied homes are different
For grid-tied solar with no battery, the inverter is sized to the solar array, usually 80 to 100% of the array’s DC wattage, because the grid supplies any extra power your house needs.
Our solar calculator gives a starting inverter size from your daily energy use, alongside panel and battery sizing. Confirm it against your simultaneous loads with the steps above.
Worked example: an off-grid home near Austin, Texas
A rural household outside Austin runs entirely on solar and batteries: 6 ceiling fans, 10 LED lights, a fridge, an 18,000 BTU (1.5-ton) mini-split, a TV, a router, a 1 HP well pump and a washing machine.
| Load | Running watts | Notes |
|---|---|---|
| 6 fans (standard, about 70 W) | 420 W | |
| 10 LED lights | 120 W | |
| Fridge | 200 W | Surge about 1,000 W |
| 18,000 BTU (1.5-ton) mini-split | 1,500 W (full speed) | Low surge |
| TV + router | 150 W | |
| 1 HP well pump | 1,000 W | Surge about 3,000 W, runs briefly |
| Washing machine | 500 W | Run separately |
Simultaneous worst case (without the washer): 420 + 120 + 200 + 1,500 + 150 + 1,000 = 3,390 W × 1.25 = 4,240 W. Surge when the motor starts: 3,000 + 2,390 = about 5,400 W.
A 5 to 6 kW hybrid inverter (48 V, 120/240 V split-phase output for the 240 V well pump) fits. Running the well pump when the mini-split is off allows a smaller inverter, but 6 kW gives freedom.
Inverter size quick table
| Inverter | Comfortable continuous load | Battery voltage |
|---|---|---|
| 1,000 W | up to 800 W | 12 V |
| 2,000 W | up to 1,600 W | 12 or 24 V |
| 3,000 W | up to 2,400 W | 24 V |
| 5,000 W | up to 4,000 W | 48 V |
| 8,000 W | up to 6,400 W | 48 V |
Inverter features worth paying for
- Pure sine output with low THD.
- High surge rating and duration for motors.
- Built-in MPPT charger (all-in-one) to simplify wiring.
- Battery communication with your lithium battery.
- Generator and grid input with automatic transfer.
- Low idle consumption and power-saving mode.
Common mistakes
- Sizing from daily kWh instead of peak watts.
- Ignoring surge from well pumps and fridges.
- Choosing 12 V for 3,000 W+ systems.
- Buying the biggest inverter possible, then paying for its idle draw every day.
FAQ
What size inverter do I need to run a refrigerator?
A full-size fridge runs on 100 to 200 W but needs about 1,000 W of surge to start. A 1,000 W pure sine inverter with 2,000 W surge is the practical minimum; 1,500 to 2,000 W if other loads share it.
Can I run a 3,000 W inverter on a 100 Ah battery?
Only for light loads. A single 12 V 100 Ah lithium battery with a 100 A BMS can supply about 1,200 W. See what a 3,000 W inverter will run.
Is a 5,000 W inverter enough for a house?
For an efficient off-grid home without central air, usually yes. See how many batteries for a 5,000 W inverter.
What size inverter for a house?
An efficient off-grid home without central AC typically uses 5 to 8 kW; with central AC, 10 kW or more.
What size inverter for a 1.5 ton AC?
At least 3 kW for an inverter-type AC alone, and 5 kW or more if other loads run at the same time.
Related guides
- Split-Phase (120/240V) Off-Grid Inverters Explained
- Inverter Efficiency Explained (and How Much Power You Lose)
- Can You Connect a Solar Panel Directly to an Inverter?
- Off-Grid Solar Grounding Basics: What You Need to Know
Sources and further reading
- U.S. Department of Energy: Inverters and Grid Services Basics: what inverters do and the main types.
- NFPA: Understanding NFPA 70, the National Electrical Code: the US wiring code that covers solar and batteries.
- ENERGY STAR: efficiency ratings and energy use of certified appliances.