Off-grid planning
What size inverter do you need for off-grid solar?
Size the inverter from the loads that run at the same time and the biggest motor that has to start, not from the size of your panels or battery.
In an off-grid system the inverter turns battery DC power into the 120V or 240V AC your appliances use. It has two limits that matter: how much power it can deliver all day (continuous) and how much it can deliver for a moment while a motor starts (surge). An inverter that is too small trips or shuts down; one that is far too large wastes money and idle power.
At a glance
The two numbers to calculate
- Battery12V, 24V or 48V DC
- Main fuseClass T, sized from the manual
- InverterDC to 120V or 240V AC
- Your loadsContinuous + surge must fit
Everything the inverter powers is limited by both its continuous and surge ratings. The cable and fuse between battery and inverter carry the highest current in the system.
Worked example: a small cabin
These are the starting loads the off-grid planner uses for a cabin's essential loads.
| Load | Running watts | Startup watts | Extra at startup |
|---|---|---|---|
| Refrigerator | 150 W | 600 W | 450 W |
| LED lights | 60 W | 60 W | 0 W |
| Internet/router | 25 W | 50 W | 25 W |
| Water pump | 750 W | 1,500 W | 750 W |
| Total | 985 W | Largest: 750 W |
- Continuous: 985 W × 1.25 ≈ 1,232 W (the planner rounds up), so a 1,500–2,000 W inverter is a sensible size.
- Surge: 985 W + 750 W = 1,735 W. The inverter must hold at least that long enough for the pump to start.
- Battery voltage: at this size a 12V bank works; 24V gives more room to add loads later.
Startup watts vary widely by model. For motors, check the nameplate or manual for locked-rotor amps (LRA), or measure inrush with a clamp meter that has an inrush mode. Do not rely on generic charts for well pumps or air conditioners.
Size your own inverter
- Loads selected
- 4
- Running total
- 300 W
- Suggested battery
- 12V
- Battery current at 12V
- ≈ 35 A
- Battery current at 24V
- ≈ 17 A
- Battery current at 48V
- ≈ 9 A
Uses the same rules as the off-grid planner: 25% headroom, one large motor starting at a time, 90% inverter efficiency.
Read surge ratings carefully
Two inverters can both advertise "4,000 W surge" and behave very differently. Some hold surge for several seconds; others for a few milliseconds, which is not long enough to start a pump or compressor. Look for the surge duration in the spec sheet. Low-frequency (transformer-based) inverters usually handle long motor starts better than high-frequency models of the same rating. For large compressors and well pumps, a soft starter can cut startup draw substantially.
Choose the battery voltage to match
The inverter pulls DC current from the battery roughly equal to watts ÷ (battery voltage × efficiency). Higher current needs thicker cable, larger fuses and better connections, and it wastes more power as heat.
Current scales with load: at 1,000 W these figures halve (about 93 A, 46 A and 23 A), and at 4,000 W they double (about 370 A, 185 A and 93 A).
Figures assume 90% inverter efficiency at nominal voltage; current rises further as the battery voltage drops. A common planning rule:
- 12V: vans, RVs and small cabins up to about 2,000 W.
- 24V: larger cabins and workshops up to about 4,000 W.
- 48V: anything larger, and every full-residence system.
Other features that matter off-grid
- Pure sine wave output. Required for motors, compressors and most electronics.
- Idle draw. An inverter using 30 W while idle burns about 0.7 kWh a day. Check the no-load figure and whether it has a power-saving mode.
- Built-in charger and transfer switch. An inverter/charger can recharge the batteries from a generator and pass generator power through to loads.
- 120V or 120/240V. Well pumps, dryers and some HVAC need 240V, which means a split-phase inverter or stacked units.
- All-in-one units. Combine the inverter, charger and solar charge controller, which simplifies wiring for larger 48V systems.
What to leave to a professional
Choosing an inverter size is a reasonable DIY planning task. Some of the installation is not:
- Connecting the inverter output to a building's wiring, subpanel or service panel.
- Any 240V or split-phase wiring, and transfer switches or interlocks.
- System grounding and bonding, which must follow the inverter manual and local code.
- Permits and inspection, which most jurisdictions require for a fixed installation.
Whoever does the work, the battery-to-inverter cable must be sized for the current in the table above and protected by the fuse type and rating the inverter manual specifies, usually a Class T fuse for lithium banks.
FAQ
What size inverter do I need for an off-grid cabin?
Add the running watts of everything that could be on at the same time, then add about 25% headroom. Check that the inverter's surge rating covers that total plus the extra startup draw of the largest motor. Many small cabins land between 1,500 and 3,000 W continuous.
What is the difference between continuous and surge watts?
Continuous watts are what the inverter can deliver indefinitely. Surge watts are a short burst, from a fraction of a second to a few seconds depending on the model, used to start motors such as refrigerator compressors, well pumps and air conditioners.
Do I need a pure sine wave inverter?
For off-grid homes, cabins and RVs, yes. Motors, compressors, chargers and many electronics run hotter, louder or not at all on modified sine wave power.
Should my off-grid system be 12V, 24V or 48V?
Higher battery voltage means lower DC current for the same power. A common planning rule is 12V up to about 2,000 W, 24V up to about 4,000 W, and 48V above that or for any whole-home system.
Can a bigger inverter hurt?
An oversized inverter costs more and draws more idle power around the clock, which matters on small battery banks. Size for real simultaneous loads plus headroom, not the largest unit you can afford.
Next, size the storage behind the inverter with the solar battery sizing guide, or see how big loads change the system in solar panels for appliances.