How the Calculator Works (The Math)
The question "what size inverter do I need?" has two answers, and a good inverter must satisfy both at once. The first is continuous power — enough capacity to run everything you switch on at the same time, indefinitely. The second is surge power — a brief burst, often two to three times higher, that a motor or compressor demands at the instant it starts. Size for only one and you either trip on startup or pay for capacity you never use.
This solar inverter sizing calculator computes the continuous requirement first. It sums the running watts of every appliance you check, multiplied by its quantity, then applies your off-grid headroom and rounds up to the next standard inverter size:
continuous = SUM( running_watts x quantity ) for each checked load
requiredContinuous = continuous x (1 + headroom / 100)
recommendedSize = round UP to next standard inverter
[1000, 1500, 2000, 3000, 4000, 5000,
6000, 8000, 10000, 12000] wattsThe headroom (default 25%) is the off-grid safety margin. It covers loads you forgot, future additions, the inverter's own efficiency loss, and the reality that you do not want an inverter running flat-out at 100% capacity for hours. Twenty-five percent is the standard cushion; bump it higher if your load list is likely to grow.
Surge is calculated separately, because surges do not all happen at once. The worst realistic case is your single largest starting load kicking in while everything else is already running:
requiredSurge = ( largest_item.running x largest_item.surge )
+ SUM( running_watts of every OTHER running load )For each appliance, surge watts equal its running watts times a surge multiplier — about 3x for a fridge, freezer, well pump, or air conditioner with a standard induction motor, 2x for a washing machine or power tools, and 1x for resistive or electronic loads such as lights, a TV, a laptop, a coffee maker, a water heater, or an induction cooktop (which has no large motor to start). The calculator scans your selected loads, finds the one whose extra surge demand (surge minus running) is largest, applies the full surge to that one appliance, and adds the plain running watts of all the others. That single appliance is reported as your surge driver.
Finally the tool suggests a system voltage from the continuous requirement — 12V below 1,500W, 24V from 1,500 to 3,000W, and 48V above 3,000W — and recommends a pure sine wave inverter, which every modern off-grid home should use.
Why surge matters more off-grid: on the grid, a momentary sag during a motor start is invisible. On battery power, an undersized inverter simply shuts down or trips when the compressor kicks in, leaving your fridge warm and your lights flickering. The surge rating is the number that keeps the system stable in the real world.
Worked Example: A Small Off-Grid Cabin
Press the Small cabin demo above to load this exact scenario. The cabin runs, at peak, a fridge, four LED light circuits, a TV, a laptop, and an occasional microwave — and the microwave overlaps the fridge cycle.
| Appliance | Running W | Qty | Surge x | Continuous W |
|---|---|---|---|---|
| Refrigerator | 150 | 1 | 3x | 150 |
| Microwave | 1000 | 1 | 1x | 1000 |
| Lights (LED) | 100 | 1 | 1x | 100 |
| TV | 150 | 1 | 1x | 150 |
| Laptop | 60 | 1 | 1x | 60 |
| Continuous total | 1,460 |
Step 1 — continuous. The running watts sum to 1,460 W. With the default 25% off-grid headroom: 1,460 × 1.25 = 1,825 W. Rounded up to the next standard inverter, that is a 2,000 W continuous inverter.
Step 2 — surge. The microwave is the biggest running load, but it is electronic (1x surge), so its surge equals its running watts. The refrigerator, at 150 W running and a 3x surge, adds 450 W when its compressor starts. So the worst case is the fridge starting while everything else runs: surge = (150 × 3) + (1000 + 100 + 150 + 60) = 450 + 1,310 = 1,760 W. A 2,000 W inverter typically advertises a 4,000 W surge, which clears this comfortably. The surge driver here is the refrigerator.
Step 3 — voltage. A 1,825 W continuous requirement falls in the 1,500–3,000 W band, so the tool suggests a 24 V system. That keeps the battery current sane: 2,000 W at 24 V is about 93 A, versus 185 A at 12 V.
Step 4 — waveform & cable. Recommend a pure sine wave inverter. For the battery feed, the continuous DC current is roughly requiredContinuous ÷ battery voltage = 1,825 ÷ 24 ≈ 76 A, which the tool runs through the shared wire-sizing engine to pick the cable gauge and a DC breaker, and lists them in the BOM.
Result: a 2,000 W pure sine inverter, 4,000 W surge, 24 V system, fridge-driven surge. That matches the calculator's output exactly.
Appliance Running & Surge-Watt Reference
These are the running watts and surge multipliers built into the picker. Real appliances vary — always confirm the nameplate. Surge applies only to the single largest starting load in the calculator, because two big motors rarely start at the same instant.
| Appliance | Running watts | Surge multiplier | Surge watts | Load type |
|---|---|---|---|---|
| Refrigerator | 150 | 3x | 450 | Compressor motor |
| Chest freezer | 120 | 3x | 360 | Compressor motor |
| Microwave | 1000 | 1x | 1000 | Electronic |
| Well pump | 1000 | 3x | 3000 | Induction motor |
| AC / mini-split | 1200 | 3x | 3600 | Compressor motor |
| Lights (LED) | 100 | 1x | 100 | Resistive / LED |
| TV | 150 | 1x | 150 | Electronic |
| Laptop | 60 | 1x | 60 | Electronic |
| Coffee maker | 1000 | 1x | 1000 | Resistive heat |
| Washing machine | 500 | 2x | 1000 | Motor + heat |
| Water heater | 4000 | 1x | 4000 | Resistive heat |
| Power tools | 1200 | 2x | 2400 | Universal motor |
| Induction cooktop | 1800 | 1x | 1800 | Electronic |
Choosing 12V, 24V, or 48V
Your inverter's input voltage must match your battery bank, and that choice has a bigger downstream effect than almost any other decision in an off-grid build. The reason is current. Power equals voltage times current, so for any given wattage, a lower-voltage system has to push proportionally more amps through the battery cables:
| Continuous load | Suggested voltage | DC current @ that V | Typical cable |
|---|---|---|---|
| Under 1,500 W | 12 V | up to ~140 A | 2–1/0 AWG |
| 1,500–3,000 W | 24 V | ~70–140 A | 4–1/0 AWG |
| Over 3,000 W | 48 V | ~70–250 A | 2–4/0 AWG |
A 3,000 W inverter pulls roughly 295 A at 12 V but only 74 A at 48 V — a quarter of the current, a quarter of the cable copper, and far less heat. That is why every serious off-grid build over a couple of kilowatts runs 48 V. For the full trade-off, see our 12V vs 24V vs 48V guide, and size the actual battery feed with the wire size calculator.
Pure Sine vs Modified Sine
For a primary off-grid power system, choose a pure sine wave inverter without hesitation. Pure sine output is electrically identical to grid power, which keeps sensitive and motor-driven loads happy: variable-speed motors, microwaves with inverter magnetrons, CPAP machines, medical devices, laser printers, and most modern appliances all expect it.
Modified sine wave inverters are cheaper but output a blocky, stepped waveform. They can cause audible buzzing in audio gear, overheating and reduced efficiency in motors, dimming or flicker in some LEDs, and outright refusal-to-run or damage in microwaves and electronics. The price gap has narrowed to the point where modified sine only makes sense for a dedicated resistive load like a job-site light. For a home, the answer is pure sine. Read more in our solar inverter types explained guide.
Battery-to-Inverter Cable & Fusing
The cable between your battery bank and the inverter is the highest-current run in the entire system, and it is where most DIY fires start. Size it to the inverter's maximum continuous DC current — roughly the continuous wattage divided by the battery voltage (the calculator uses requiredContinuous ÷ battery V as a conservative figure that already includes your headroom).
- Keep it short. Mount the inverter within a few feet of the battery bank. Every foot of high-current cable adds voltage drop and cost.
- Fuse close to the battery. Place a DC-rated fuse or breaker (ANL, MEGA, MRBF, or Class-T for lithium) within inches of the positive terminal. Its job is to protect the cable, not the inverter.
- Use fine-stranded cable and torqued lugs. Welding-grade or marine-rated copper, with crimped and heat-shrunk lugs torqued to spec, handles vibration and the heavy current without loosening.
The BOM above lists the recommended cable gauge and DC breaker for your result, both derived from the same engine that powers our dedicated wire size calculator. For the diagrams showing where this run sits in a full system, see the complete wiring diagrams guide.
Common Inverter-Sizing Mistakes This Calculator Prevents
- Sizing for continuous only. A 1,500 W inverter can run a 1,200 W mini-split steadily, then trip the instant the compressor starts and demands 3,000 W. This tool always reports the surge requirement and the driving appliance.
- Adding every surge together. Beginners stack the starting watts of every motor as if they all kick in at once. In reality only one large motor starts at a time, so the calculator surges the single worst load and adds the others' running watts.
- Ignoring headroom. Running an inverter at 100% of its rating for hours shortens its life and leaves no room for the load you forgot. The 25% cushion is built in.
- Choosing 12 V for a big inverter. A 3,000 W inverter at 12 V needs 4/0 cable and pulls nearly 300 A — the tool steers large loads to 24 V or 48 V automatically.
- Buying modified sine to save money. The savings vanish the first time a microwave or CPAP refuses to run. Pure sine is recommended for every off-grid home.
- Oversizing wildly. A 6,000 W inverter idling to power a phone charger wastes standby power every hour it is on. Size for the loads you run, add headroom once, round up once.
Frequently Asked Questions
What size inverter do I need?
Add up the running watts of every appliance you might run at the same time, add about 25% off-grid headroom, then round up to the next standard inverter size. Just as important, the inverter's surge rating must cover the largest single motor or compressor starting while everything else runs. A cabin running a fridge, lights, a laptop, and an occasional microwave totals roughly 1,400 running watts, so a 2,000 W inverter with a 4,000 W surge is a comfortable fit. This calculator does both totals for you and names the appliance that drives surge.
What is the difference between continuous watts and surge watts?
Continuous (running) watts is the power an appliance draws while it operates steadily — the number you add up to size for normal use. Surge (peak or starting) watts is the brief spike a motor or compressor demands at the instant it starts, often two to three times its running watts for a fridge, well pump, or air conditioner. An inverter must handle both: enough continuous capacity for everything running at once, and enough surge headroom for the single largest starting load on top of the others.
How do you size an inverter for solar?
Size the continuous rating to the sum of running watts of all loads you run simultaneously, multiplied by roughly 1.25 for off-grid headroom, then rounded up to a standard size such as 2,000 W or 3,000 W. Size the surge rating to the largest single appliance's starting watts plus the running watts of everything else. Then pick the system voltage: 12V under 1,500 W, 24V for 1,500–3,000 W, and 48V above 3,000 W, because higher voltage cuts DC current and cable cost dramatically.
Do I need a pure sine wave inverter?
For an off-grid home, yes. A pure sine wave inverter produces clean power identical to the grid, required by sensitive electronics, variable-speed motors, CPAP machines, inverter microwaves, and most modern appliances. Modified sine inverters are cheaper but can cause buzzing, overheating, reduced motor efficiency, and outright failure of some devices. The small price difference is not worth the risk for a primary power system.
Can I run an air conditioner on an inverter?
Yes, with the right sizing. A 12,000 BTU mini-split draws roughly 1,200 running watts but can surge to 3,000 watts or more on a standard compressor start. That means at least a 3,000 W inverter, and ideally a soft-start kit or an inverter-driven mini-split that eliminates the surge. Because the surge load is large, a 24V or 48V system is strongly preferred so the battery cable and inverter are not pushed to their limits.
What system voltage should I use for my inverter?
Use 12V only for small systems under about 1,500 W, such as a van or small RV. Use 24V for mid-size systems of 1,500–3,000 W. Use 48V for anything over 3,000 W. The reason is current: a 3,000 W inverter pulls about 295 A at 12V but only 74 A at 48V, so 48V uses a quarter of the copper and runs far cooler. This calculator suggests a voltage automatically from your continuous load.
Why does the inverter draw so many amps from the battery?
Power is voltage times current, so a low-voltage battery must supply high current to make the same wattage. A 2,000 W load at 12V draws about 185 A from the battery once you account for roughly 90% inverter efficiency, which requires 2/0 AWG cable and a large DC fuse close to the battery. Moving to 24V halves that current and to 48V quarters it — the single biggest reason serious off-grid builds run 48V.
Should I oversize my inverter?
A small margin is wise, which is why this tool adds about 25% headroom before rounding up. But a heavily oversized inverter wastes energy: most inverters draw a fixed idle or no-load current whenever they are on, so a 6,000 W unit powering a single laptop bleeds far more standby power than a right-sized 1,500 W unit. Size for the loads you actually run together, add headroom, round up once, and stop there.
Related calculators & guides
- Off-Grid Solar System Calculator — size panels, battery, controller, and inverter together.
- Battery Bank Calculator — how much battery your inverter and autonomy need.
- Wire Size Calculator — exact gauge and fuse for the battery-to-inverter run.
- Load / Appliance Calculator — turn your appliance list into daily watt-hours.
- Solar Inverter Types Explained — pure sine, modified sine, hybrid, and all-in-one.
- 12V vs 24V vs 48V Systems — picking the right battery voltage.