⚡ Inverter Sizing Tool

Solar Inverter Size Calculator:
Surge Watts, DC Cable Sizing, PSW vs MSW, Brand Picks

The only inverter calculator that computes your surge watt requirement appliance-by-appliance, sizes your DC battery-to-inverter cable for the actual current load, warns you which devices MSW inverters will damage, and recommends specific models by brand, voltage, and price.

⚡ Size Your Inverter

Step 1 — Inverter Wave Type
Pure Sine Wave (PSW): Required for CPAP machines, variable-speed motors, sensitive electronics, audio equipment, and most modern appliances. Matches utility grid power quality.
Step 2 — Battery System
kWh
%
Why voltage matters for cable sizing: A 2,000W load at 12V draws 167A from the battery. At 24V it draws 83A. At 48V only 42A. Higher voltage = smaller DC cables and less heat loss from battery to inverter.
Step 3 — Your Loads (Watts + Surge Multiplier)
Motors surge 2-5x on startup. Enter running watts and the surge multiplier for each load. The biggest single surge determines your required inverter surge rating.
LoadWattsSurge
🔌 Essential Loads
🏠 Home Comfort
⚙ High-Surge Loads

📈 Your Inverter Specification

⚡

Select your wave type, battery voltage, and loads. The calculator sizes your inverter for both continuous and surge watts, sizes your DC battery cable, estimates runtime, and recommends specific models.

Why Surge Watts Are the Most Dangerous Number in Inverter Sizing

Most people size an inverter by adding up their running watts and buying the next size up. This works until the moment a motor starts, and the inverter shuts off with a fault code, a blown fuse, or a burned output stage. Understanding why this happens — and how to prevent it — is the most important thing you can learn about off-grid power systems.

Electric motors require dramatically more power to start than they do to run. A refrigerator compressor that draws 150 watts continuously might surge to 450-600 watts for the one to three seconds it takes to start spinning. A window air conditioner that runs at 500 watts might surge to 2,500 watts. A well pump that runs at 750 watts commonly surges to 3,000 watts or more. If your inverter’s surge rating is lower than the starting surge of the motor, the inverter’s overload protection trips and everything shuts off.

The surge requirement compounds when multiple devices can start simultaneously. If your refrigerator compressor kicks on while your AC is already running, the inverter sees the surge of the refrigerator on top of the running watts of the AC. This is the simultaneous surge scenario — and it is what actually determines the minimum safe inverter size for a real household.

Pure Sine Wave vs Modified Sine Wave: Which Devices Are at Risk

A modified sine wave (MSW) inverter approximates AC power with a stepped square wave rather than a smooth sinusoidal curve. For simple resistive loads like incandescent bulbs and basic electric heaters, this makes no practical difference. But for any device with a microcontroller, transformer, or motor, the difference can range from reduced efficiency and buzzing noise all the way to permanent damage.

CPAP and BiPAP machines are the most important example. The manufacturers of all major CPAP machines explicitly state that their devices must not be used with modified sine wave power. The PSU inside a CPAP will overheat and fail, sometimes damaging the machine in ways not covered by warranty. For anyone who depends on a CPAP for sleep apnea — a health-critical device — a pure sine wave inverter is non-negotiable, and the extra cost is trivial compared to the cost of replacing the machine.

How the Solar Inverter Size Calculator Works

Select your wave type, battery voltage, usable battery capacity, and inverter efficiency. Then check each load you plan to run and adjust the wattage and surge multiplier per appliance. The calculator finds the biggest single load surge, adds it to the remaining continuous loads (worst-case simultaneous scenario), applies safety margins of 15% for continuous and 25% for surge, sizes your DC battery-to-inverter cable per the actual current draw, estimates runtime from your battery bank, and recommends specific inverter models from Victron, EG4, Renogy, Giandel, and Xantrex that meet all your requirements.

Three Real Inverter Sizing Examples from Across the US

🏠

Off-Grid Homestead — Basic Loads, 24V System

Rural Tennessee | 24V LFP | 10 kWh usable | PSW required for CPAP

The Hendersons live full-time off-grid and run a refrigerator, LED lights, laptop, CPAP machine, and a fan. They need pure sine wave because of the CPAP. Their 24V LFP bank holds 10 kWh usable.

LoadRunning WSurge xPeak Surge W
Refrigerator150Wx3450W
CPAP machine30Wx1.545W
LED lighting80Wx180W
Laptop + phone100Wx1.5150W
Ceiling fan60Wx2120W
Total420W575W simultaneous
Required inverter: 485W cont (420×1.15) / 720W surge (575×1.25). A Victron MultiPlus 24/3000/70 at $970 is the right choice — massively oversized for surge protection, has a built-in battery charger, and provides UPS transfer so the CPAP never sees a power interruption when the generator kicks on. DC cable at 24V: 420W / (24V x 0.92) = 19A — 6 AWG copper is more than sufficient.
⛽

Van Build — High Efficiency, 12V System

Full-time travel | 12V LFP | 3 kWh usable | PSW for laptop and audio

Marco’s Sprinter van has a 12V LFP system with 3 kWh usable. He runs a laptop, LED lights, 12V fridge, phone charging, and a fan. No AC or heavy motors. He records music so PSW is required for his audio interface.

LoadRunning WSurge xPeak Surge W
12V fridge (compressor)45Wx3135W
Laptop + monitor120Wx1.5180W
LED lights30Wx130W
Audio interface + speakers60Wx160W
Phone + USB charging30Wx130W
Total285W375W
Required inverter: 328W cont / 469W surge. A Victron MultiPlus 12/500/20 at $380 is ideal — barely 500W but handles all loads with margin, includes a 20A battery charger, and the UPS transfer is seamless. At 12V: 285W / (12V x 0.92) = 25.8A DC — 8 AWG copper is safe. Runtime: 3 kWh / (310W DC draw) = 9.7 hours — a full night of work and sleep with power to spare.
🏛

Emergency Backup — AC Unit, 48V System

Phoenix, AZ | 48V LFP | 20 kWh usable | PSW for variable-speed AC

The Garcias in Phoenix have a grid-tied solar + battery system. During summer outages they need to run their 5,000 BTU window AC, refrigerator, and basic lighting. Their AC has a variable-speed compressor — requires pure sine wave.

LoadRunning WSurge xPeak Surge W
Window AC 5,000 BTU500Wx52,500W
Refrigerator150Wx3450W
LED lighting100Wx1100W
Phone + laptop80Wx1.5120W
Total830W2,500+330 = 2,830W
Required inverter: 955W cont / 3,538W surge. This is the dangerous scenario — the AC surge alone (2,500W) will kill any 2,000W inverter. They need at minimum a 3,000W continuous / 6,000W surge PSW inverter. The Victron MultiPlus 48/3000/35 at $920 handles it with margin. At 48V: 830W / (48V x 0.92) = 18.8A — 8 AWG is fine. Runtime: 20 kWh / (902W DC draw) = 22.2 hours — comfortably covers a full day and night of Arizona summer outage.

Expert Tips for Inverter Selection and Installation

1

Size for the Surge, Not the Running Watts

The most common inverter sizing mistake is buying based on running watts alone. A 2,000W inverter sounds like more than enough for a fridge (150W) and an AC (500W). But the AC’s starting surge of 2,500W will trip a 2,000W inverter’s overload protection every time the compressor cycles on — which is every 10-15 minutes in summer. Always size to at least 125% of your worst-case simultaneous surge load. For AC-heavy loads, a 3,000W inverter with a 6,000W surge rating is the realistic minimum. The Victron MultiPlus family has excellent surge handling — often 2x continuous — which is why professional installers default to it for any serious system.

2

The DC Cable Is the Most Overlooked Fire Risk

A 2,000W inverter at 12V draws over 180A from the battery at peak load. If the DC cable between battery and inverter is undersized, it heats up, melts insulation, and can start a fire — in a van, RV, or home electrical cabinet. Use the largest practical cable (at minimum the recommended AWG from this calculator), keep the run as short as possible — ideally under 18 inches — and install an appropriately rated fuse or breaker within 18 inches of the battery positive terminal. Use flexible welding cable (rated for high-flex) or SGX automotive cable, not Romex or standard building wire, which is too stiff and not rated for the vibration of mobile applications.

3

Consider an Inverter-Charger Instead of a Standalone Inverter

An inverter-charger (Victron MultiPlus, EG4 6000XP) combines a pure sine wave inverter with a battery charger and an automatic transfer switch in one unit. When shore power or a generator is connected, it automatically switches the loads to grid power and charges the battery simultaneously. When grid power fails, it switches the loads back to battery in milliseconds — fast enough to keep a CPAP running without interruption, fast enough to keep computers from rebooting. For any system with a generator backup or occasional shore power access, an inverter-charger costs roughly the same as a quality standalone inverter and eliminates the need for a separate transfer switch, separate charger, and separate wiring. It is the right choice for 90% of off-grid and backup power applications.

16 Frequently Asked Questions About Solar Inverters

What is the difference between continuous watts and surge watts?+
Continuous watts is the power the inverter can deliver indefinitely without overheating. Surge watts is the peak power it can handle for 1-3 seconds to start a motor. A Victron MultiPlus 12/3000/120 is rated for 3,000W continuous and 6,000W surge — it can handle a 3,000W refrigerator compressor starting (surge) without tripping, because 3,000W surge is well within the 6,000W surge rating. Buy based on the surge requirement for your motor loads; the continuous rating is secondary for most household applications unless you run high-draw loads like electric ovens continuously.
Why does my inverter shut off when my AC or fridge starts?+
This is almost always a surge watts problem. Your inverter’s surge rating is lower than the starting surge of the motor. A 1,500W inverter with a 3,000W surge rating will shut off when a 5,000 BTU window AC starts — that AC’s compressor surges to 2,500-3,500W for 1-3 seconds. The inverter’s overload protection trips and it shuts down. Solutions: buy a larger inverter with a higher surge rating, or add a Soft Starter device (around $30-50) to the AC, which reduces the starting current draw by 50-70% and often allows smaller inverters to handle AC loads they previously couldn’t start.
Can a modified sine wave inverter damage my appliances?+
Yes, for certain devices. CPAP and BiPAP machines: manufacturers state these cannot be used with MSW power and it may void the warranty and damage the device. Variable-speed motor tools and appliances: MSW causes additional heat in the motor windings, reducing efficiency and potentially shortening motor life. Laser printers: typically will not work at all on MSW. Audio equipment: a characteristic hum from the MSW waveform is audible through speakers and amplifiers. Battery chargers for power tools and laptops: some will charge normally, others will run hotter than normal. Simple incandescent bulbs, basic electric heaters, and older non-electronic devices typically work fine on MSW. When in doubt, use PSW — the price difference between PSW and MSW inverters at the same wattage has shrunk dramatically in the past few years.
What voltage system should I use — 12V, 24V, or 48V?+
The higher the system voltage, the lower the DC current for the same power output, which means smaller (and cheaper) wiring, lower resistive losses, and more efficient operation. General guidelines: 12V systems are appropriate for small to medium mobile applications (vans, RVs, boats) up to about 2,000W of inverter capacity. 24V systems are appropriate for medium off-grid cabins, larger RVs, and systems from 2,000W to 5,000W. 48V systems are the professional standard for off-grid homes, large battery banks, and any system with a solar array over 2 kW. Most charge controllers, inverters, and lithium battery banks are available in all three voltages. Once you commit to a system voltage it is difficult and expensive to change, so choose carefully based on your realistic long-term power needs.
What is an inverter-charger and should I get one?+
An inverter-charger combines a pure sine wave inverter, a battery charger, and an automatic transfer switch in one unit. When connected to shore power (campground hookup, house outlet) or a generator, it automatically switches all connected loads to that source and simultaneously charges the battery bank. When shore power is removed or the generator shuts off, it switches back to battery power in milliseconds. The Victron MultiPlus is the most popular inverter-charger among professional off-grid installers. For van builds with shore power access, RVs, and off-grid homes with generator backup, an inverter-charger is almost always the better choice over a standalone inverter plus separate charger plus transfer switch. The cost is similar and the integration is far cleaner and more reliable.
How long can I run my loads on battery with a given inverter?+
Divide your usable battery capacity (in watt-hours, not rated capacity) by your actual DC draw from the battery. The DC draw equals your total AC load divided by the inverter efficiency. For example: 5 kWh usable battery (5,000 Wh) with 800W total AC loads at 92% inverter efficiency draws 800/0.92 = 870W from the battery. Runtime = 5,000 / 870 = 5.75 hours. Note that this is continuous full-load runtime — most real loads cycle on and off (fridge compressor, lights), so actual runtime is typically 25-50% longer than the full-load estimate. Our calculator gives you the full-load baseline, and you can mentally adjust based on your actual usage patterns.
What size fuse or breaker do I need between my battery and inverter?+
Size the fuse or breaker to protect the wire, not the inverter. Use the wire ampacity as your guide, not the inverter’s rated output. For a 2,000W inverter at 12V, the theoretical max DC current is 2,000/(12 x 0.90) = 185A. If your DC cable is 2/0 AWG rated for 175A, install a 200A Class T fuse. If your cable is 4/0 AWG rated for 230A, install a 250A Class T fuse. The fuse must be installed within 18 inches of the battery positive terminal. Use ANL fuses for currents up to 350A and Class T fuses for higher currents. Both are available at RV and marine supply stores. Never omit the fuse — a short circuit in the DC cable between the battery and inverter can generate tens of thousands of amps and cause a fire within seconds.
Why does inverter efficiency matter?+
Inverter efficiency determines how much power the inverter draws from your battery to deliver a given AC output. A 90% efficient inverter drawing 1,000W of AC output takes 1,111W from the battery. A 95% efficient inverter takes only 1,053W — a difference of 58W that adds up to 696 Wh per day for continuous loads, or about 14% of a 5 kWh battery bank. Quality pure sine wave inverters like the Victron MultiPlus achieve 94-96% efficiency at typical loads. Budget MSW inverters often run at 85-90% efficiency. For off-grid systems where every watt-hour counts, inverter efficiency has a meaningful impact on daily autonomy and the required battery bank size. Higher efficiency also means less heat generated inside the inverter, which improves long-term reliability.
What type of cable should I use for the battery-to-inverter connection?+
Use flexible welding cable (also sold as welding wire or battery cable) or SGX-rated automotive cable. Both are designed for high current, vibration, and frequent flexing. Standard building wire (Romex, THHN) is too stiff, not rated for the current densities involved, and not appropriate for any mobile application. Marine wire (tinned copper) is ideal for marine environments. Use the largest practical gauge — the voltage drop across even a short cable run at high current causes real losses. Keep the total battery-to-inverter cable run (positive + negative combined) under 3 feet if possible — every additional foot at 200A is a significant resistive loss. Use proper ring terminal lugs crimped with a hydraulic crimper, not pinched with pliers — a bad crimp on a high-current connection generates heat and is a fire risk.
Can I run an air conditioner on a solar inverter system?+
Yes, with the right inverter and battery bank. The critical considerations are: first, the surge rating must handle the AC compressor startup (typically 4-6x running watts). A 5,000 BTU window AC that runs at 500W can surge to 2,500-3,000W on startup — you need at least a 3,000W continuous / 6,000W surge PSW inverter. Second, the battery bank must be large enough for meaningful runtime. A 5,000 BTU AC consuming 500W plus other household loads might pull 800W total. A 10 kWh battery bank provides about 12 hours of AC runtime. Third, if you run AC on solar, your array must be sized to keep up with the consumption — a 5,000 BTU AC running 8 hours per day consumes 4 kWh per day, which requires roughly 1.5-2 kW of solar panels to replenish. Many off-grid homesteaders use mini-split heat pumps, which are more efficient than window ACs and have lower startup surges when equipped with soft starters.
What is a soft starter and does it help with inverter sizing?+
A soft starter is a small electronic device wired in series with a motor (typically an AC compressor) that ramps up the voltage slowly during startup instead of applying full voltage instantly. This reduces the starting current surge by 50-70%. A 5,000 BTU window AC that normally surges to 3,000W on startup might only surge to 900-1,200W with a soft starter installed. This can allow a 2,000W inverter to successfully start an AC that would normally require a 4,000W inverter. The SoftStartRV and MicroAir EasyStart are two popular soft starters designed for RV and off-grid use, costing $150-$300. The payback is immediate if you can downsize your inverter as a result. Even if you already have a large inverter, a soft starter extends AC compressor life by reducing mechanical stress during startup.
How do I read an inverter’s specifications to compare models?+
The key specs to compare: Continuous power (W) — the maximum output indefinitely. Surge power (W) — peak output for 1-3 seconds. Efficiency (%) at typical load — check the efficiency curve at 25%, 50%, and 100% load; quality inverters are most efficient at 25-50% load. Input voltage range — the battery voltage and the range the inverter can handle (important for LFP batteries with wider voltage swing). Output voltage/frequency — should be 120V/60Hz for US residential. Transfer time (ms) — for inverter-chargers, how quickly it switches between grid and battery. Idle consumption (W) — how much the inverter draws from the battery when no AC loads are active; important for overnight standby. Operating temperature range — relevant for installations in uninsulated spaces. Warranty — Victron offers 5 years; most budget brands offer 1-2 years.
Should I size my inverter for all my loads or just the critical ones?+
For whole-home backup or off-grid systems, size for all loads you plan to run simultaneously — but be realistic about which loads actually run at the same time. You probably do not run the washing machine, air conditioner, and power tools simultaneously. Use the calculator for your realistic worst case: the loads you would run simultaneously during a hot afternoon or a power outage. For minimal budget systems (van builds, cabin weekenders), size for only the critical loads and run high-draw items one at a time. Many experienced off-grid users adopt a load management discipline: check battery state before running heavy loads, stagger motor starts rather than running everything simultaneously, and shed non-essential loads during low-production weather. A slightly undersized inverter with good operating habits outperforms an oversized inverter bought without considering daily usage patterns.
Can I parallel two inverters for more power?+
Yes — the Victron MultiPlus and MultiPlus-II series support parallel operation (up to six units) and three-phase operation. Two Victron MultiPlus 48/3000/35 units paralleled provide 6,000W continuous output from a 48V battery bank, and each unit can independently handle its own surge load. This approach is popular for homesteads that need more than 3,000W but want modular redundancy — if one unit fails, the system continues to operate at reduced capacity on the second. However, parallel inverter setups require compatible models (ideally identical), proper synchronization (handled automatically in Victron’s system), and careful wiring. For most residential off-grid systems, a single large inverter (5,000-8,000W) is simpler and often cheaper than two smaller units in parallel.
What is idle consumption and why does it matter at night?+
Idle consumption (also called no-load or standby power) is the wattage the inverter draws from the battery when it is powered on but no AC loads are running. Budget 1,000-2,000W inverters typically draw 15-30W in standby. The Victron MultiPlus draws about 9-11W in standby. Over 10 hours of night, a 30W standby draw consumes 300 Wh — 6% of a 5 kWh battery bank for doing nothing. On a small van build with a 2 kWh battery, this is a meaningful overnight drain. Solutions: use an inverter with a search/sleep mode that wakes up only when a load is detected (Victron has this feature, called search mode), or manually switch the inverter off when not needed. Victron’s search mode can reduce standby to under 3W at the cost of a brief delay when a load powers on.
Where can I find the official standards for inverter installations?+
The National Electrical Code (NFPA 70) Article 690 covers photovoltaic systems including inverters. Article 705 covers interconnected power production systems. For RV applications, NFPA 1192 and RVIA standards apply. For marine, ABYC E-11 and E-12 cover electrical systems including inverters. The US Department of Energy Solar Energy Technologies Office publishes guides for residential solar system design including battery storage and inverter selection. Most permanent inverter installations require a permit and inspection — verify your local AHJ (Authority Having Jurisdiction) requirements before installation.

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Legal Disclaimer and Editorial Transparency

The Solar Inverter Size Calculator on USCalculators.com provides estimates for educational and planning purposes only. Actual surge requirements vary by specific appliance model, motor condition, and ambient temperature. Surge multipliers are industry approximations and may differ from your specific equipment. Always verify with the manufacturer’s datasheet for motor-driven appliances before sizing a final system.

DC cable ampacity values are based on NEC Table 310.15(B)(1) for copper conductors at 75C in free air; bundled or enclosed runs require additional derating. Fuse sizing is the responsibility of a licensed electrician. Inverter brand and model recommendations are based on publicly available specifications and approximate retail pricing — no affiliate or commercial relationship exists with any manufacturer. All permanent electrical installations must comply with NFPA 70 (NEC) and local codes. See NFPA.org for the National Electrical Code.

Editorial policy: USCalculators.com is an independent educational resource with no affiliate relationships with Victron, EG4, Renogy, Giandel, Xantrex, or any inverter manufacturer.