Inverter Sizing Calculator

What size inverter do you need? Enter your load and get a recommendation with surge rating and expansion headroom.

Load requirements

Total continuous load?

Largest surge load?

Future expansion?

Recommended inverter size

1,500W

Minimum size1,000 W

With expansion1,250 W

Surge rating needed2,000 W

Minimum: 1,000W with 25% headroom. Recommended: 1,500W including 25% future expansion. Ensure the inverter has a peak/surge rating of at least 2,000W.

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How to Use This Calculator

Enter your total continuous load

Add up the running watts of every appliance you expect to operate simultaneously. Running watts is the steady-state power draw — not startup/surge. For example, if you run a 150W refrigerator, 100W of LED lighting, and a 120W TV at the same time, your continuous load is 370W. Check appliance nameplates or use our Solar Inverter Calculator for a detailed appliance-by-appliance breakdown.

Enter the largest surge load

The largest surge load is the peak startup wattage of the biggest motor in your system. Motors draw 3-7× their running watts for about 1-2 seconds when starting. Check your appliance specs for "starting watts" or multiply running watts by the surge multiplier. If your system has no motors, enter 0 — the surge rating will default to 2× the continuous recommendation.

Set future expansion %

Adding 20-30% for future expansion means you won't need to replace your inverter when you add loads later. A 25% buffer is the residential standard. For a fixed, never-changing installation (RV with known loads), 10-15% is fine.

Read the output

The calculator shows the next standard inverter size above your minimum requirement, the recommended size including expansion headroom, and the minimum surge/peak rating your inverter must have. Choose an inverter that meets both the continuous and surge specifications.

The Formula

With headroom = Continuous load × 1.25 With expansion = With headroom × (1 + expansion %) Minimum size = Next standard size above "with headroom" Recommended size = Next standard size above "with expansion" Surge needed = Largest surge load × 1.10 (10% safety margin)

Standard inverter sizes follow a rough doubling pattern: 300W, 500W, 750W, 1,000W, 1,500W, 2,000W, 3,000W, 4,000W, 5,000W, and up. The calculator rounds to the nearest standard size so you can shop for real products rather than hypothetical custom wattages.

Example

Weekend cabin — Colorado mountains

The Hendersons are building a solar cabin with a 24V system. Their continuous load is 650W (fridge + lights + electronics). Their largest motor is a well pump with 2,250W surge (750W running × 3×). They want 25% future expansion room.

Total continuous load650 W

Largest surge load2,250 W (well pump)

Future expansion25%

With headroom (×1.25)813 W

With expansion (+25%)1,016 W

Minimum inverter size1,000W (next standard)

Recommended size1,500W (next standard)

Surge rating needed2,475 W

The Hendersons choose a 1,500W pure sine wave inverter with a 3,000W surge rating. This covers their current loads with comfortable headroom, handles the well pump startup, and leaves room to add a small washing machine or additional appliances later.

FAQ

Is it OK to oversize my inverter? ▼

Yes, but with caveats. Oversized inverters draw more standby/no-load power (typically 10-30W just staying on) which reduces your battery runtime when loads are light. They're also more expensive upfront. The sweet spot is 125-150% of your peak simultaneous load — enough headroom for safety and expansion, not so much that you're wasting power and money on idle consumption.

What happens if my inverter is undersized? ▼

An undersized inverter will trip its overload protection and shut down when loads exceed its rating. This is automatic and won't damage the inverter or connected appliances. However, frequent overload trips indicate a mismatch and can shorten inverter life over time. If surge loads exceed the inverter's peak rating, the inverter shuts off during motor startup — the appliance never turns on successfully. The fix is a larger inverter or soft-start capacitors on the motor load.

Do I need a separate inverter and charger or a combined unit? ▼

Inverter-chargers combine a pure sine wave inverter with a battery charger and automatic transfer switch in one unit. They're ideal for off-grid systems and home backup because they seamlessly switch between solar/battery and grid or generator power. Standalone inverters are cheaper but require separate charging. For any home backup or off-grid system with a generator backup, an inverter-charger is strongly recommended.

How efficient are solar inverters? ▼

Modern pure sine wave inverters are 90-95% efficient at typical loads. A 2,000W inverter delivering 1,500W to AC loads draws about 1,600-1,650W from the battery. Efficiency drops at very low loads (under 20% of rated capacity) and at very high loads (above 90%). Maximum efficiency is usually achieved at 50-75% of rated load. Factor in 90% efficiency when calculating your battery capacity needs — you need to store roughly 10% more than you consume.

Can I connect two inverters in parallel for more power? ▼

Some inverter brands support parallel stacking — connecting two or more units to multiply capacity. Victron MultiPlus and Quattro, Schneider Electric XW+, and SMA Sunny Island all support this. However, paralleling requires units from the same product line, identical firmware, and a communication cable. You can't simply connect two random inverters in parallel. If you anticipate needing 6,000W+, buy a single large inverter or choose a stackable model from the start rather than trying to expand later.