Do I need a pure sine-wave inverter?

Yes for any home with variable-speed motors (modern fridges, well pumps, induction cooktops), sensitive electronics (laser printers, CPAP), or LED dimmers. Modified-sine-wave inverters save ~30% cost but can cause motor buzzing, early motor failure, and electronics damage.

How much battery bank does my inverter need?

Rule of thumb for lithium: battery bank in amp-hours ≥ inverter continuous watts ÷ system voltage ÷ 2. A 3kW inverter on a 48V bank wants at least 31 Ah of lithium. Undersized banks brownout under surge.

What's a diversity factor?

The fraction of total nameplate load that actually runs simultaneously at peak. A house with 8kW of appliances rarely runs more than 5-6kW at once because the coffee maker, toaster, and oven don't all draw peak together. 70% is typical; oversize to 80-90% for shops/workshops with several workers.

Inverter Sizing Calculator

Size an inverter for an off-grid home, RV, boat, or backup system. Accounts for diversity factor (not all loads run at once) and motor starting surge (3× continuous for ~100 ms on induction motors).

Your AC loads

Nameplate total: 2,750 W across 5 loads

Diversity factorFraction that runs simultaneously. 0.7 is a residential default.Motor surge factor3× continuous is the NEMA default for induction motors.DC bus voltageDrives DC bus current for cable + fuse sizing on the inverter input.

Recommendation

Recommended continuous rating

3,000 W

Continuous (at diversity)

1,925 W

Required (+25% headroom)

2,406 W

Peak surge

2,375 W

Largest motor

Fridge (modern) (450 W surge)

DC bus current — continuous (48V)

67.2 A

DC bus current — surge (48V)

53.2 A

Recommended W ÷ 48 V ÷ 93% inverter efficiency. Size battery cables + the inverter input OCPD against the continuous-amp figure with a 1.25× ampacity multiplier.

Summed AC nameplate is 2,750 W. At a 70% diversity factor, continuous is 1,925 W. Adding 25% headroom yields 2,406 W continuous. The Fridge (modern) motor adds a 200% starting surge (450 W for ~100 ms). Inverter peak efficiency assumed 93%. Rounded up to a standard 3,000 W inverter. On the DC side at 48V, that draws 67.2 A continuous (53.2 A on surge) — size battery cables and the inverter OCPD against this continuous-amp figure with the 25% headroom + 1.25× cable-ampacity multiplier baked into your wire calc.

Design the full system →

Why rounding up matters

Inverter manufacturers rate continuous power at 25 °C. In a sealed battery compartment at 40 °C, the same inverter de-rates to about 80% of nameplate. A 3 kW inverter run at 2.9 kW continuous in summer is a ticket to premature failure. Add 25% headroom minimum; more if it lives somewhere hot.

Motor surge is the silent killer

A 1 HP (746 W) well pump pulls ~2.2 kW for 50–150 ms when it starts. Adding it to a 2 kW inverter running 1.5 kW continuous trips the surge protection. The calculator above accounts for the largest single motor — but if you have two motors that might start within the same 100 ms window (rare, but happens with bad pump-controller logic), size for both surges.

120V vs 120/240V split-phase

If your home has any 240 V appliance (well pump, some dryers, electric water heaters, AC condensers), you need a split-phase inverter or a pair of inverters stacked in series. Split-phase units cost roughly 30% more but eliminate the hack of rewiring 240 V loads to run on 120 V legs.

Want this inverter sized against your actual loads, battery bank, and solar array — with a permit-ready PDF? Design the full system.

Code references

The Inverter Sizing math above sits inside a regulatory framework. Inspectors and underwriters look at the citations below — your installation has to satisfy them, not just produce a number that looks reasonable.

NEMA AB-1 (2014, R2024)
Molded-case circuit breakers feeding inverter loads must coordinate with the inverter's peak-surge waveform. The standard quantifies thermal-magnetic trip envelopes; inverters with brief 3× surges typically need a magnetic trip ≥150% of continuous rated current to avoid nuisance trips during motor start.
Why it matters: The 3× motor-surge factor this calculator applies is the same envelope NEMA AB-1 uses to size upstream OCPDs. Pulling continuous + surge from the same model keeps your breaker and inverter selections consistent.
NEC 690.8(A)(3) (2023 / 2026)
Maximum AC output current of an interactive inverter equals the inverter's continuous output current at nominal AC voltage. DC battery-cable ampacity must be sized at 1.25× this current, then again at 1.25× per 690.8(B) for continuous-duty conductors.
Why it matters: The DC bus continuous-amp output of this calculator already includes the 25% headroom; multiply by 1.25 again when sizing battery cables for continuous-duty AC inverters per NEC.
ABYC E-11 §11.10.1 (2018) / Supplement 65 (2025)
Each ungrounded conductor must be protected by an overcurrent device sized to ≤150% of the conductor ampacity. The 2025 Supplement 65 adds §11.5.1.4: battery banks >500 Ah at 12 V (or proportional energy at higher voltages) require a per-battery OCPD with sufficient AIC.
Why it matters: For marine inverter installs, the DC bus continuous current this calculator emits is the input to the OCPD sizing rule — not the AC continuous rating. Banks larger than 500 Ah additionally need per-battery protection under E-11 (2025).
IEEE 1547-2018
Grid-interactive inverters (when used in hybrid systems with utility tie-in) must respond to anti-islanding, voltage and frequency ride-through within bounds defined per Category I/II/III response.
Why it matters: Stand-alone inverter sizing (this tool) does not require IEEE 1547 compliance; if your design becomes grid-interactive, the inverter SKU you pick must carry an IEEE 1547-2018 certification on its nameplate.

Citations are paraphrased for brevity. Always work from the most recent published edition of each standard for permit submittals.

Load	Watts	Qty	Motor?