How do I size an inverter for solar?

Add up the running watts of all appliances you expect to run simultaneously, then multiply by 1.25 for a safety margin. This gives your minimum continuous inverter rating. Also check that the inverter's surge rating (typically 2–3× continuous) covers the startup current of your largest motor — refrigerators, well pumps, and HVAC units draw 3–6× their running watts on startup. Round up to the next standard inverter size.

What is the difference between kW and kVA for inverters?

kW (kilowatts) is real power — the actual work done by electricity. kVA (kilovolt-amperes) is apparent power, which includes reactive power drawn by motors and other inductive loads. The ratio between them is the power factor (PF). Pure resistive loads like heaters have a PF of 1.0, so kW = kVA. Motors and electronics often have a PF of 0.8, meaning a 5 kVA inverter delivers only 4 kW of real power. Always size by kW for loads, but check the kVA rating for inverter comparisons.

What is inverter surge capacity and why does it matter?

Surge capacity is the peak power an inverter can supply for a short period (typically 5–30 seconds) to start motor loads. A refrigerator compressor may draw 1,500W to start despite only using 200W when running. If your inverter cannot deliver the surge current, the motor will stall, potentially damaging both the appliance and the inverter. Always verify that the inverter's surge rating exceeds the highest single-appliance startup demand on your system.

Do I need a different inverter for off-grid vs grid-tie?

Yes. Grid-tie (or grid-interactive) inverters synchronize with utility frequency and phase — they cannot operate without a grid signal, making them useless during outages unless paired with a battery system. Off-grid inverters are standalone units that generate their own AC waveform from a battery bank. Hybrid inverters (the most popular choice today) combine both functions: they grid-tie when utility power is available and switch to battery backup automatically during outages.

Quick answer: Running 3,000W of simultaneous loads (kitchen appliances, lights, heater) with a refrigerator compressor that surges to 1,500W requires a continuous 3,750W (3kW × 1.25) inverter with at least 5 kVA surge capacity.

Solar & Battery

Solar Inverter Sizing Calculator

Calculate the minimum inverter size for your solar or backup system — with surge headroom for motors, compressors, and inductive loads.

Inputs

Total continuous load (W)

System type

Motor/surge loads present?

Minimum inverter size

4.0 kVA

grid-tie · manual 1.5× surge factor

Continuous load

2,000 W

Startup load

3,000 W

Min continuous

2.50 kW

Recommended size

5.0 kVA

How we calculate inverter size

Total running watts are summed across all simultaneous loads, then multiplied by 1.25 to arrive at the minimum continuous inverter rating. The peak surge requirement is calculated separately by identifying the single highest motor-start draw and adding it to all other running loads.

Min Inverter (kW) = Total Running Watts × 1.25 / 1000

kVA is derived by dividing kW by the assumed power factor (0.8 for mixed loads), which is what most inverter spec sheets use for their continuous rating.

Frequently Asked Questions

How do I size an inverter for solar?: Add up the running watts of all appliances you expect to run simultaneously, then multiply by 1.25 for a safety margin. This gives your minimum continuous inverter rating. Also check that the inverter's surge rating (typically 2–3× continuous) covers the startup current of your largest motor — refrigerators, well pumps, and HVAC units draw 3–6× their running watts on startup. Round up to the next standard inverter size.
What is the difference between kW and kVA for inverters?: kW (kilowatts) is real power — the actual work done by electricity. kVA (kilovolt-amperes) is apparent power, which includes reactive power drawn by motors and other inductive loads. The ratio between them is the power factor (PF). Pure resistive loads like heaters have a PF of 1.0, so kW = kVA. Motors and electronics often have a PF of 0.8, meaning a 5 kVA inverter delivers only 4 kW of real power. Always size by kW for loads, but check the kVA rating for inverter comparisons.
What is inverter surge capacity and why does it matter?: Surge capacity is the peak power an inverter can supply for a short period (typically 5–30 seconds) to start motor loads. A refrigerator compressor may draw 1,500W to start despite only using 200W when running. If your inverter cannot deliver the surge current, the motor will stall, potentially damaging both the appliance and the inverter. Always verify that the inverter's surge rating exceeds the highest single-appliance startup demand on your system.
Do I need a different inverter for off-grid vs grid-tie?: Yes. Grid-tie (or grid-interactive) inverters synchronize with utility frequency and phase — they cannot operate without a grid signal, making them useless during outages unless paired with a battery system. Off-grid inverters are standalone units that generate their own AC waveform from a battery bank. Hybrid inverters (the most popular choice today) combine both functions: they grid-tie when utility power is available and switch to battery backup automatically during outages.

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Inverter Sizing Calculator

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Reviewed April 2026

Methodology and source note

PowerSizing calculators use shared formulas, documented assumptions, and current planning inputs that are summarized on the methodology page. Use these tools for first-pass planning, comparison, and sanity checks, then confirm local code, pricing, utility tariff, and installer specifics before you buy equipment.