What Size Inverter Do I Need? 2026 Sizing Guide + Calculator

What size inverter do I need for a 400W solar system?

Size your inverter based on your AC loads — not your panel wattage. A 400W solar system pairs well with a 1,000W pure sine wave inverter for most van and RV builds. Add up the watts of every AC device you plan to run simultaneously, add 25%, and that is your minimum inverter size. If you only run a laptop (65W), phone charger (30W), and CPAP (50W), a 600W PSW inverter is sufficient and more efficient than a 2,000W unit idling at low load.

Pure sine wave vs modified sine wave inverter — which do I need?

Always buy pure sine wave (PSW) for solar, RV, van, and home backup. Modified sine wave (MSW) inverters damage CPAP machines, overheat laptop chargers, cause hum in audio equipment, and shorten the life of variable-speed motors. Pure sine wave inverters cost $20–$60 more and are worth every dollar. The only scenario where MSW is acceptable is running simple resistive loads (basic power tools, incandescent bulbs) from a temporary car inverter — and even then PSW is the safer choice.

What is the difference between continuous watts and surge watts on an inverter?

Continuous watts is what the inverter sustains indefinitely — size your inverter by this number. Surge watts is the short burst (1–5 seconds) for starting motor-driven appliances. A refrigerator that runs at 150W may surge to 1,200W on compressor startup. Your inverter’s surge rating must exceed this peak startup draw. Most quality inverters have a surge rating of 2× their continuous rating — a 1,500W inverter typically surges to 3,000W, which handles most standard refrigerator startups.

How do I calculate what size inverter I need?

Add the running watts of every AC appliance you plan to run at the same time. Add 25% as a safety buffer. That is your minimum continuous watt rating. Then check the highest-surge device (usually a fridge, AC, or power tool) — your inverter’s surge rating must exceed its startup draw. Example: laptop (65W) + coffee maker (900W) + phone (30W) = 995W × 1.25 = 1,244W minimum. Buy a 1,500W inverter. Use the free Inverter Size Calculator above for precise results.

What size inverter do I need for a refrigerator?

A standard household refrigerator (15–20 cu ft) runs at 100–400W but surges to 800–1,500W on compressor startup. You need a 2,000W pure sine wave inverter with at least 4,000W surge to run a standard fridge safely. If you are building a van or RV system, consider a 12V DC compressor fridge (BougeRV, ARB, Dometic) instead — it runs directly from your battery with no inverter, uses 50–70% less energy, and eliminates the startup surge problem entirely.

Can I run an air conditioner on an inverter?

Yes, but it is demanding. A 5,000 BTU window AC draws 450–600W running and surges to 1,800W — a 2,000W pure sine wave inverter handles it. A 12,000 BTU RV rooftop AC draws 1,200–1,500W running and surges to 4,500W — you need a 3,000–4,000W inverter. For van and RV use, a 12V/24V mini-split designed for DC systems (Fogatti, Zero Breeze) is far more efficient because it eliminates the DC-to-AC-to-compressor conversion loss.

What size inverter do I need for a 200Ah battery?

A 200Ah LiFePO4 at 12V holds 1,920Wh of usable power (at 80% DoD). A 1,000W inverter at full load drains it in about 1.9 hours. At a more realistic 500W load, you get about 3.8 hours of runtime. A 1,000–1,500W pure sine wave inverter is the practical sweet spot for a 200Ah battery — large enough to handle most AC loads, sized so you get meaningful runtime without excessive current draw stressing the battery cells.