Inverter Size Calculator (Continuous & Surge Watts)
Enter the total running watts of everything you would power at once and the largest single surge load to find the continuous inverter rating you need, with a 25 percent headroom.
Results are estimates for planning and education, based on your inputs and standard engineering values (AWG resistance, NEC ampacity, resistivity). Electrical work can be dangerous and is governed by the NEC and your local code — verify all sizing with a licensed electrician and your authority having jurisdiction (AHJ). Not a substitute for professional design.
Calculator
| Continuous rating needed | 2,250 W |
|---|---|
| Surge to handle | 0 W |
Formula
Continuous inverter rating = total running watts × 1.25 (a 25 percent headroom). The surge figure is the largest single starting load the inverter must also tolerate for a few seconds.
Worked example
For 1,800 W of running load, the continuous rating is 1800 × 1.25 = 2,250 W. If a well pump adds a 3,000 W start-up surge, the inverter must also handle that brief peak, so a unit rated around 2,500 W continuous with a higher surge rating is appropriate.
Frequently asked questions
What is the difference between continuous and surge watts?
Continuous watts is the power an inverter can supply indefinitely, while surge watts is the much larger power it can deliver for a few seconds. Motors, compressors and pumps draw a brief inrush several times their running watts when they start, and the inverter must ride through that spike without shutting down. A good inverter lists both numbers, for example 2,000 W continuous with 4,000 W surge. Size the continuous rating to your steady load and confirm the surge rating covers your biggest starting appliance.
Why add 25 percent headroom?
Running an inverter at its absolute limit is inefficient and shortens its life, and real loads are often a little higher than estimated. A 25 percent margin lets the inverter run cool, absorb small load variations and leave room to add a device later. It is the same spirit as the continuous-load rule in electrical work. If you have measured your loads precisely and never run near the maximum, a smaller margin can work, but 25 percent is a safe, common default.
How do I find my total running watts?
List only the devices you would realistically run at the same time, not everything you own, and add their running watts. A microwave plus some lights and a laptop might total 1,800 W, even though the house has far more plugged in. Be honest about simultaneous use: sizing for every appliance at once buys a needlessly large, costly inverter. The appliance-wattage reference table gives typical running and starting watts to build this total.
Does the inverter need to match my battery voltage?
Yes. Inverters are built for a specific DC input such as 12, 24 or 48 V and must match the battery bank. For a given output power, a higher input voltage draws less current, so 24 and 48 V inverters use smaller, cheaper DC cabling and fuses than a 12 V unit of the same wattage. Large inverters above roughly 2,000 to 3,000 W are usually 24 or 48 V for exactly this reason. Pick the inverter voltage to match the bank you sized.
Can one inverter run my whole house?
Only if its continuous rating covers your simultaneous load and its surge rating covers your largest starting appliance, such as a well pump or air conditioner. Many off-grid homes either choose a large inverter or stage loads so the big surges do not all hit at once. Heavy 240 V loads like electric ranges and dryers may need a split-phase inverter or a dedicated circuit. Size to realistic simultaneous use, and verify the install with a qualified installer.
Source: Continuous rating = running watts × 1.25 (25% headroom); surge rating must cover the largest motor start-up inrush. · All sources