What the Inverter Size Calculator does
This calculator sizes an inverter from the appliances you will run at the same time. It adds up their running watts, converts them to volt-amperes using each appliance's power factor, works out the start-up surge that motors and compressors demand for a moment, adds a safety margin and suggests the nearest common inverter size.
Two ratings matter on every inverter data sheet - the continuous rating and the surge (peak) rating - and an inverter that meets one but not the other will trip. The calculator gives you a figure for both, plus the current the inverter will draw from its battery.
How to use it
- List the appliances that could be on together. Leave out ones you will never run at the same time as the rest - that is the biggest lever on inverter size.
- Enter each appliance's running watts from its rating plate, and the quantity.
- Enter a surge factor: 1 for lights, chargers and electronics; about 3 for a fridge or freezer compressor; 3 to 7 for pumps, air compressors and power tools. Motor data sheets sometimes give locked-rotor or starting current, which is the better source.
- Enter a power factor if you know it (on the plate or data sheet). Many inverters are rated in VA as well as watts, and loads with a low power factor use more of the VA rating than their watts suggest.
- Set the safety margin - 20-30% is common so the inverter is not run flat out - and your battery voltage to see the battery current.
- Tick the simultaneous-start box only if several motors can genuinely start at the same moment, for example after a power cut when everything restarts together.
Reading the results
Continuous load is what the inverter supplies all the time. The suggested size covers it plus your margin, in both watts and VA, rounded up to the next common size.
Surge needed is the continuous load plus the biggest start-up extra. The inverter's surge rating must exceed it, usually for a few seconds; many inverters manage about twice their continuous rating.
Battery current is continuous watts divided by battery voltage and inverter efficiency. Above roughly 150 A the cable and fuse become large and expensive, which is why bigger systems move to 24 V or 48 V.
Worked example: a home backup inverter
The loads are a fridge (150 W, surge x3, power factor 0.8), three 75 W ceiling fans, eight 10 W LED lights, a 100 W television and a 12 W router. The running total is 150 + 225 + 80 + 100 + 12 = 567 W.
Dividing each by its power factor gives 643.7 VA. With a 25% margin that is 708.8 W and 804.6 VA, so a 1,000 VA class inverter is the smallest common size that covers both (an 800 W unit would be only just under the VA figure).
The fridge compressor needs an extra 150 x (3 - 1) = 300 W while it starts, so the surge rating must exceed 567 + 300 = 867 W. On a 24 V battery at 90% efficiency, the continuous current is 567 / (24 x 0.9) = 26.3 A.
Formulas and scoring rules
- Continuous load
W = sum(running watts x quantity)- Apparent power
VA = sum(running watts x quantity / power factor)- Surge
surge = continuous + max(running x (surge factor - 1)); simultaneous start uses the sum instead of the max- Suggested rating
continuous x (1 + margin) and VA x (1 + margin), each rounded up to the next size in 300, 500, 600, 800, 1000, 1500, 2000, 2500, 3000, 3500, 4000, 5000, 6000, 8000, 10000, 12000, 15000- Battery current
A = continuous W / (battery V x inverter efficiency)
Limitations: what the result does not prove
- Surge factors are typical, not measured. A particular compressor or pump can start harder, and soft-start or inverter-driven appliances start much more gently. The data sheet or a clamp meter reading beats any rule of thumb.
- The common-size list is a convenience, not a catalogue. Check the actual continuous, VA and surge ratings of the model you intend to buy.
- It does not choose between modified-sine and pure-sine inverters. Motors, medical equipment and many chargers need pure sine wave output.
- Connecting an inverter to a building's wiring needs a proper changeover or transfer switch and must follow local electrical rules; that work is for a qualified electrician.
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Standards and sources
Frequently asked questions
What size inverter do I need to run a fridge?
A typical household fridge runs at 100-200 W but needs about three times that for a second or two when the compressor starts. A 150 W fridge on its own therefore needs an inverter with a surge rating above roughly 450 W; with lights and a TV added, a 1,000 VA class unit is typical.
What is the difference between VA and watts on an inverter?
Watts are the real power the load uses; volt-amperes are voltage times current. They are equal only for purely resistive loads. Motors and many electronics have a power factor below 1, so they draw more current - and use more VA rating - than their watts suggest.
How much safety margin should I add?
20-30% is common. It keeps the inverter away from its limit, where it runs hot and less efficiently, and leaves room for a small appliance you forgot. Adding more than that mainly buys headroom for future loads.
Do all motors start at the same time?
Usually not, so the calculator adds only the largest single start-up surge. The exception is restoring power after an outage, when a fridge, freezer and pump can all start together. Tick the simultaneous-start box if that can happen in your setup.
Why does battery voltage matter for inverter size?
The inverter's power comes from the battery as current, and current is power divided by voltage. A 2,000 W load draws about 185 A from a 12 V battery but only 46 A from 48 V. High current needs thick cables and big fuses, so larger inverters usually run from higher-voltage banks.
Can I run an air conditioner or a water heater on a home inverter?
Only with an inverter and battery sized for it. A 1.5-ton air conditioner can draw 1,500-2,000 W running with a large starting surge, and a water heater 2,000-3,000 W continuously. Add them as rows to see the effect; they usually push the size and battery current up sharply.
Last reviewed by the A2Z.Tools team against the sources listed above.
