What This Calculator Does
This inverter size calculator tells you how strong your inverter needs to be.
An inverter is a kind of translator box. It turns the DC power from your battery into the AC power your sockets and appliances understand. Too small, and it shuts off the moment the kettle and the fridge start together.
How To Use This Calculator
- In Solar array size, type the total watts of all your panels in kWp. 5 kWp is 5,000 W. This only feeds a warning for hybrid inverters, so a rough number is fine. No solar, just a generator or the grid? Type 0.
- In Continuous load, type the most power that runs at the same time for minutes or longer. Not your daily average: add up the watts of everything that can be on together, like the kettle while the fridge and lights run.
- In Peak load, type the biggest burst for a few seconds: your continuous load plus the extra kick of a motor that starts, like a water pump or an older fridge.
- In Battery system voltage, pick 12, 24 or 48 V, the voltage of your battery bank.
- In Phases, pick single-phase unless your home already has three-phase wiring.
- In System type, pick Off-grid only or Grid-tied hybrid. Not sure which you have? Read how grid-tied and off-grid systems differ.
- Read the result card: the minimum continuous rating in watts and in kVA, the minimum peak (surge) rating, and the DC current at the battery. Below it you get a suggested standard model, advice on voltage and phases, and any warnings.
The Formula, In Plain Words
The calculator takes your continuous load and adds 25% headroom. That's the minimum continuous rating in watts. The headroom matters because an inverter gives less power when it gets warm.
Inverter makers often print the size in VA instead of watts. On Victron's 3,000 VA and bigger models, each VA gives 0.8 W (a "power factor" of 0.8), so a 3,000 VA model gives 2,400 W. Smaller models give a little more. The calculator divides the watts by 0.8 to show the matching VA, which keeps you on the safe side.
The peak rating must be at least your peak load. Scientists call this short burst a "surge". It's the extra kick a motor needs to get spinning, a bit like pushing a bike from standstill.
The DC current is your continuous load divided by the battery voltage, and divided again by 0.9 because the inverter loses some power as heat. Victron lists 93 to 95% at best, so 0.9 leaves a little margin. Then it picks the smallest Victron MultiPlus model for your battery voltage whose watts and peak both cover your numbers.
A Worked Example
Say at most 800 W runs at the same time for long stretches, and a pump starting pushes that to 2,000 W for a moment. You pick 24 V, single-phase and off-grid.
The continuous rating is 800 × 1.25 = 1,000 W. On a datasheet that is 1,000 ÷ 0.8 = 1,250 VA. The peak rating must be at least 2,000 W.
At the battery, 800 ÷ (24 × 0.9) = about 37 A flows.
The smallest model that fits is a 1,200 VA unit: 1,000 W continuous and 2,400 W peak. The tool also says 12 V would work at this size, but 24 V keeps the current lower. Single-phase is fine.
What To Watch Out For
Watts and VA are not the same. Always compare your watts with the inverter's watts, not with its VA number. A "3,000 VA" inverter is a 2,400 W inverter.
A motor that starts straight on the supply can pull about 6 to 8 times its running current for a moment. Check the datasheet of your pump or fridge if you can, or look at a soft starter.
Higher voltage means lower current, and lower current means thinner cables and smaller fuses. That's why the tool nudges you toward 48 V above about 4 kW.
Victron's own wiring guide prefers 12 V up to 3,000 VA (2.4 kW), 24 V up to 5,000 VA (4 kW) and 48 V above that. Read more about inverters, then size the battery cable with the DC cable size calculator.
Hard Words
| Term | What it means |
|---|---|
| Inverter | A box that turns battery DC power into the AC power your sockets use. |
| Continuous load | The power your home pulls for long stretches. |
| Peak load | The highest short burst of power, like a kettle and microwave running together. |
| Surge | The short extra kick of power a motor needs when it starts. |
| kVA | Kilovolt-amps, the size unit on many inverters. At a power factor of 0.8, 1 kVA gives 0.8 kW. |
| Power factor | How much of the VA number turns into real watts. On Victron's 3,000 VA and bigger inverters it is 0.8. |
| Phase | One of the lines that carry AC power. Most homes have one; some have three. |
Common Questions
What size inverter do I need?
Take your continuous load and add 25% headroom: that's the continuous watts you need. Then make sure the peak rating covers your biggest burst. For 800 W continuous and a 2,000 W peak, you need 1,000 W continuous and 2,000 W peak, which a 1,200 VA model covers.
What is the difference between continuous and surge rating?
The continuous rating is the power an inverter can deliver for hours without overheating. The surge rating is the higher power it can give for a few seconds, for example while a fridge or pump motor starts. You need both numbers to be high enough, or the inverter will shut down under load.
Should I choose a 12 V, 24 V or 48 V inverter?
Pick based on size. Victron prefers 12 V up to about 2.4 kW (3,000 VA), like vans and boats, and 24 V up to about 4 kW (5,000 VA). Above that, 48 V is the practical choice, because lower current means thinner cables, smaller fuses and less energy lost as heat.
Can an inverter be too big?
Yes, a little. A very large inverter uses some power just by being switched on, which drains a small battery all day for nothing. It also costs more. Sizing close to your real peak load with sensible headroom gives the best balance between safety and efficiency.
Do I need a single-phase or three-phase inverter?
Most homes are fine with single-phase up to about 5 kW continuous. Above that, or if your house already has three-phase wiring, a three-phase setup spreads the load. It uses one inverter per phase, so it costs more and is more complex. Stay single-phase if your loads allow it.