What the inverter size calculator works out
This calculator estimates the inverter output needed for a realistic group of appliances that may operate at the same time. Each appliance row uses running watts, the maximum quantity operating together and an optional starting or short-duration surge value.
The output is an equipment-selection guide rather than a product approval. A suitable inverter must still meet the manufacturer’s continuous-output conditions, surge duration, waveform requirements and the current limits of the battery and battery-management system.
- combined running load for all active appliances;
- the highest modeled starting-load scenario;
- the number of watts added as reserve capacity;
- minimum recommended continuous inverter power;
- minimum recommended surge power;
- approximate current drawn from the battery system;
- the appliance responsible for the most demanding start;
- warnings where the entered data may understate the real requirement.
Build a realistic operating scenario
Enter the appliances that could genuinely be supplied together during the intended use. Adding every appliance in a property can overstate the inverter requirement, while omitting a pump, compressor or other essential load can make the result unsafe or impractical.
Entering each appliance
Appliance-row fields
| Field | What to enter | Useful check |
|---|---|---|
| Appliance | A clear name used in the results and warnings | Use names such as refrigerator, water pump, boiler, laptop or workshop tool. |
| Running power per unit | The power used by one appliance during normal operation | Use the nameplate, technical manual or a suitable measurement rather than a broad guess. |
| Maximum units running at once | The greatest number of identical units that may operate together | Enter 4 for four lights that may all be on, even if more lights exist elsewhere. |
| Starting or surge power per unit | The short-duration demand when one unit starts or energises | This is especially important for motors, compressors, pumps and some electronic power supplies. |
Why the simultaneous quantity matters
The quantity is not the number owned; it is the number expected to be on at the same time. The calculator multiplies one unit’s running watts by this quantity before adding the row to the continuous load.
For example, two freezers that may operate together should use quantity 2. Three pumps arranged so that only one can run at any moment should normally use quantity 1 for that row.
Continuous inverter power
- Running watts per unit are multiplied by the maximum simultaneous quantity.
- All appliance-row running loads are added.
- The selected reserve percentage is applied to the combined load.
- The result becomes the minimum recommended continuous inverter rating.
How the peak-load model works
The calculator tests a separate start for every appliance row. Other selected loads remain at their normal running demand, while one unit in the current row changes from running watts to its entered surge watts. The largest result is used as the modeled peak load.
Loads behave differently at startup
Common load characteristics
| Load type | What may happen | Examples |
|---|---|---|
| Mainly resistive | Starting demand is often close to running demand, although total watts may be high | Kettle, portable heater, incandescent lamp. |
| Electronic | A short inrush may occur as internal capacitors charge | Computer, television, charger and some LED drivers. |
| Motor or compressor | Starting demand can be much higher than the normal running figure | Refrigerator, freezer, pump, air conditioner. |
| Variable or cyclical | Power changes with speed, load, heating stage or operating cycle | Power tool, washing machine, compressor. |
A brief startup surge can determine the inverter size
Lighting, routers and laptops often create a relatively steady demand. Refrigerators, pumps, compressors and some power tools can draw much more power for a short time when they start.
The calculator keeps the other appliances at their normal running load and tests one startup event at a time. The most demanding result becomes the basis for the recommended surge rating.
A large surge figure is useful only when the inverter can sustain it for long enough to start the actual appliance under real battery conditions.
Where several motors may start together, test a separate combined scenario and replace the original rows so the same appliances are not counted twice.
Choosing a sensible reserve
The editable default reserve is 25%. It is an estimating assumption, not a universal requirement. A larger margin may be appropriate when appliance data are uncertain, additional loads are planned or the inverter would otherwise operate close to maximum output for long periods.
Oversizing also has trade-offs. A much larger inverter may cost more, consume more power while idle and place greater demands on the battery system. The aim is a justified margin rather than the largest possible unit.
Approximate battery current
Estimated battery current is calculated from the recommended continuous watts divided by nominal battery voltage and inverter efficiency. An efficiency entry of 90% is used as 0.90 in the calculation.
Understanding the results
Result guide
| Result | Meaning | How to use it |
|---|---|---|
| Total continuous load | The combined normal demand of all appliances operating together | Compare it with the inverter’s continuous-output capability. |
| Highest calculated peak load | The most demanding modeled single-appliance start | Compare it with surge power and the time for which that surge is available. |
| Added reserve | The extra watts created by the selected percentage | Use it to see how much headroom has been included. |
| Recommended continuous rating | The minimum calculated guide for sustained operation | Choose an inverter whose continuous W rating is not lower. |
| Recommended surge rating | The minimum calculated guide for startup demand | Confirm both the wattage and surge duration in the product documentation. |
| Estimated battery current | Approximate DC demand at the recommended continuous load | Use only for early system planning, not final protection design. |
Watts, volt-amperes and power factor
The calculator uses watts. When an inverter is promoted mainly by a VA figure, do not assume that the same number of real watts is always available. Find the separate continuous W rating, any stated power-factor conditions and the manufacturer’s surge specification.
Worked example
A refrigerator uses 180 W and 900 W at startup, a pump uses 300 W and 900 W at startup, lighting uses 120 W and a laptop uses 90 W. All may operate together and the reserve is 25%. What is the result?
Answer: The combined running load is 690 W. A refrigerator start produces a 1,410 W scenario, while a pump start produces 1,290 W. The highest modeled peak is therefore 1,410 W. With a 25% reserve, the continuous guide is about 863 W and the surge guide is about 1,763 W.
Explanation: The selected inverter should meet or exceed the continuous guide and sustain at least the calculated surge for long enough to start the refrigerator under real battery conditions.
Checks before choosing a model
- confirm that continuous output is specified in watts, not only by a product name or VA figure;
- check surge wattage and the period for which it can be delivered;
- confirm DC input voltage matches the battery system;
- check the battery and BMS continuous and peak discharge limits;
- confirm the required output waveform and frequency;
- consider ventilation, ambient temperature and any manufacturer derating;
- allow for genuinely planned future appliances rather than selecting exactly at today’s load.
Frequent sizing mistakes
Problem and better approach
| Mistake | Why it distorts the result | Better approach |
|---|---|---|
| Adding every appliance in the building | Creates an unrealistic load that may never occur | Model the busiest credible simultaneous-use scenario. |
| Ignoring a compressor or motor start | The inverter may trip even though normal running watts appear acceptable | Enter a documented or measured surge value. |
| Using total owned quantity | Overstates normal demand | Enter only the maximum number operating together. |
| Matching watts to VA without checking the specification | The inverter may provide fewer usable watts than expected | Use the continuous W rating and stated power-factor conditions. |
| Treating estimated battery current as a cable calculation | Length, heating, voltage drop and installation conditions are missing | Design DC wiring and protection separately. |
| Leaving no room for likely additions | The system may become undersized immediately | Include planned loads or retain a reasonable reserve. |
What the calculator does not determine
- whether a particular appliance is compatible with the inverter waveform;
- complex overlapping starts involving several motors automatically;
- DC voltage drop or cable heating;
- battery, BMS, terminal or connector current limits;
- battery capacity or expected runtime;
- DC cable size, fuse, breaker or isolator rating;
- earthing or grounding, bonding, neutral treatment, or electrical-panel and distribution-board arrangements;
- compliance with local installation requirements.
Questions users often have
Can the surge field be left blank?
Yes, but the calculator then treats running watts as the minimum peak figure. That may be acceptable for a preliminary resistive-load estimate, but it can materially understate refrigerators, pumps, compressors, air conditioners and power tools.
How should simultaneous motor starts be entered?
Run a separate calculation for the overlapping start. Replace the individual appliance rows with one combined row, enter their total running watts, quantity 1 and combined surge watts, then compare the result with the ordinary one-at-a-time scenario.
Does efficiency change the required AC rating?
Not in this model. Efficiency is used to estimate battery-side current. Continuous AC power is based on running loads plus reserve, while surge power is based on the highest startup scenario plus reserve.
Can this result select the battery as well?
No. Inverter power and battery capacity answer different questions. Runtime requires load energy, battery voltage and capacity, allowable depth of discharge, conversion losses and the desired operating time.
Final check: continuous watts and surge duration both matter
A dependable estimate starts with the appliances that can truly operate together. Running watts set the sustained-output requirement, while the most demanding start sets the surge requirement. After calculating, verify waveform, surge duration, battery-current capability and installation requirements before buying or connecting equipment.