Solar Inverter Size Calculator
Work out the inverter size you need from your simultaneous load, starting surge and safety margin.
What this tool does
A solar inverter size calculator estimates inverter capacity using simultaneous load, starting surge and safety margin. Add up everything that can run at once, multiply by a safety margin (25% is common), and make sure the surge rating covers the largest motor's startup draw. The result is given in watts and, at the inverter's power factor, in kVA.
Estimates inverter capacity using simultaneous load, starting surge and safety margin.
Applies the safety margin so the inverter is not running permanently at its limit.
Checks the largest starting surge separately, because motors and compressors draw several times their running watts for a moment.
Reports the result in both watts and kVA, using the power factor that inverter ratings assume.
How to use it
- Add up the running watts of every load that can be on at the same time.
- Enter that simultaneous load in watts.
- Enter the highest starting surge of any single load, if you know it.
- Choose a safety margin - 25% is the usual figure.
- Read the recommended continuous size in watts and the equivalent in kVA.
Why use this calculator
Estimates inverter capacity using simultaneous load, starting surge and safety margin, because the three are usually quoted separately and only their combination tells you the size to buy. An inverter that exactly matches your running watts will still trip the first time a fridge or pump starts, and one sized only for the surge wastes money and runs less efficiently most of the year.
Why we built it
We built the solar inverter size calculator to make the surge check part of the sizing rather than an afterthought. The solar panel system size calculator sizes the array, the solar battery size calculator sizes the storage, and the battery backup time calculator tells you how long the pair will carry your loads.
If your task goes further, try the kVA to kW converter and the kW to kVA converter to move between real and apparent power, and the Power Factor Calculator when a rating plate only gives one of the two.
Who this tool is for
- Homeowners choosing an inverter for a solar or battery system
- Off-grid and backup builders sizing for whole-house loads
- Installers checking a quote against the actual load list
- Anyone comparing inverter adverts that quote watts and kVA differently
When to use it
- Choosing an inverter size before buying solar or battery hardware.
- Checking whether an existing inverter can take a new appliance.
- Comparing quotes where one inverter is rated in kW and another in kVA.
- Sizing a backup inverter for essential loads during outages.
How it is calculated
Continuous requirement = simultaneous running load × (1 + safety margin). Surge requirement = the largest starting draw of any single load. The inverter you choose must satisfy both: continuous rating at or above the first figure, surge rating at or above the second. To express the result in kVA, divide the watts by the inverter's power factor: kVA = kW ÷ PF, and 0.8 is the rating power factor most manufacturers assume.
Worked example: a simultaneous load of 3,600 watts with a 25% safety margin needs 3,600 × 1.25 = 4,500 watts continuous. A 1,500 watt pump starting at 3.6 times its running draw pulls 5,400 watts for a moment. A 5,000 W / 6 kVA inverter with a surge rating of 10,000 watts covers both: 4,500 W continuous is inside 5,000 W, 5,400 W surge is inside 10,000 W, and 4,500 ÷ 0.8 = 5.6 kVA rounds to the 6 kVA unit.
Loads that never run together should not be added together. A kettle, a shower and an oven might each be 3,000 watts, but if only one is ever on at a time the simultaneous load is 3,000 watts, not 9,000.
What you enter, and what you get back
| Item | Details |
|---|---|
| Simultaneous load | Running watts of everything on at once. |
| Starting surge | Highest startup draw of any single load. |
| Safety margin | Headroom above the running load, usually 25%. |
| Power factor | Used to convert watts to kVA, usually 0.8. |
| Result | Recommended continuous watts, required surge and kVA. |
A worked example
A worked example you can repeat in the calculator above. Change the load, the surge or the margin and the recommended size updates from the same calculation.
| Input | Value |
|---|---|
| Simultaneous running load | 3,600 W |
| Starting surge of largest load | 5,400 W |
| Safety margin | 25% |
| Power factor | 0.8 |
With those inputs, the calculator returns:
- Continuous requirement: 4,500 W
- Surge requirement: 5,400 W
- Apparent power: 4,500 ÷ 0.8 = 5.6 kVA
- Practical choice: a 5 kW / 6 kVA unit with at least 6,000 W of surge
Why the surge figure often decides the size
Motors, compressors and pumps draw several times their running current while the rotor is still stationary and there is no back-EMF to oppose the flow. A refrigerator motor may pull three to five times its running watts for a second or two; a well pump or air conditioner compressor sits around three to four times. Purely resistive loads - kettles, heaters, lights - have no surge at all, so a house of heating loads can be sized on running watts alone.
Inverters publish a surge rating, usually one to two times their continuous rating for a few seconds. If your largest motor's startup exceeds it, the inverter will shut down and restart, sometimes repeatedly, which is why the surge check is done separately from the continuous calculation rather than folded into the margin.
Very large motors are better started by a soft starter or by an inverter that explicitly advertises a high surge multiplier. Check the datasheet rather than assuming.
Issues you may run into
| What you see | Why it happens | What to do |
|---|---|---|
| Inverter trips when the pump starts | Starting surge exceeded the rating. | Choose a unit with a higher surge rating. |
| kVA figure seems bigger than the load | kVA is apparent power, before power factor. | Compare like with like using the kW figure. |
| Inverter runs hot at low load | Efficiency falls when an oversized unit idles. | Size within about 1.5× of your normal load. |
| Loads do not add up the way I expected | Appliances that never run together were summed. | Base the total on realistic simultaneity. |
Assumptions and limitations
- Assumes you know your simultaneous load and the largest surge - an incomplete load list gives an undersized result.
- Uses the standard 25% margin as a default; local wiring rules and manufacturer guidance take precedence.
- Does not model temperature derating, altitude, generator start-up or harmonic loading on the inverter.
- An estimate for planning, not a design for a live installation.
Frequently asked questions
What size inverter do I need for solar?
Add up every load that can run at the same time, multiply by about 1.25 for safety margin, and make sure the surge rating covers the largest motor's startup. A 3,600 W simultaneous load therefore needs roughly 4,500 W continuous, and an inverter advertising 5,000 W with 10,000 W surge fits comfortably.
What is surge power?
Surge power is the short burst an inverter must supply while a motor, compressor or pump starts. It is typically two to five times the load's running watts and lasts a second or two. A motor sitting still behaves almost like a short circuit until it is turning, so the inverter must survive that moment even though normal operation is far lighter.
Can I oversize my inverter?
Within reason, yes. Extra headroom improves margin and future expansion, and running an inverter at 40% to 60% of its rating is efficient. Going far too large wastes money and can reduce efficiency at light load, and you must still respect the array voltage, battery voltage and the manufacturer's maximum DC input.
What is the difference between watts and kVA?
Watts are real power - the electricity that does the work. kVA is apparent power - the product of voltage and current flowing through the equipment, which includes the reactive component. They are related by the power factor: kVA = kW ÷ PF, so at a power factor of 0.8 a 5,000 W load is 6.25 kVA.
How does inverter efficiency affect my system?
Inverters are typically 90% to 96% efficient, so 4% to 10% of the energy is lost as heat on the way from DC to AC. On a battery system that means your bank drains faster than the load suggests; on a solar system it means slightly less usable output. Efficiency also varies with load, peaking around half to three-quarters of capacity.
What happens if my inverter is too small?
It will overheat or trip under load, often shutting down when a motor starts. Repeated overloading shortens its life, and some inverters derate in hot weather, so a marginal unit fails first on the hottest day. Undersizing also limits expansion - there is no headroom for an added appliance or a larger array.