Choosing an inverter starts with one question: what do you actually need to run, and how much of it need to be running at the same time? Get that answer wrong and the consequences show up quickly. An undersized inverter will not power everything you want it to, and leaves no room to add to the system later. An oversized one costs you more money than you need to spend and takes up space you didn’t need to use.
The right choice is an inverter large enough to run every load you intend to use together, with a little redundancy so it is never working flat out and there is room to expand if your loads change in the future. The most common mistake we see is underestimating how many appliances or circuits will be running concurrently. The second is misreading how appliances behave: some draw constant power, some have high start-up loads, and some vary with load or temperature.
This guide covers how to calculate system load, the inverter types available and when each applies, the specifications that matter when matching an inverter to that load, and how much headroom to build in.
Understanding System Load and Why It Comes First
System load is the amount of power, measured in watts, that each appliance uses when it is operating. Total system load is the sum of those figures for everything you intend to run.
To calculate it, add up the wattage each appliance is rated at. This is usually printed on the equipment itself or listed in the paperwork supplied with it. That total gives you the running load of your system.
What the Ratings Will Not Tell You
Surge loads and peak demand are rarely advertised on the product. These are the figures that catch people out, and they are the point at which professional advice is worth seeking rather than estimating.
A Worked Example of a System Load Calculation
Take a system with a 2000W kettle, a 400W fridge, a 150W television and a 1500W hair dryer. Added together, that is a total load of 4050W.
In practice, not everything runs at once. The television and fridge can be assumed to be on constantly, but the hair dryer will not be used at the same time as the kettle. A 3,000W inverter is therefore sufficient, provided it is accepted that the kettle and hair dryer cannot be used together.
Looking closer at how each load behaves:
- The television is a constant load.
- The fridge fluctuates slightly depending on how full it is and the temperature around it.
- The kettle draws a consistent 2000W, but only for a short period.
- The hair dryer has several heat settings, so it may draw less than its rated power for much of its operating time.
This is what a thorough power audit looks like. It is not just a list of wattages, it is an understanding of what runs together and for how long, critical when designing your battery bank.
Types of Inverter and When to Use Each
Pure Sine Wave and Modified Sine Wave
Mains electricity in homes, workshops and offices is a pure sine wave AC supply. Inverters produce either a pure sine wave or a modified sine wave output. The difference is the quality of the waveform generated.
Electrical equipment with electronic circuitry may not perform properly on a modified sine wave. As equipment becomes more sophisticated, more of it demands a pure sine wave inverter. In practice, only passive loads such as the heating elements in kettles and boilers are unaffected by modified sine wave output, and even then, if that kettle has a Bluetooth connection to a smartphone, a pure sine wave supply will generally be required.
To remove any doubt and eliminate problems during installation or use, we recommend always specifying a pure sine wave inverter.
Hybrid Inverters and Standard Inverters
A standard inverter supplies all the power itself. A hybrid inverter combines a second source of power internally, either a solar system or a mains supply, which allows the user to run higher loads than the inverter could support on its own.
As an example, a workshop with a 16A supply (roughly 4000W) and a 3000W inverter can support a load of up to 7000W by combining the two. Mains is fed into the inverter and the result is delivered using what is known as power boost: two inputs, one output.
Why the output must be combined
The output has to be a combination of both supplies so the sine wave of the electrical system stays the same. Two outputs from two separate inverters would produce two separate sine waves.
The system would not work, and it would not be safe.
Key Specifications to Match to Your Load
Continuous Power Rating
The continuous power rating of the inverter should exceed the constant running power of every appliance you want to run at any one time. A rule of thumb is to allow 15% above that figure.
Surge and Peak Power Rating
Surge power ratings are generally twice the continuous rating, and are usually sustained for a maximum of five seconds. This matters because motors, pumps and certain types of lighting have a high start-up draw, sometimes up to five times their continuous rating. An inverter sized only to the running load may not start the equipment at all.
Efficiency
Efficiency describes how much DC input power from your batteries is needed to deliver the required AC output. Anything at or above 90% is considered good. Roughly, an output of 1,000W AC requires around 1,111W of DC power at the input.
VA Is Not the Same as Watts
Failures and underperformance in the field can usually be traced back to incorrect specification of the inverter and the battery bank. One common mistake is assuming VA and watts are the same figure.
Equipment is given a continuous rating in watts, which is the energy actually used, known as real power. VA is the total energy drawn from the inverter, combining real power and unused reactive power, known as apparent power. As a rule of thumb, an inverter rated in VA will have a watts rating of 80%, so a 1000VA inverter is typically an 800W inverter.
Sizing the Battery Bank Alongside the Inverter
Start-up loads and surges need to be considered, and so does how long the equipment needs to run. Battery banks must be designed carefully to deliver enough DC power to maintain the output load for the required length of time.
To run a 900W load for two hours, you would need a 1000W inverter and a battery bank of around 330Ah:
- 900W / 12V = 75A
- Allowing for 90% efficiency: 75 / 0.9 = 83A
- Over two hours: 83A x 2 = 166Ah
- At 50% usable capacity: 166 / 0.5 = 332Ah
Future-Proofing and Headroom
A safety margin of 10% to 20% above the calculated load is a sensible starting point. It allows for expansion, and it keeps the inverter from working flat out in normal use, which is better for the equipment and for the system as a whole.
If you expect to add loads later, size for the system you expect to have rather than the one you are installing today. Some inverters are also stackable, meaning additional units can be connected in parallel to increase the available power of the system safely and efficiently. Where growth is likely but hard to quantify, a stackable model is worth specifying from the outset.
Frequently Asked Questions
What size inverter do I need?
The inverter must deliver enough power in watts to run every appliance you want to use at the same time. Add up the running wattage of those appliances, then select an inverter above that figure. If your appliances total 1,800W, a 2,000W inverter would be the sensible selection.
What size batteries do I need?
The battery bank must be large enough for the inverter to deliver power for the time you need it. Running 1,500W for three hours requires approximately 375Ah of usable power from a 12V bank. The battery type and its recommended depth of discharge then determine the total bank size.
What is surge or start-up power?
Certain equipment, including motors, pumps, compressors and fluorescent lighting, requires a large instantaneous burst of power to start before reverting to its normal running power. That burst is the surge or start-up load, and the inverter has to be able to supply it as well as the running load.
Are VA and watts the same?
No. VA is the total power drawn from the inverter, combining real power and apparent power. Watts is the energy actually consumed by the device, which is real power. Generally, an inverter rated in VA will have a watts rating of 80%, so a 1000VA inverter is an 800W inverter.
Key Takeaways
Specifying an inverter correctly comes down to a few decisions made in the right order:
- Audit your loads first. Work out what runs together, not just what you own.
- Allow around 15% above your concurrent running load, and check the surge rating against the start-up draw of any motors, pumps or compressors.
- Specify pure sine wave unless you are certain every load is passive, and size the battery bank alongside the inverter rather than after it.
- Build in 10% to 20% headroom so the system has room to grow and the inverter is never running flat out.
If you are unsure about surge figures, battery bank sizing or which inverter best suits your installation, our team can work through the calculation with you.