Solar inverter size: Calculate the right size for your inverter

What is a solar inverter?

A solar inverter converts electricity between “direct current” (DC) and “alternating current” (AC). Electricity produced by solar panels and electricity stored in batteries is DC. Energy for use at home is AC. Therefore, any solar installation requires an inverter for the conversion of currents.

Why does the size of a solar inverter matter?

The size of a solar inverter is crucial because it determines how much energy can flow to your home and battery at any given time. More specifically, the inverter ensures that enough energy can flow from your solar panels to the grid and load or if installed with a battery, from and to the battery. If the inverter is too small, any electricity produced above the inverter’s capacity will not be converted and the inverter itself might experience higher degradation.

How to calculate the right size for my solar inverter?

Getting the inverter size right depends on two key factors:

1. Solar array size

2. Household load, if installed with a battery

Sizing the inverter based on the solar array

Inverters work most efficiently when operating near their maximum capacity and are typically sized to be roughly the same size as your solar panels. Inverters are usually sized lower than the kilowatt peak (kWp) of the solar array because solar panels rarely achieve peak power.

The solar array-to-inverter ratio is calculated by dividing the direct current (DC) capacity of the solar array by the inverter's maximum alternating current (AC) output. For example, a 4 kWp solar panel system paired with a 3.6 kW inverter has a ratio of 1.1. Most solar systems are designed with a ratio between 1 and 1.25, to maximise efficiency without overloading the inverter.

Sizing the inverter for use with battery storage

You might have a solar battery to store excess solar production for use during darker hours and import cheaply during the night. In this case, it’s important that the inverter will work for both solar panels and battery. If you have a battery, you need to make sure that the inverter can:

• Charge up the battery from the grid quickly enough during off-peak times with the cheaper electricity rates. Divide the battery storage capacity (kWh) by the inverter capacity (kW) to get the number of hours (h) it would take to charge the battery.

• Discharge quickly enough from the battery to fulfil household demand during peak times. The peak demand is driven by large electricity consumers such as an oven, electric heating, etc.

Therefore, you may want a larger inverter if you would like to regularly run several high-powered devices at the same time from your solar system or battery. You should think about which devices you regularly run at the same time:

• Kettle = 500-1,000 W

• Washing Machine = 300-500 W

• Dishwasher = 1,200-2,400 W

• Oven: 800-1,500 W

• Cooking hob (per ring) = 1,200-1,800 W

• Microwave = 600-1,200 W

Additionally, if you have big consumers in your home, like an EV or a swimming pool, a 3.6 kW inverter will probably be insufficient.

What is the size of a typical inverter in the UK?

Typically, 3.68 kW inverters have been and are still recommended in the UK because of the easier G98 approval process from the distribution network operator. If your inverter is more powerful than that, your installer will need to apply for a slightly different, G99, permission, which can be a slightly longer process.

Objectively, a 3.68 kW inverter is sufficient for a small UK household, a home with 2-3 people. It can cover a large part of their consumption from this size of inverter. However, for most households the load is larger than 3.68 kW at peak times and, if you want to satisfy this from the battery too, a larger inverter is needed. Additionally, if you have big consumers in your home, like an EV or a swimming pool, a 3.6 kW inverter will probably be insufficient. Therefore, we typically recommend 5 kW inverters which cater even to the peak demand of most British households.

Most inverters charge and discharge at the same rate. However, this is not always the case. For example, the Tesla PW3 has a charge capacity of 5 kW and discharge capacity of 11.5 kW. This means that it takes more than double the time to charge than to discharge. This has been designed for the consumption of the average American rather than British household.

While it is always good to do your research to know what inverter size you need, it is always best to check with an expert. Get a tailored proposal by quickly answering the following form:

Frequently asked questions

What is the difference between the size of a battery and inverter?

The size of a battery refers to its energy storage capacity, measured in kilowatt-hours (kWh), and determines how much energy can be stored for later use, such as during peak hours, when electricity prices are highest. In contrast, the size of an inverter refers to its power conversion capacity, measured in kilowatts (kW), and determines how much energy can flow from the solar panels or battery to your home or grid in real-time. While the battery focuses on how long energy can be used, the inverter focuses on how much energy can be used or supplied at a given moment.

Does my solar inverter also work with a battery?

Yes, your solar inverter can work with a battery, but compatibility depends on the type of inverter. A “hybrid inverter” is designed to manage solar panels, batteries, and grid power seamlessly. When you have a hybrid inverter, it is called a “DC-coupled” system and has both AC and DC outputs. The electricity generated by your solar panels can be transferred to the battery directly as DC, without having to convert it to AC and back to DC again. It is very efficient given it only has 1 conversion point (~5% loss in conversion).

If you have a string inverter, you need to either replace it with a hybrid inverter or add another inverter for the battery. An “AC-coupled system” has 2 inverters, 1 for the battery and 1 for the solar panels. The electricity generated by your solar panels is converted to AC and then is converted back to DC again when passing through the battery inverter to be stored in your battery. Since battery and solar connect with the fuse board independently, there are more conversion points, lowering the efficiency (10-15% lost in conversion).