What’s in a Home Solar Kit: Every Component Explained, from Panel to Transfer Switch

The three kinds of kit

• Off-grid: the home has no grid connection or doesn’t want to depend on it. It includes batteries.
• Grid-tied: the panels help and the grid covers the rest. It may have no batteries, and then it gives no power in an outage. The U.S. Department of Energy explains that the inverter must be one your power company accepts, and that you need to talk to them before connecting.
• Grid-tied with batteries: uses the grid, the panels and the batteries, and keeps working in an outage.

This article focuses mainly on kits with batteries, which have the most parts. To find out what size each part should be, see our kits sized by consumption.

1. The panels

They turn sunlight into electricity. Their power is given in watts (Pmax) and measured in the lab; on a roof they produce less, as we explain in what limits each part. How many you need depends on your usage and your city’s sunshine: the solar panel calculator works it out.

2. The mounting or panel racking

It holds the panels on the roof or the ground at the right direction and tilt. According to the World Bank’s Global Solar Atlas, the most productive direction is facing the equator: south in the northern hemisphere and north in the southern hemisphere. Letting air flow behind them matters too: according to PVEducation, panels mounted flat against a roof with no air gap can run 10 °C or more hotter, and they perform worse the hotter they get. Many kits don’t include it, because it depends on each roof.

3. The charge controller

It sits between the panels and the batteries. According to the U.S. Department of Energy, it lets energy into the battery without overcharging it, and many cut off the loads when the battery has been drained too far, which greatly extends its life. NREL’s guides add that it can include the fuses or breakers that protect against a short circuit. There are two types:

• PWM (the conventional type): the simplest and cheapest, but it doesn’t run the panels at the point where they give the most power.
• MPPT: constantly finds the point where the panels give the most power, which shifts with sunlight and temperature. In a test published in the scientific journal Protection and Control of Modern Power Systems (2018), with two identical panels, the MPPT harvested more energy than a conventional PWM controller in every measurement.

A controller is chosen by the amps it can handle. A rough calculation: the panels’ watts ÷ the battery voltage. With 900 W of panels and a 12 V battery, about 900 ÷ 12 = 75 amps.

4. The batteries

They store energy for the night and cloudy days. They must be deep-cycle or lithium; a car battery won’t do. The U.S. Department of Energy says to size them for the cloudy spells expected where you live, and to keep them in a ventilated space separate from living areas, protected from temperature extremes and easy to reach. Which type to choose is explained in home battery types.

5. The inverter

It turns the batteries’ power into the kind your outlets supply. For computers and electronics, the Department of Energy notes you need steady power with a smooth waveform: that of a pure sine wave inverter. It also advises planning for loads you may add in the future when choosing its size.

6. The transfer switch

You need one when the inverter will power your home’s circuits rather than just its own outlets. Its job is to make the house take power from the grid or the inverter, never both at once. The U.S. workplace safety agency (OSHA) explains why it’s essential with a generator, and the same applies to an inverter: without it, power can flow out into the lines outside and electrocute whoever is repairing them. Some inverters do the switching internally (NREL’s guides describe inverters that alternate between one source and another); others need a separate transfer switch. Check the inverter’s spec sheet, and have an electrician decide and install it.

7. Cables and connectors

They link the panels to the controller, the controller to the batteries and the batteries to the inverter. A lot of current flows between battery and inverter, so those cables need to be thick: we explain it with numbers in the inverter article. Many kits include only short cables, or none.

8. Protection: breakers, fuses, disconnects and grounding

The U.S. Department of Energy lists these safety parts:

• Fuses and breakers: they cut the power if there’s a short circuit or too much current.
• Disconnects: manual switches to cut the power and work safely on the system, and automatic ones that protect it from faults.
• Grounding: a path to earth for currents from lightning or a fault. The panels, the rest of the equipment and any exposed metal someone could touch are connected to it.
• Surge protection: against spikes from nearby lightning. We cover it in how to protect your appliances.

9. Meters

They show battery voltage, how much you’re using and how much charge is left. Many newer devices come with their own display or a phone app; Australia’s CSIRO mentions this for batteries. On the grid, your power company may require a meter for what the panels produce.

What kits often leave out

Before you buy, ask exactly what’s included. What’s most often missing: the mounting, long cables, breakers and fuses, grounding, the transfer switch and, in kits designed for grid connection, the batteries. And check that the inverter and controller match the battery voltage (12, 24 or 48 V).

You can buy the parts, but installing them isn’t a DIY job: connecting to your home’s wiring, the protection devices and the grounding are work for a qualified electrician.