7 things to check before you sign a solar quote
Two quotes can look similar and hide very different systems. Use this checklist to compare them like for like.
Read article 12 Oct 2026What size solar system do I need? A simple way to work it out
Daily energy, peak load, sun hours and backup time. Four numbers decide your panels, inverter and battery, and you can work them out yourself.
Most people ask for "a 5 kW system" because that is what their neighbour has. But the right size has nothing to do with the neighbour. It depends on four numbers that belong to your own home. Once you know them, quotes stop looking like a mystery.
The short answer
Your system is sized by:
- Daily energy use in kilowatt-hours (kWh), which decides how many panels you need.
- Peak load in kilowatts (kW), which decides the inverter size.
- Sun hours where you live, which decide how hard each panel can work.
- Backup time, which decides how big the battery is.
If you would rather skip the maths, the Solar-Rescue load calculator does all four steps for you. Reading on will help you understand what it is doing, and why.
Step 1: Work out your daily energy use
Every appliance uses energy equal to its power (watts) multiplied by the hours it runs. Divide by 1,000 to get kWh.
Fridges and freezers are a special case: they only draw power part of the time, so use roughly a third of the day for a typical unit.
| Appliance | Power | Hours used | Energy per day |
|---|---|---|---|
| 10 LED bulbs | 10 W each | 5 h | 0.5 kWh |
| Fridge / freezer | 150 W | about 8 h effective | 1.3 kWh |
| Television | 100 W | 5 h | 0.5 kWh |
| Kettle | 2,000 W | 0.5 h | 1.0 kWh |
| Electric geyser | 3,000 W | about 1.5 h effective | 4.5 kWh |
| Total | about 7.8 kWh |
Notice what happens: one electric geyser is more than half of this household's energy. That is why we always look at the geyser first. Switching to a heat-pump or solar geyser can shrink the system you need more cheaply than adding panels and batteries to feed it.
You can also check last month's electricity bill. Divide the kWh used by 30 for a rough daily figure.
Step 2: Find your peak load
The inverter has to cope with everything that could run at the same time, plus the start-up surge of motors. A pool pump, borehole pump or air conditioner can pull several times its running power for a moment as it starts.
A simple approach is to add up the power of everything that might run together, then allow for the biggest surge. For example, a kettle (2 kW), microwave (1.2 kW) and fridge (0.15 kW) running at once is about 3.4 kW, so a 5 kW inverter gives comfortable headroom.
An inverter that is too small trips when you switch on a heavy load. One that is much too large costs more for no benefit, unless you plan to grow the system later.
Step 3: Work out how many panels
Panels only produce full power while the sun is strong, so installers use peak sun hours: the number of hours per day the sun delivers its full-strength equivalent. In much of South Africa this is roughly 5 to 6 hours in summer, but it can drop to around 3 hours in a Cape winter.
The formula is:
Solar array size (kW) = daily energy (kWh) ÷ (sun hours × 0.75)
The 0.75 allows for real-world losses in wiring, heat, dust and the inverter.
Worked example (illustrative): a home using 10 kWh a day in a region with 5 winter sun hours needs 10 ÷ (5 × 0.75) = 2.7 kW of panels. With 550 W panels, that is 5 panels.
Size for winter, not summer, if you will rely on the system in June and July. The calculator lets you choose either. For a fully off-grid home, winter is the safe choice.
Step 4: Size the battery
A battery is sized by how much of your day you want it to cover:
- Load-shedding cover (a few hours): roughly 30% of daily energy.
- Evening and night: roughly 60% of daily energy.
- A full day or more (off-grid): 100% to 200% of daily energy.
Then allow for usable capacity. Lithium batteries typically allow around 90% of their rated capacity to be used each day. The battery comparison guide explains why lead-acid is usually used much more gently.
Continuing the example: 10 kWh a day with evening and night cover is 6 kWh, divided by 0.9 usable, so about 7 kWh of lithium storage.
Common sizing mistakes
- Sizing for the best month. Winter output is what keeps the lights on.
- Forgetting the geyser, stove and dryer. They are often the biggest loads in the house.
- Ignoring surge. Pumps and air conditioners need an inverter that can start them.
- Not allowing for growth. An electric car, a second fridge or a new pool pump can add a lot.
- Comparing quotes by price alone. Check that each quote is sized for the same loads. Our quote checklist shows what to compare.
Next steps
Put your own appliances into the load calculator to see your panel, inverter and battery size, and an indicative price. If you want to understand what each part does first, start with how a solar system works.
This article is general information. A registered installer should confirm sizing after a site visit, because roof space, shading and local conditions all matter.