Solar
What Size Solar System Do I Need for My House? Worked Example
Short answer
To size a solar system, add up the energy your appliances use per day (watts × hours), divide by about 4 kWh per kWp per day to find panel size, add a 20% margin, then size the inverter to your peak simultaneous load and the battery to your night and outage needs. In our example home, 5.3 kWh per day needs about 1.6 kWp of panels, a 3 to 5 kW inverter and roughly 5 kWh of battery.
Step 1: List your appliances
Write down every appliance, its power in watts, how many you have and how many hours per day it runs. Watts are on the label or in the manual. Multiply watts × quantity × hours to get watt-hours (Wh) per day.
Example home
| Appliance | Watts | Qty | Hours/day | Wh/day |
|---|---|---|---|---|
| LED bulbs | 10 | 8 | 5 | 400 |
| Fridge (daily average) | 50 | 1 | 24 | 1,200 |
| TV | 100 | 1 | 5 | 500 |
| Decoder | 20 | 1 | 5 | 100 |
| Wi-Fi router | 12 | 1 | 24 | 288 |
| Laptops | 60 | 2 | 6 | 720 |
| Phone chargers | 10 | 4 | 3 | 120 |
| Washing machine | 500 | 1 | 1 | 500 |
| Iron | 1,000 | 1 | 0.5 | 500 |
| Water pump | 750 | 1 | 1 | 750 |
| Microwave | 1,200 | 1 | 0.2 | 240 |
| Total | 5,318 (≈ 5.3 kWh) |
Typical figures. Check your own labels. Heating loads such as geysers and electric cookers use far more.
Step 2: Size the panels
Divide daily energy by 4 (kWh per kWp per day in Nairobi, a conservative figure): 5.3 ÷ 4 = 1.33 kWp. Add 20% for losses and growth: about 1.6 kWp, which is four 400 W panels.
Step 3: Size the inverter
Add up what can run at the same time. If the pump (750 W), iron (1,000 W), washing machine (500 W), fridge, lights and TV all run together, the peak is about 2,600 W. Multiply by 1.25 for headroom: about 3.2 kW. Choose a 3 to 5 kW inverter, and check it can handle motor starting surges from the pump and fridge.
Step 4: Size the battery (hybrid or off-grid)
If about 45% of use is after sunset, that is roughly 2.4 kWh. With a lithium battery you can use about 90%, so about 2.7 kWh. Add reserve for outages and cloudy evenings and you land near 5 kWh of battery.
Check it with a survey
This is a planning estimate. Roof space, shading, panel direction and actual appliance behaviour change the final design. Try our solar calculator for a quick estimate, then get a site survey.
Common questions
How many solar panels do I need for a 3-bedroom house?
Many 3-bedroom homes without electric heating use roughly 5 to 12 kWh per day, which means around 1.5 to 3 kWp, or four to eight 400 W panels.
Why 4 kWh per kWp per day?
Nairobi gets around 5 peak sun hours per day, and system losses reduce usable output. Using 4 gives a conservative design.
Can I run a geyser or air conditioner on solar?
Yes, but they are large loads and raise system size and cost sharply. Solar water heating is often a better way to cover hot water.