Sizing a Solar System
How the number of panels, the inverter rating and the battery capacity are actually arrived at, so you can sanity-check any quote you are given — including ours.
Step one: daily energy, from your bill
Your KPLC bill states units consumed. Units are kilowatt-hours. Divide a month's units by thirty and you have your average daily consumption in kWh. Use twelve months if you have them, because consumption moves with the seasons and with occupancy, and a system sized on your quietest month will disappoint in your busiest.
Step two: array size
Most of Kenya receives roughly 5 to 6 peak sun hours a day. Peak sun hours are not hours of daylight; they are the equivalent hours at full rated irradiance, which is why the figure is far lower than the time between sunrise and sunset.
Divide daily kWh by peak sun hours to get a nominal array size, then add around 30% to cover inverter losses, cable losses, dust, temperature derating and the fact that panels do not sit at their rated output for long. Ten kWh a day, at 5.5 peak sun hours, is 1.8kW nominal — call it 2.4kW after losses. That is six 440W panels.
Step three: inverter rating
The inverter is sized on simultaneous load, which is a different question from daily energy. Add up what can run at the same moment and pay particular attention to motor starting current: a fridge compressor or a borehole pump draws several times its running figure for the first fraction of a second. Systems that trip under load are almost always systems sized on running current.
Step four: storage
Battery capacity covers the energy you need when the panels are not producing, plus reserve for overcast days. Work out the evening and overnight portion of your consumption specifically rather than taking a fraction of the daily total. Then size on usable capacity, not nameplate capacity — lithium units are specified with a depth of discharge that protects cell life, and the usable figure is what you actually get.
The single most effective way to reduce system cost is to move heavy resistive loads — water heating, cooking, ironing — into daylight hours, or off the system entirely. Every kilowatt-hour you do not have to store is storage you do not have to buy.
What a survey adds
The arithmetic above gets you to the right order of magnitude. A survey establishes the things arithmetic cannot: which roof planes are unshaded between mid-morning and mid-afternoon, whether the structure will carry the array, where the inverter and battery can be mounted safely and ventilated, how the cable runs and how the system ties into your existing board.
Questions
How do I know what size solar system I need?
Work from your KPLC bill. Units consumed per month divided by thirty gives daily kWh. Divide that by the peak sun hours for your area — around 5 to 6 in most of Kenya — and add roughly 30% for system losses. That gives an array size in kW.
What size solar system runs a 3-bedroom house in Kenya?
Commonly 3 to 5kW of panels with a 3.5 to 5kW inverter, and 5 to 10kWh of storage depending on how much of the evening load you want covered. The figure moves sharply if there is an electric shower, a borehole pump or air conditioning.
Do I need a battery?
Only if you need power when the sun is not on the panels. A business running in daylight can get most of the benefit with little or no storage. A household that consumes in the evening cannot.
Have us do the sizing
Send your KPLC bills and a photo of the roof. The survey and the sizing are free.