What size solar system does a Kenyan home actually need?

Ask three solar companies to quote for the same house and you will often get three different system sizes, sometimes varying by a factor of two. That is not because solar is mysterious. It is because sizing is arithmetic, and arithmetic needs inputs — and many quotes are produced without ever asking for them.

This article walks through the actual calculation. By the end you will be able to sanity-check any quotation you are given, including ours.

Start with what you use, not what you want

The single number that drives everything is your daily energy consumption in kilowatt-hours. There are two ways to find it.

From your KPLC bill. Your bill shows units consumed for the month. A unit is a kilowatt-hour. Divide the monthly figure by 30 and you have your daily consumption. If your bill shows shillings but not units, divide the amount by roughly 30 — an approximation of the effective cost per unit once levies and fixed charges are included — and then by 30 again for the daily figure.

From your appliances. Multiply each appliance's wattage by the hours you run it, add them up, and divide by 1,000. A 150W fridge running effectively 10 hours a day is 1,500 watt-hours, or 1.5kWh. Ten 10W bulbs for 6 hours is 600Wh. A 1,800W kettle for 12 minutes is 360Wh.

The appliance method is more work but far more useful, because it shows you where your energy goes. Most Kenyan households are surprised to find that lighting and entertainment are trivial, and that a handful of heating appliances dominate.

The heating appliances are the whole story

Here is the uncomfortable truth that separates a realistic quotation from an optimistic one. Anything that makes heat draws enormous power:

• Electric kettle: 1,500–2,200W
• Iron box: 1,000–1,500W
• Microwave: 900–1,200W
• Instant shower heater: 3,000–4,500W
• Air conditioner: 1,000–2,000W

Compare that with the things people assume are expensive: a TV at 80W, a Wi-Fi router at 15W, an LED bulb at 10W. You could run every light in a large house, the TV and the router all evening for less energy than fifteen minutes of ironing.

This matters because a system sized for lights, TV, fridge and Wi-Fi is a modest, affordable installation. A system sized to also run a kettle, an iron and a shower heater is a substantially larger and more expensive one. Neither is wrong — but you must decide which you are buying, and any quotation that does not ask the question is guessing.

Peak load decides the inverter

Daily consumption tells you how much energy you need. It does not tell you how fast you need it. That is peak load — the largest combination of appliances running simultaneously — and it decides your inverter size.

Add up what genuinely runs at once. Then add a margin for starting surge: motors draw far more when they start than when they run. A fridge compressor, a water pump or a washing machine can pull three times its running wattage for a second or two as it starts. An inverter that cannot supply that surge will trip, even though the running load is well within its rating.

For most Kenyan three-bedroom homes running normal domestic loads, this lands somewhere between 5kW and 8kW. Add a borehole pump, a workshop or air conditioning and it climbs.

Array size: the sun does the rest

Now the panels. The calculation is:

Array (kW) = daily kWh needed ÷ (peak sun hours × system efficiency)

Peak sun hours is the number of hours per day the sun delivers full-strength irradiance, averaged across the year. Across Kenya this ranges from roughly 5.0 in the cool highlands to 5.8 in the arid north and coastal areas. Nairobi sits around 5.2.

System efficiency accounts for everything lost between the panel and the socket: heat, dust, cable resistance, inverter conversion. A realistic figure is 0.8 — that is, expect to actually receive about 80% of the theoretical output. Anyone quoting you on 100% efficiency is selling you a shortfall.

So a household using 10kWh a day in Nairobi, wanting solar to cover it fully:

10 ÷ (5.2 × 0.8) = 2.4kW of panels, rounded up to about 2.5kW.

At 615W per modern module, that is four panels. This is usually the point at which people realise solar is more attainable than they assumed — and also the point at which they discover their roof faces the wrong way, which is what a site survey is for.

Battery size: how long, and for what?

Battery sizing depends entirely on what you are asking it to do.

For backup only, you are carrying essential loads through an outage. Work out the running load of just those circuits, multiply by the hours you want to survive, and add about 10% because you should not discharge even a lithium battery completely.

Essentials of about 400W for 8 hours is 3.2kWh, so a 5kWh battery covers it with headroom.

For full solar, the battery must carry the evening and overnight consumption — typically 40–55% of your daily total. A household using 10kWh a day therefore wants somewhere around 5–6kWh of usable storage.

For off-grid, add a margin for consecutive cloudy days, because there is no grid to fall back on.

Sanity-check the quote you have been given

Armed with the above, three questions will tell you whether a quotation was engineered or guessed:

1. 1Did they ask what you run, or just how big your house is? House size is a proxy, not an input. Two identical houses with different families use wildly different amounts.
2. 2Does the inverter rating exceed your realistic peak load, with surge headroom? An undersized inverter trips; an oversized one wastes money.
3. 3Is the battery sized to a stated number of backup hours? "10kWh" means nothing on its own. "10kWh, which carries your stated 600W of essentials for around 15 hours" is a design.

Where people go wrong in both directions

Undersizing is the more painful failure. The system works beautifully for a week, then someone plugs in an iron during a blackout and the inverter trips. Confidence never fully recovers.

Oversizing is quieter but expensive. A system generating far more than the household can use, with a battery that never cycles fully, has cost money that will never come back. This happens most often when a system is sized on the bill of a household that has since changed — children moved out, a business closed, a water heater replaced an electric shower.

The honest answer is that sizing should be done on measured or carefully estimated current usage, with a modest allowance for growth you can actually name. Not on a round number, and not on what the neighbour installed.

Do it yourself, then have it checked

If you would rather have someone run this arithmetic for you, ask us for a quotation with your appliance list or a recent KPLC bill. We apply the peak sun hours for your county, derate for real-world losses, and map the result to actual products with quantities and prices — stating every assumption we have made, and rounding savings down rather than up.

It is still an estimate. Roof orientation, shading, roof structure and the state of your existing wiring can all change the design, and none of those can be assessed from a web form. That is what a site survey is for — but you should arrive at the survey already knowing roughly what you need, and now you can.