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Is That Solar Panel Worth It? A Simple Guide to Return on Investment

Say your neighbour installs solar panels on their roof and tells you, "This thing pays for itself." What does that actually mean, and how do you check if they are right — or if they are just excited about the new gadget? You don't need an accounting degree to work this out. You need three simple ideas: Payback Period, Net Present Value (NPV), and Internal Rate of Return (IRR). Let's walk through them the way you'd explain them to a friend over a cup of tea.

The Basic Picture

A small solar PV system costs money upfront — panels, inverter, batteries (if you want them), wiring, and labour. Let's say the whole job costs 150,000 shillings. Once it's installed, it starts saving you money every month because you're buying less electricity from the grid, or none at all. Over the years, those monthly savings add up. The question every investment tool tries to answer is simple: does the money you save eventually outweigh the money you spent, and is it worth it compared to just putting that cash in the bank?

1. Payback Period — "When Do I Break Even?"

This is the easiest one, and it's probably what your neighbour is already doing in his head.

If the system costs 150,000 shillings, and it saves you 2,500 shillings a month on your electricity bill, then:

150,000 ÷ 2,500 = 60 months, or 5 years.

That's your payback period — the time it takes for your savings to equal what you spent. After that point, every shilling saved is pure gain.

Why it's useful: It's simple, and it tells you how long your money is "at risk" before you start coming out ahead.

Why it's not the whole story: It ignores what happens after year 5. A system that keeps producing savings for 20 more years is clearly better than one that dies right after payback — but the payback period alone can't show you that difference. It also doesn't account for the fact that money today is worth more than money tomorrow.

2. Net Present Value (NPV) — "Is It Worth More Than the Cash in My Pocket?"

Here's a truth most people feel instinctively: 10,000 shillings today is worth more than 10,000 shillings five years from now. Why? Because today's money can purchase more than the same amount next year. Inflation measures the rise in the price of goods. As long as inflation is more than zero, things are getting more expensive in shilling value. NPV is simply a way of respecting that truth when comparing costs and savings that happen at different times.

Here's how it works in plain terms:

• List out your yearly savings from the solar system (say 30,000 shillings a year for 20 years).
• "Discount" each year's savings — shrink it slightly to reflect that future money is worth less than present money. The further into the future, the more you shrink it. The rate you use for this shrinking is called the "discount rate," and a sensible choice is roughly what you'd earn if you put your money in a safe bank deposit or a SACCO or money market account or Treasury bond instead — often somewhere around 10-12% per year in Kenya, though your own numbers may differ.
• Add up all those shrunk, future savings to get the total savings in today’s shilling value.
• Subtract your original cost (150,000 shillings) from your total savings.

If the answer is greater than zero, congratulations — your solar investment gives you more value than just leaving the cash in a savings account. If it's less than zero, you'd technically have been better off putting the money in an interest-earning account. However, there may be other reasons to install the solar PV system, like energy independence or reliability during blackouts.

Why it's useful: NPV looks at the entire lifespan of the system, not just the breakeven point, and it fairly weighs money earned now against money earned later.

Why it can feel tricky: You have to guess a reasonable discount rate, and different people might reasonably choose different numbers. But even a rough estimate is far more honest than ignoring time value altogether.

3. Internal Rate of Return (IRR) — "What Interest Rate Did My Money Earn?"

IRR answers a different, very natural question: if I think of this solar system as if it were a savings account, what interest rate would that account need to pay me to produce the same stream of savings I'm getting from my panels?

Technically, IRR is the discount rate that would make the NPV exactly zero — the break-even discount rate. On this website, the Sizer, Auditor, and Cost Analyser calculator computes the payback period, NPV, and IRR for you.

Note on “discount rate” versus “interest rate”:

• Interest rate = how fast money grows going forward in time. You put 10,000 shillings in the bank today at 10% interest, and in a year it grows to 11,000 shillings.
• Discount rate = the same idea, but run backwards. If you know you'll receive 11,000 shillings a year from now, and you use a 10% rate, the discount rate tells you that the future 11,000 is only "worth" 10,000 in today's money.

Suppose the IRR comes out to 18%. That means your solar investment is effectively behaving like a savings account paying 18% a year — which almost certainly beats any bank deposit or government bond available to you. That's a strong signal the investment makes sense.

Why it's useful: IRR gives you one clean percentage number you can compare directly against other opportunities — a bank's fixed deposit rate, a government bond, or even a different-sized solar system.

Why it needs care: If your savings pattern is unusual (for example, the cost of a big battery replacement happens midway through the system's life), IRR calculations can occasionally behave oddly or produce more than one answer. For a simple household system, this rarely causes trouble.

Bringing It Together

Think of these three tools as three different questions a careful buyer asks:

• Payback Period: "How soon will my savings on electricity bills equal the initial cost of the solar PV system, ignoring inflation?"
• NPV: "Over the system's whole life (usually about 25 years), what is today’s value of my savings in electricity bills minus my initial cost of the system?"
• IRR: "What effective interest rate is my money earning, and does that beat my other investment options?"

None of them lies to you, but each tells only part of the truth on its own. A short payback period feels good, but a proper NPV and IRR calculation tells you whether the investment is genuinely sound — not just quick.

Before you commit your hard-earned money to any solar installation, it's worth taking a few minutes with the Solar Kenya Hub Sizer, Auditor, and Cost Analyser. Your neighbour's excitement might be justified — or the panels may be just a nice decoration on their roof. The numbers will bring you closer to the truth!