I get a lot of questions about rooftop solar photovoltaic panels. Even people who haven’t really started investing in energy efficiency are eager to consider spending their savings on solar PV. I understand. There is something sexy about generating your own power.

I rarely write about the subject here because I haven’t purchased PV panels for one very good reason: photovoltaics should always be the last energy investment for a home, not the first. You can save more electricity at a lower cost with other expenditures: LED lighting, solar hot water, heat pumps and variable-speed pool pumps, to name a few.

But this is a good time to look closely at the numbers for two reasons.

1. PV panel prices have dropped significantly in the past few years.

2. My Green Home has given me a window on real-world results for a house with solar PV.

Not too many years ago, an executive in the solar industry in South Africa admitted to me that even at wholesale prices, it would not pay for him to put panels on his own roof. But for the past several years, PV module prices in South African have been falling by about 22 percent a year. Many installers are also keeping prices down by recommending systems with no battery storage, since batteries greatly increase costs both up-front and down the line. (Batteries generally do not last as long as PV panels.) Without batteries, you are not protected from load-shedding and blackouts, but you are supplying some of your own clean electricity at a more reasonable cost.

This is the kind of simple, grid-tied system installed for My Green Home at the Ngewanas’ home in Cape Town. I have access to detailed metering data from the house and have used it to calculate electricity production for the last three months and to extrapolate those results to create a projection for the annual kilowatt hours to be generated by the system. (Seasonal variability was factored in using the monthly data from this Soventix commercial PV installation at Blaauklippen Vineyards.)

The Ngewana family has a small system, with one 255 watt panel connected to a 230 watt inverter that creates AC current from the panel’s DC output. On every sunny day, it produces at least 1 kWh. My calculations project that it will produce 400 kWh in a year. The Ngewana family pays R1.54 per kWh, so the panel will save them R615 in the current year.

That’s mahala money for the Ngewana family, since the panel was sponsored by Citrine Energy. But if you had to pay for it yourself, Citrine would charge about R10,500, and the system would take 10 full years to pay for itself at the anticipated rates of tariff inflation. It’s a fairly safe investment, since polycrystalline silicon panels generally last much longer than 10 years. But you will need patience if you are counting the days until you’ve paid off your investment.

The rate of return would be better for South Africans who live in sunnier regions or pay a higher tariff, but at todays PV prices, it will take several years to cover the outlay under most circumstances. The situation will improve dramatically when South African utilities begin to pay homeowners for any excess electricity they feed into the grid, which is called a feed-in tariff. This will also allow homeowners to buy larger panel sets that cost less per watt. (Some homeowners do this already by running their disc meters backwards, but this is illegal.)

At My Green Home, we had to carefully size the system to produce no more electricity than the home would ever use during the sunny part of the day. (Approximately equal to their 300 watt variable-speed pool pump.) With the pre-paid meter at that house, kilowatt hours add up no matter which direction the electricity is flowing. They would have paid for any excess kWh they contributed to the grid!

So has the time arrived yet for generating electricity on your roof? It depends. If saving money is your only priority, wait a couple of years until prices fall further or your utility offers to buy your excess electricity with a feed-in tariff. If, however, you have already invested in making your home energy efficient, and it would give you pleasure to know that some of your electricity is clean and carbon-free, the time is now.