I spent some time researching hot water heating systems last week. Obviously, we want to use renewable energy wherever possible.

It seems logical that renewable energy such as solar will reduce the CO2 emissions relating to electricity generation.

While it seems logical it turns out it’s not as true as we might think, at least not on a national level.

Jan Wright was the commissioner for the environment in 2012 when she wrote a report called, Evaluating solar hot water heating.

12 per cent of the electricity generated in NZ is used by households to heat hot water. Hot water heating accounts for approximately 30 per cent of a households power bill.

For this reason, many people install solar hot water systems & they benefit greatly for it.

Most debate about solar hot water systems is around the cost savings and whether the installation costs are worth it for a household.

We tend to just assume that because solar is renewable that it is a more sustainable option. Clearly, it is sustainable on an individual householders perspective.

But at a national level, solar doesn’t have the CO2 saving that we would expect.

How can that be logical?

Jan Wright points out that in the New Zealand context, the time of day in which the power is saved is almost more important than the amount of power saved.

The lowest power usage is overnight when we’re all sleeping. In the morning the power consumption spikes upward as we turn on the showers and kettles. Consumption stays relatively steady and then drops off again at about 9 pm at night.

New Zealand has lots of renewable electricity production such as hydro, geothermal & wind. The rivers run & the wind blows throughout the day & night and our electricity system uses these renewable sources as the base supply. Often there is excess electricity being generated at night.

But in the morning the coal-fired plants are ramped up to meet the spike in demand that is created during the day.

New Zealand’s baseload electricity is very low in CO2 emissions, but the power to supply the peaks in demand are high CO2 emissions.

The CO2 emissions generated in the early evening are 70% higher than that generated in the small hours of the morning.

The key to reducing the CO2 emissions from electricity generation is to flatten the peaks in the demand curve. The more demand we can move from day time to overnight supply the lower the CO2. 

Because we are moving the demand from peak coal-fired power to renewable baseload generation.

Solar hot water heating does reduce the load over that peak daytime period when the sun is up. But only in the summer months.

During the winter months, solar systems are affected by lower sunlight hours and the inlet temperature of the cold water is lower too. 

The result is not much hot water gets heated by solar hot water systems in the winter. Instead, the backup electric element kicks in.

Even if all NZ households installed solar hot water systems, planners who manage the power grid. Would still need to build new coal-fired plants in order to meet the winter peak in demand.

The report suggests that households should set their hot water systems to heat only at night time.

That will have the effect of moving 12 per cent of the power demand from peak day time hours to night time when there is plenty of renewable, low CO2 generation taking place.

That would result in a significant reduction in emissions.

The technology to do this has been around since the 1950s and its quite possible to do this with the smart meters installed in many homes now. But most lines companies don’t offer night heating as an option.

This finding appeals to me as it backs up my default position, that many of the answers to our modern problems, do not require some magic silver bullet technology that has yet to be discovered.

Instead, going back and doing things the way we did them in the past can have significant advantages.

Remember in the 1960s when we had one car, we reused our bottles, fixed things when they broke & heated our water at night time.