One topic I do not see being discussed are Heating and Cooling degree days. For those not in the know, it boils down to how many days of the year you require heating or cooling when the temp is above or below a baseline temp. Normally 18degC is used as the base.

This will obviously vary dependent on your location. This info is available for SA. The link is here: Heating/Cooling degree days

When using Solar to generate energy, this may be helpful in planning heating and cooling and its affect on consumption and your system sizing. South African homes in general, there are obviously exceptions, are badly designed for our climate. Most homes are leaky, not well insulated and the Solar control from overhangs are nonexistent. This results in a draughty home with high summer Solar loads and lots of heat loss during winter. With a few exceptions, homes do not use double glazing and most do not have sun blocking overhangs above windows to keep summer Sun out and let winter Sun in. Most windows are also not even using Solar heat control coated glass.

Apart from all the above, having a basic understanding of the climate ito Heating/Cooling degree days can help to plan your solar systems. Lets use space heating as the prime example. In good ol SA energy pricing are controlled. Nersa will ensure that you pay about as much for any energy source, as any other type of source. So we are left with subsidising paraffin, while paying about the same per Btu for electricity or Gas or any other source of Btu’s.

Space heating, keeping warm during winter, and space cooling or keeping cool during summer, consumes a large % of your budget. Gas and Resistive type space heating are about the same efficiency per Btu, both being close to 100% of incoming energy converted to heat. Electricity is about 5% more efficient than Gas due to the fact that you must exhaust some heated air to replenish the consumed oxygen for gas heating systems.

Currently, the only realistically used technology with efficiencies above 100% are Air Conditioners. In general, their efficiency are expressed in CoP (Coefficient of Performance). 100% =CoP of 1 (actually 1:1 for 1 unit of energy in = 1 unit of energy out)

Modern Air conditioners are very efficient with CoPs of 2-3.8 on cooling and if the are heat pumps as well, heating may be about the same if slightly less than for cooling.

The below was grabbed from a typical, if a bit more expensive, inverter type air conditioner system brochure. Considering a 1000W heater will produce about 3400Btu/h or 1W = 3.4Btu/hW.

So we can expect roughly between 2 and 3 times more heating from heat pump type Air conditioners, depending on ambient temps, than from resistive or gas heaters. Cooling are about the same efficiency with 2-3 times more cooling then energy consumed.

Some more cool and hot stuff later….

COP is for heating performance
EER is for cooling performance

But that is now being pedantic.  🙃

Tell me more about the 18C figure please.  Between 18 and say 20 C I would still heat my home.  About 22C is the tip over point for me. 20-24C would be comfortable in my opinion.

@Pietpower you Sir are correct, I was not very clear in my explanation to highlite the difference.

So the concept had it origins in Agriculture.  HDDs and CDDs are defined relative to a base temperature (the outside temperature) below which a building is assumed to need heating or cooling, this is generally 18degC. If the outside ambient is way above or somewhat below, we normally intervene by cooling or heating.

Hdd and Cdd are proxies for energy demand and are based off of outside temperatures. 18degC are used as a standard so that wide areas can be compared. As we are all different, you can use whatever temp to base your calculations on.

You can use 22degC as the base temp as I do. Remember, there will be a range of temps (depending on the season) where you may not want cooling or heating indoors. For me and my home, the heating point is below 18.5degC and cooling point is above 28degC. Yours will be different.

In my case, for my location and house, I will only look at active cooling above that temp, and active heating below my minimum comfort temp.

For clarity follow this link here: Hdd and Cdd explained in some more detail.

The below are from the link posted in message above.

left image is Cdd and to the right are Hdd

If you follow the link above, you can select different layers. This layer shows Heating energy demand relative to location.

More clarity is required. A degree day is if the outside temp is 1degC less than your base temp (say 20degC) for 1 day. Thus if the outside temp is 1degC less for 1 hour and then 2degC less for 2 hours, that will be 3deg days of heating required. next day rinse and repeat. It can get intricate and complicated.

Lets start with a temp base of 18degC. The temp dives lower than base temp at 18:00 and continues to decline to -2degC at 07:00 next morning. That is about 20degC over 14 hours, or 1.4degC per hour. So then about 11.66deg Heating days.

So to be factually complete and considering this started in Agriculture, I need to add growing degree days (GDD) in here. This is not relevant to what I think is most valuable from an energy consumption perspective, and that is heating degree days (HDD).

Lets get some proper definitions in here, nicely summarised:

• The heating base temperature (or heating balance point) of a building is the outside air temperature below which the building needs heating.
• The cooling base temperature (or cooling balance point) of a building is the outside air temperature above which the building needs cooling.

It would be erroneous thinking in that the heating base temperature is the inside temperature below which the building needs heating, which is not the case, it is indeed the outside temperature.

Most buildings have other heat sources like people (about 140W per sedentary person), refrigerators and freezers, TVs etc. etc. All buildings have leakage paths like windows, doors ceilings and cracks around windows and doors or otherwise. The type of construction and insulation (or lack thereof) have a huge influence on losses and gains. These have to be considered, (and rectified if possible and economical) as having and influence on how fast heating loss will occur.

The opposite is true for CDD as heat gain happens via the same mechanisms. Without getting into the Air conditioning rabbit hole, these are the things in addition to room volume that will make or brake the sizing of the cooling and heating systems. All this not withstanding, heat pump split Air conditioning units are still between 2 and 3 times as efficient at heating a space than most other heating methods. 

That efficiency translates to using between 1/3 and 2/3ds less energy for space heating compared to other means. Now to the real point of this for me, apart from storing excess Solar PV generated energy in the water heater, we can use some excess Solar to do space heating during daytime. This will offset some night time energy usage to daytime as the building will be on average warmer this way.