What is convection and how it effects your heating costs

Solar power storage battery costs are dropping globally and many competing battery technologies have entered the market. Already a few battery companies have gone to the wall in the initial marketing positioning as people try to work out what is the best battery for their needs.

In this article we hope to clear away some of the confusion and provide a proven way to establish which battery is right for you and if the current crop of batteries meet your needs long term.

Power usage

Well before deciding which battery you need you will need to determine what is your typical daily power usage. We are assuming your planning on hooking up the battery to a solar panel, so during day light hours you can charge the battery from the solar panel, then during the evenings run off the battery.

In countries which employ 'time of use' charging, such an approach makes lots of sense, in effect you are saving yourself the difference between the cost to charge the battery and the price you would have paid taking power from the grid. Although it is important to realise that the full saving is only possible if you make full use of the capacity of the battery, too small a battery (or too small a solar panel array) and you will quickly drain the battery, yet too big a battery (or solar array) and you won't make full use of the capacity.

What you are looking for is a size of battery just bigger than the amount of power you would use during the peak time. Now determining exactly how much power you have used can be quite a challenge, there are a few options:

Direct Measurement: If you have a meter that provides time of day usage reporting directly, then you can read the KwH (kilowatt hours) figure straight off the meter day to day and the difference will tell you the power usage directly.
Quarterly Averaging: You may find your quarterly bill will tell you the average daily usage at peak, if not divide the total for the peak usage by 120 (this assumes peak is only charged week day evenings)
Power Consumption Calculation: This is where, by hand, you establish the power usage of all the devices in your house and work out the KwH usage yourself - this might not be that precise.

Once you have this figure, add 20% onto it to give you a 'buffer' against differences in the pattern of energy usage - you may want to go higher than this as it now becomes worth 'time shifting' the usage of devices into the now cheap evening time period.

Also divide this total power figure by the number of hours in an evening it is peak power, this gives you the average per hour power consumption - or rather how much 'energy' the battery has to pump out to keep up with demand in the evenings. Ideally your battery should be more than capable of providing this constantly.

Storage size & Usable Capacity

Batteries are rated by two figures: Nominal capacity and Usable Storage Capability. It's important to understand the difference. Nominal capacity is the maximum charge the battery is physically capable of storing; whereas Usable Storage Capacity is the capacity you can actually use before damage to the battery occurs. Usually, in order to prevent such damage, the battery management system will prevent you using the remaining charge. Most solar batteries have this 'feature', although flow batteries do not.

Rate of discharge

Something else to be aware of that the battery cannot give up all it's charge instantly, due to chemical and electrical limits batteries often come with two rate of discharge figures: a sustained rate of discharge and a peak discharge figure. The sustained is the power rate (kWh) the battery can keep pumping out until empty, where as the peak discharge figure (kWh) is a higher rate of discharge the battery can sustain for a short period of time.

You want to ensure your peak energy consumption is ideally below the sustained rate of discharge, or worst case below the peak discharge rate.

Battery Memory Effects

Many battery chemistries (like Lithium Ion) have what are termed 'memory effects', in that the battery capacity slowly decreases over time as the actual batteries themselves degrade. Remember, this is a chemical process more often than not and its very hard indeed to create chemical processes that can be repeated day in day out without some form of degradation. Most batteries will typically degrade by 30% over their warranted life time.

Note: Flow batteries are quite unique in this regard, they do not suffer memory effects and will give pretty much 100% of their rated storage at the end of their warranty period.

Grid connected Solar or not

Another factor you need to consider is if you want a system that is Grid Connected or not. Grid Connected means you can buy (or sell) electrical energy with the Grid. The practical financial upshot of this is that you do not need a big solar and battery system to cover you for the whole 24 hour period (or more). The downside is that unless you have it specifically wired to support it, if the grid goes down, so does your power! This is done to protect those working on the grid from being electrocuted by solar system pumping power into the grid, and the easiest way to do this is to completely disconnect all power sources, including yours in your own property.

Now if you have the money (and the roof and storage space) you can choose to go 'off-grid' - this will typically cost 3 to 4 times as much, as you will to ensure you have enough generating and storage capacity to cover the periods with little Sun. Plus ideally another backup power source (like a petrol generator).

Useful Resources

Battery Comparison Table - Easy to use interactive table to compare different solar storage batteries

Ash said:

kiwi boy here that was looking for information about keeping our houses warm, great article on convection. thanks alot! i would like to see you do an article on under floor insulation too! will you get into trialing products? i hear you can drill holes in walls and fill them up with expanding unsulation foam! that sounds like a treat.

ON Mon, 14 Sep 09, 5:22am probably from Australia Reply to this comment

Leith Nance said:

Do you have a system suitable for a very small home (double mobile)? Something easily installed and easy to manage for an elderly person.

ON Thu, 28 Jun 12, 9:48pm probably from Canada Reply to this comment

Jordan said:

So good it was amazing

ON Tue, 23 Oct 12, 5:52pm probably from United Kingdom Reply to this comment

Pelmets Melbourne said:

i really like this article very much... i am looking forward for some more articles regarding this topic only...

ON Tue, 25 Dec 12, 9:01am probably from India Reply to this comment

i really like this article very much and i am also looking forward for some more articles regarding the Pelmets Melbourne...

ON Wed, 2 Jan 13, 9:04am probably from India Reply to this comment

Andrew From Melbourne said:

I have a question sir if I may.

How does double glazing windows infront of a thermal mass impact the passive solar heating of a home in winter?

My initial thought is to consider double Glazing every window in my new build home but if that's done, does that mean my passive solar ability is reduced? If so what is the solution? Only double glaze windows in the winter sunpath? If so what happens at night due to convection?

Thank you.

ON Sat, 2 Feb 19, 9:36pm probably from Australia Reply to this comment

> What is convection and how it effects your heating costs