Is It Time To Get A Battery?

I got my Electricity Bill the other day which set me wondering if I should invest in a battery for our house.

Our Situation

We live in a retirement unit which is reasonably compact.

As a result our 3.25 kW of Solar Panels takes up most of the available roof space.

The advantage of having a small house is that although we are all electric our daily usage is quite small.

The below graph shows our daily usage and solar export.

From the figures above it would appear that with a battery we should be able to store, and then use overnight 3kw of power almost every day..

Power Company Rates

Our current rates from Powershop are:

$0.2893/kWh Consumption Charge
$0.052/kWh Feed in Tariff

Annual Savings

The annual savings by installing a battery  will be:

Consumption Charge minus the feed in Feed In Tariff  multiplied by 3kWh multiplied by 365.

$(0.2893 – 0.052) x 3 x 365 = $259.84/year

NB There will be some efficiency loss in the battery but this may be compensated by the battery partially discharging and being recharged during summer days.

Conclusions

Recent industry figures (September 2023) indicate the installed cost of batteries is $1,000 to $1,300 per kW.h

This gives a cost for a 3kWh of $3,000 to $3,900,

Even with the cheapest battery the payback at $259/year is going to be over eleven year,.by which time the battery will have gone through over 4,000 charging cycles.

As I am not convinced that the battery will be in good condition after those 4,000 cycles I think I might not bother with a battery for a while yet.

If I see battery prices drop to around $500/kWh I might rethink..

 

 

Insulation Basics – Brick Veneer Walls

This post will help you understand how much heat you lose through walls. A previous post has explained ‘R’ and ‘U’ values

When considering insulation a typical Australian brick veneer wall would be:

Element

R value

Outside surface air layer

0.03

110mm brick

0.08

25mm cavity

0.12

R1.5 Insulation

1.5

Plasterboard 10mm

0.06

Inside surface air layer

0.12

Total R value

1.91

U value = 1/R

0.51

The heat losses or gains for 150 sq m (fairly typical external wall area) of this type of brick veneer wall at 15 degrees above, or below, outside temperature will be:

Area x ‘U’ x temperature difference = watts per hour

150 m2 x 0.51 x 15 degrees = 1178 watts per hour

Heating/Cooling Requirement = 1.17kw/hour

To change the U value calculation simply change the value of the element or add an element in.

Example 1 Changing the Insulation to R 2.0

New Total R = 2.41

New U = 0.41

Reduced Heating/Cooling requirement to 0.92kw/hr

Example 2 Adding a reflective building wrap to example 1 (increases cavity R by 0.18

New Total R = 2.59

New U = 0.39

Reduced Heating/Cooling Requirement to 0.87kw/hr

Remember this isn’t the total heating requirement as heat is also lost through windows, ceilings floors and ventilation.

 

See Insulation for similar Posts

For Posts about Green Building see Sustainability

Solar Power – Reasons for Low Output

For each kw of installed capacity of solar panels on average you can expect to generate 3.5kwhrs per day.

This can however vary considerably for a variety of reasons.

Monthly Variation

On a peak summer day you may be able to generate 6 or 7kw hrs per day.

In winter it can be quite different. typical reasons include:

  • Shorter Days – The panels may only get around 6 hours of sunlight.
  • Weather – For most of us winter means more clouds and rain which reduce the sunlight.
  • Less than Optimum Angle With the sun low in the sky you won’t have the panel fully facing the sun unless you can change the angle.

Partial Shading

One of the things that most people don’t realise is that solar panels are ‘team players’.

The power that each panel in a string(circuit) generates is limited by the performance of the worst performing panel in the string.

This means that if one 200w panel in a string of 6 is shaded by as little as 10% the loss can be 120w.

That’s 10% of the total string 6 x the 20w of the single panel)

To minimise this problem you need to ensure:

  • All obstructions are minimised. Even a TV aerial, or toilet vent pipe can make a difference.
  • If you have panels on different areas ( for example on each on a different roof) they should be on different ‘strings’ with separate inverter inputs.

Dirt

Any dirt on the panels will reduce the effectiveness so a regular clean will help retain efficiency.

It’s also worth thinking about keeping birds from perching on the top edge and crapping on the panels. (You can get some plastic spikes to fasten to the panels)

Heat

Typically panels are at their highest efficiency at a panel temperature of 25degees C.

In the middle of summer the panel temperature can be 80 degrees.

If you panel is mounted to allow better air movement around the panel this will help keep the temperature down,  and help efficiency.

 

See Solar Electricity for more posts

 

Eaves

If you go back 39 years most houses had proper eaves but now they are less common.

As I travel around I sometimes see new houses with eaves on the front facade but non elsewhere, which I think looks weird.

If you are thinking about ‘eaves’ on your new house here are some advantages and disadvantages.

Advantages

  • The eaves keep the rain off the walls. As well as improving the weather proofing this helps improve the thermal performance of the walls in winter.
  • They will shade north facing windows in summer while letting the winter sun in. (This effect is negligible for windows facing in other directions and only about 50% effective for full length windows and patio doors)
  • Appearance. I think they give a more finished appearance and the shadow line adds interest.

Disadvantages

  • Cost. A typical cost is around $80/m2 so 600mm eaves all the way round a typical house could add around $4,000 to the cost. 450mm eaves will be a little bit cheaper.
  • You can’t build as close to the boundary which can be important if you have a narrow block.

As for me my first Australian house had eaves, but my next house did not so I have a foot in both camps.

I didn’t mind the look of the design without eaves and I built a pergola on the north side for shading.

Are you for, or against, having eaves on your new house?

 

To find out how big your eaves need to be… see Shading Northern Windows

 

Double Glazing or Smaller Windows?

I’ve posted on Double Glazing but that not the only way to save heat loss through windows so I thought I would do a numerical comparison of the various options for glazing treatment of windows.

In a bedroom of our last house the South facing windows were approximately 4m square (We are in Australia so these windows don’t get any sun).The  basis of my calculations is a difference of 15oC between internal and external temperature.

The equation used to calculate heat loss is:

Heat Loss  =  Area  x  Temperature Difference   x   ‘U’

for

‘U’ single glazing = 7*

‘U’ double glazing = 3*

‘U’ brick veneer = 0.51

* ‘U’ value includes effect of frame.

Option 1 Do Nothing

Heat loss through sinle glazing  =  4 x 15 x 7  =  420watts  =  0.42kw/hour

Remember this heat loss is for one room only.

Option 2 Reduce window by 40% to 2.4 m

Heat loss through single glazing  =  2.4 x 15 x 7  =  252watts  =  .25kw/hour

Heat loss through new brick vineer   =  1.6 x 15 x .51  =  12 watts  = 0.012kw/hour

Total heat loss  =  0.25kw/hr  +  0.012kw/hr  =   0.262kw/hour

With our builder this was a no cost option that has reduced the heat loss by 38%.

Option 3 Double Glazing

Heat loss through double glazing = 4 x 15 x 3 = 180watts = 0.18kW/hour

This is a heat loss reduction of 57% but at a significant cost.

Option 4 Reduce Window by 40% and Double Glazing

Heat loss through double glazing = 2.4 x 15 x 3 = 108watts = 0.108kW/hour

Heat loss through replacement brick wall = 1.6 x 15 x .51 = 12 watts = 0.012kW/hour

Total  =   0.108kW/hour  +  0.012kW/hour  =  0.12kW/hour

This final option has reduced the heat loss by over 70% and will be around 30% cheaper than double glazing the original large windows.

I hope this has given you some food for thought!

See Insulation for similar Posts

For Posts about Green Building see Sustainability

Round or Slimline Tanks?

These days many states require you to have rainwater storage , unless you have access to recyled water.

But which type of tank will be best?

Round Tanks

The standard is round tanks.

These cost less because they are easier to make and use less material per litre of stored water.

It is easy to see the entire contents of the tank from the top access so you can check on sediment level.

Also, it is easy to hose sediment to the tanks drain point when the tank needs cleaning. . . . NEVER GET INSIDE YOUR TANK.

Slimline Tanks

A popular shape is the slimline tank which will fit in a narrow space.

I have even seen a row of them used as a boundary fence.

A disadvantage is it is much more difficult to to clean out sediment.

Before you make a choice it could be worth thinking about finding out if you can find room for a round tank.

Cost

Typically the cost increase for a slimline tank over a round tank of the same volume is at least 50%.

A comparison from my local builders merchants:

    • $1,200 for a Slimline 3,000 Litre tank
    • $800 for a Round 3,000 Litre tank

If you haven’t the width to fit a 3000L tank you could perhaps look at getting two round 2,000Litre tanks for a similar price as the 3,000Litre Slimline tank.

 

 

See this post to find out How Much Rainwater Storage You Need

 

Rainwater Tank Problems

Rainwater Tanks aren’t always trouble free, so here is some advice on a couple of problems that you can encounter.

Cleaning

Even with pre-filters there will eventually be a build up of dirt and cloudy water at the bottom of the tank.

There are professional tank cleaning companies that will provide the safest way of cleaning a tank.

As they can filter the water out of the sludge they minimise the water lost from your tank.

If you are going to do it yourself the best time is to do it when the tank is low.

Working from out side the tank use a pressure washer to wash the sides and agitate the sludge in the bottom, then flush it out of the drain.

Keep Out of the Tank

Whatever you do DO NOT Climb Inside The Tank.  

Working inside tanks (known as Confined Space Work) is a very risky thing and several people have died inthe last few years.

Chlorination

Regular chlorination of your rainwater tank should not be necessary, although there will be some bugs in the water you will develop an immunity.

However if:

  • People are getting ill, or:
  • You suspect the water in your tank is contaminated.
  • You have a new baby, or
  • Small children from the city are visiting

. . . .you can chlorinate the water as it would be sad if your visitors became ill because they weren’t used to drinking tank water.

To chlorinate add either swimming pool chlorine (calcium hypochlorite 60-70%) or sodium hypochlorite (bleach) 12.5% by volume.

The dose to treat the contamination should be 7 grams of calcium hypochlorite or 40mL of sodium hypochlorite per 1000 litres of water in the tank at the time of treatment.

Rather than just add the chlorine fill a plastic bucket with water to 2/3 full and then add the chlorine, in the open air.

DO NOT add Water to Chlorine. Empty the bucket into the tank, being careful not to spill it, and then mix the contents of the tank.

An easy way of mixing if you have a pressure pump is to put a hose into the tank and leave it running for 15 – 20 minutes.

Once the water is mixed leave it to stand for at least 24 hours to allow the chlorine taste and smell to dissipate.

 

Have you had rainwater tank problems?

 

For more about tank water quality see Rainwater Safety

For similar posts see Sustainability

 

Insulation Basics – Ventilation

A significant factor in the comfort of you house is the ‘Ventilation Rate’.

No one likes cold draughts in winter.. . . and hot air creeping in during summer can be equally unwelcoming.

So how do you calculate heat losses due to ventilation?

Well it starts by deciding how many air changes per hour (ac/hr) you have.

That is how often is the air in the areas you want to heat, or cool, replaced.

Typical ventilation rates

Its hard to measure the actual changes without specialist equipment however here are some typical values:

    • Old weatherboard property – More than 2ac/hr
    • Typical new house – Around 1 ac/hr
    • Well draft sealed house – 0.5ac/hr

Calculations

To calculate the ventilation heat loss the following formula is used:

Qv = 0.36 x V x N

Qv – Ventilation heat loss in in W/degree C
V – Volume in cubic m of space being heated or cooled
N – Number of air changes per hour

So for a typical house with a floor area of 250m2 with 2.3m ceilings:

Qv = 0.36 x (250 x 2.3) x 1 = 207W/degrees C

To keep this house at 20 degrees with an outside temperature of 10 degrees would need 207 x 10 watts = 2.07kw/hour…… just for ventilation losses

A well draft sealed house will reduce the heating required to 1.04kw/hour. (You would be saving $1 every five hours, with more saving on colder nights)

WARNING

If you have got a flue less heater (such as portable gas heaters, kerosene heaters or bio ethanol heaters) you do need some ventilation to keep a safe level of oxygen in the room.

See Insulation for similar Posts

For Posts about Green Building see Sustainability

 

Insulation Basics – Reflective Finishes

There are a number of insulation products with reflective surfaces now available but how do they work?

Why do the instructions tell you to put the more reflective surface on the underside rather that the top?

Here is some information to help you understand what’s happening.

All surfaces have different levels of ability to reflect heat with black and matt finishes being very poor and shiny metal surfaces being very good. Most reflective surfaces are also poor emitters of heat. This characteristic is called emissivity (normally written as ‘e’).

An example is clean aluminium foil has an ‘e’ value of around 0.03. This is much less than common building materials such as:

  • bricks ‘e’ = 0.9 ; or
  • Building membrane ‘e’ = 0.5

Low emissivity finishes are generally considered to have an ‘e’ value of less than 0.05.

As dust can increase emissivity by 0.25 Aluminium foil is only considered to be low emissivity on vertical surfaces or the underside of a inclined or horizontal surface such as a roof or ceiling, where it will remain clean.

The emissivity acts with the surface layer of air on the building material to give an overall R value for the surface.

The following table shows how the ‘R’ values increase with a Reflective (low emissivity) surface compared with a high emissivity surface.

 

Cavity Width

Direction of

Heat Flow

Low Emissivity

R value

High Emissivity

R value

Horizontal

Up

0.23

0.11

Down

0.8

0.16

Sloping 45 degrees

Up

0.24

0.11

Down

0.39

0.13

Vertical

Horizontal

0.3

0.12

You will see that low emissivity insulation such as aluminium ceiling insulation has better performance (Higher R values) when the the Hotter air is above the insulation than the reverse.

This means that in summer it resists heat during the day but allows the house to cool more quickly at night. In winter, however the performance in keeping the heat in the house will not be as good.

 

See Insulation for similar Posts

For Posts about Green Building see Sustainability

 

Buffer Zones

Planning a house layout? or considering builders standard layouts?

Thinking about buffer zones when you are will save you on heating and cooling costs.

So what are buffer zones?

Buffer Zones are basically rooms and spaces that may be heated to a lower temperature, only heated occasionally, or even left unheated in winter.

In summer the situation is reversed and these rooms and spaces will not need to be cooled to the same extent as the main rooms.

As these rooms and spaces are at a temperature between the main rooms and the outside they act as additional insulation reducing the cost of keeping the main rooms at a comfortable temperature.

Examples of Buffer Zones

Typical  ‘Buffer Zones’ are:

      • Roof space
      • Garage
      • Guest bedrooms
      • Laundry
      • Study
      • Toilet
      • Bathrooms
      • Porch
      • Conservatory

An example of using buffer zones can be seen on the following floor plan.

The study, laundry, main bathroom, toilet, en-suite are all enclosed rooms on the South side of the house.

They don’t need to be heated/cooled all the time.

For instance in winter the bathroom only needs to be heated for around half an hour in the morning.

All this reduces the volume to be heated in winter and stops heat loss to the North.

A popular buffer zone in England is a Porch.

We haven’t had one in an Australian house yet and they don’t seem to be very common in standard designs. . .but if you live in the High Country, or Tasmania, one could be well worthwhile.

Conservatories are also less common in Australia probably because of the overheating risk in summer.

They can however be useful for increasing ventilation through the house if properly designed with large top vents.

Has a buffer zone worked for you?

For more posts about plans see the Design Category.

To save money on Heating and Cooling see Insulation

 

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