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How much solar do you actually need?

This is not too hard to figure out.

Grab a hold of one of your power bills and find the average number of units you use each month.  Most bills actually have a graph on them that shows you your total consumption each month for the past year.  You can use this graph to guess the average pretty easily with a quick glance.

Once you have your average units, (also called kwh) put that number into the following calculator by sliding the green thingy until it matches roughly your average monthly consumption.

Ecosmart Solar Calculator ©

Drag the slider to select your average total units by month, day or year.

Total Number of Units

Per Day

Per Month

Per Year

Panel Size (watts)

Panels Needed

KW Solar System

Disclaimer: This calculator is intended as a guide only. There are many factors which will determine the actual production of a solar system.

That calculator at least gives us a starting point.  It gives us a rough idea on what size solar system you will need to generate a comparable quantity of electricity to what you consume each month.  You may or may not want to go that big but the rest of the article will help you decide on that.

Many things will affect your actual monthly production like summer, winter, quantity of sunny vs rainy days, panel direction, panel angle etc - but let's just keep it simple and work on averages here.


Now how much of that energy are you going to actually use?

That is the most important question.

When it comes to solar, the aim is to use your own generated energy as much as possible.  This is because of the significant difference in price between what you buy your energy for and what you sell it for.  The reason for this difference is explained fully in one of my other articles here.

So unless you are using your own generated energy at the moment it is being generated you are going to be buying it for a lot more than you sell it for (unless you install batteries).

Therefore, the utilisation of your own generated energy is key (self use).  You are aiming to reduce your consumption of electricity from the grid as much as possible with whatever budget you have.  Which gets me to my next question.



What is your goal and what is your budget?

Your goal is is rather obvious.  You are trying to get rid of your power bill completely and maybe even earn a little income from selling your surplus energy.

Right? If you could afford it you would do it right?

So really the only factor we need to consider is your budget.  I can get rid of anyones power bill provided they have enough space for the panels and are willing to pay for it.



Maximising your return

If you filled in your average units into the calculator above we know what size system you will need to generate, on average, a comparable quantity of electricity.

Step 1: Economise

This has to be said.  The first thing I always do when looking at someone's place is find out how they are using their energy and whether they are wasting any.  Things like efficient heating (heat pumps) and LED lights etc go a long way.  Fix any existing problems first.  There's no point installing an oversized system to counteract the fact you are throwing electricity down the drain.

I recently had a client who wanted to install solar because he was using 6000 units per month in winter.  The idea of trying to offset that with solar is just ridiculous - you'd probably need a small nuclear power plant!  It turns out he had a 15kw underfloor heating element!  So of course the first thing we did was install a heat-pump unit that uses a quarter of the energy.  This alone would have given him bigger savings than installing solar.  Of course, after installing the heat pump, we then installed the solar.


Step 2: Figure out how much of your electricity you consume (and can make yourself consume) during sunlight hours. 

We want to figure out how much of your energy you can consume while the sun is out.

  1. Generally a premise has what I like to call a "base load".   This is a relatively constant draw of electricity which is caused by things running such as fridges, freezers, alarms etc.  
  2. Figure out what appliances could be put on timers during the day such as spa pools, hot water cylinders etc
  3. Find out how much you could force yourself to use electricity when it's sunny.  e.g. doing drying etc during the day.

You could put each of these items into a spreadsheet something like follows:

Note: KWH (kilo-watt-hours) is (Power Consumption in watts / 1000) x hours.

 Appliance Power Consumption (watts)Sunshine Hours On (Hours) 

Total KWH

Base Load  200w 9 1.8 kwh
Spa Pool 3000w 2.5 7.5 kwh
Hot Water 3000w 2 6 kwh
Total Daytime Consumption 15.3 kwh


I know what you're thinking, they should dump the spa pool?  Too right!  Well, at least swap the element out for one of our heat-pumps!



I've been doing this so long I can practically take a cursory glance at a place and tell you what you will need.  So if you're getting stuck, don't hesitate to get in touch via our contact page.


Step 3: Decide on what you can afford

Take a look at your average monthly consumption from your power bill and your total daylight consumption from the spreadsheet.  We have a current specials page which lists different systems and their rough prices.  


Solutions by Affordability

1.  The best solution would be to install a system that generates a similar quantity of electricity to what you use and install a suitably sized battery bank that covers your night usage.  This will get you close to being self-sufficient.  There is nothing quite like have a zero power bill or even a credit!

2.  The second best solution would be to install a system that generates a comparable amount of energy (the figure calculated at the start).  The reason I say this is because you could at least install a battery later and become self-sufficient.

3.  The next best solution would be to install a system that covers your daytime loads as calculated in the spreadsheet.  You can use the calculator in reverse so to speak until your "Units per Month" figure from your power bill matches your "Total Daytime Consumption" value from the spreadsheet multiplied by 30 (days).  e.g. in the above spreadsheet the total is 15.3kwh and this multiplied by 30 = 460 units per month.  Using the calculator until it shows about 460 units gives us a system size of around 3.8kw.

4. The final solution would be to put in whatever you can afford!  You are going to reduce your bill proportionately to whatever you spend anyway.


Is it a worthwhile investment?

At the moment you are paying for electricity and don't get anything for it.  When you install solar your are getting an asset.  The money you would have spent on electricity goes directly to paying off that asset and you should have money to spare (if you do it properly and with us anyway).

For example, say you are paying $300 per month on electricity and for $25k you could get rid of that bill.

Using the ASB mortgage calculator, today it tells us that you will pay $177 per month over a 20-year period at an interest rate of 5.8%.

Therefore if you didn't install it it will cost you an extra $123 per month

The other major point is that at the moment you are not getting anything back for the money you are currently spending on electricity.  You are "throwing it away" really.  In the previous example, at $300 per week, that's $3600 wasted per year which you're get nothing for.  When you install solar it is a major asset and increases the value of your property.  Recently one of our "no power bill" clients sold his house.  He advised me that the solar system was the hottest topic for discussion with each prospective buyer.  If you were looking at purchasing a new property how much would the fact you would not have to pay for electricity appeal to you?

The last point I would like to make is that the price of electricity is increasing at an alarming rate.  I would even go as far as to predict that the price of electricity will at least double in the next short while.  The rate of any loan would decrease over time and your actual savings would multiply.




If we can be certain of something, we can be certain of this: the price of power isn't going to get any cheaper.   In fact, in the last two years the price of electricity has soared.  With the increase in electric cars and other high-energy demanding devices, we are in for some horrendous power prices indeed.  We are in a world going electric.  Fossil fuels are out and electricity is in.  

If you've been thinking about getting solar, the time is certainly now!

I hope this helps and I wish you lots of sun!

About Us

We are an electricity retailer and solar installation company.

By combining wholesale energy with solar, we make people self-sufficient while still being connected to the grid.

Some of our partners

Trina Solar
Canadian Solar

Contact Information

    +64 7 595 0001

8 Hounsell Road
Burbush, Hamilton
New Zealand 3200

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