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Detailed Instructions for the Constants Tab


This is one of the two tabs you’ll spend most of your time on.  We explain first the various values you should enter into this page, and then secondly, the results it returns back to you, based on your inputs.

Constants Tab Inputs (the orange values in the grey cells)

Solar Array Maximum Generating Capacity in kW Enter the theoretical maximum generating power capacity of your array in this box, in kW.
Cost per installed kW ($) This is an optional value.  If you want to get a feeling for cost issues when doing ‘what if’ scenarios, put in a figure here that is a reasonable approximation for the total installed cost of your solar array, per kW, including the cost of the panels, the mounting, labor, wiring, and some controller functionality too.  You can be reasonably approximate here, no need to get things to the penny, and of course the price will vary a bit anyway depending on total array size, etc.
Efficiency Factor from 0 to 100% for your array Enter in any number from 0 to 100 here.  Don’t add the percent sign, just enter the number.  There are several things to consider when entering this value – first the difference between stated and actual power per panel, second the losses through wiring, third the losses through occasional shading, and fourth the losses when converting the power from DC to AC.  We suggest you should consider 90% as a reasonably optimistic number, and then vary up or down from that based on your actual experience.
Roundtrip Efficiency for Power Storage from 0% to 100% If you are going to be using power to charge batteries or in some other way to store it, you will have power losses when converting from the electrical power to however the power is stored, and then more losses when converting back again.  We suggest you start with a value of 50% (which is probably a bit pessimistic for batteries) and then vary it based on the specifics of whatever system for power storage you are using (for example, hydrogen might have you with only a 15% efficient system).  The more efficient your system and the less power you lose, the greater this number would be.
Note there are additional factors below for the daily power losses from your storage system.
Enter this number without showing the percent sign, it will appear automatically.
Max Power Surplus Store (kWh) Be careful with the number you put in here.  This is the net amount of power stored, not the gross.  If you are storing power in batteries, and if you are planning to discharge your batteries to 75% of their rated capacity, then don’t show their full rated capacity, show only the 75% value.
Cost per kWh of storage ($) This is another optional value, if you want to do cost efficiency calculations.  Show the cost of the net storage of power you have – in other words, if you have a 10 kWh battery that you can buy for $3000, but you will only use 7.5 kWh of its capacity, then its cost is $400 per kWh, not $300.
Minimum wasted power when storing (kWh) We’re not quite sure what this might be!  But we sort of sense there may be some switching inefficiencies as between when the system decides to activate its power storing feature and when it says ‘oh heck, the trickle of spare power is so small it isn’t worth trying to capture it’.So we simply show a flat amount of power that is not captured by this process.
Avg hours of full sunlight equivalent Stick in the average monthly values here for your location.  We suggest you round the numbers down a bit (ie probably by at least 10%) so as to allow for worst case months and years, because the thing about an average month is that half the time, your actual experience will be worse than this!
Daily power consumption (kWh) How much electrical power will you use in each 24 hr period?  You can change this from month to month – for example, in the hottest months (which are also when you have the most power) you can treat yourself to a bit of a/c, perhaps.
You can also see just how far you have to cut back in the winter months to avoid running out of power by tweaking these numbers.
Daily storage loss (%) If you have any sort of energy storage system, you’ll probably be losing some of the stored energy every day.  Batteries both lose their charge and also need to be continually maintained with extra current flowing into them.  A reservoir will have its water evaporate.  And so on.  Some of these power losses will be a percentage of the power stored, and you can enter a value here for the amount of power lost every day.
Daily storage loss (kWh) As well as losing some of your stored energy each day on a percentage of total energy stored, you’ll probably also be losing other energy each day at a fixed rate.  Indeed, you could also consider the energy consumed by your energy management system to also be a fixed energy drain/cost.  You can enter a fixed daily energy loss here.

Constants Tab Results (the green data in the green cells)

Totals (On the right of the monthly input values) It might be interesting to see what your rounding down of each month results for total annual hours of sunlight.  How does that compare to adding up the official numbers you started from?
Avg (On the right of the monthly input values) For your convenience, you can see what the calculated average values for the four data series you just entered are.
Min (On the right of the monthly input values) For your convenience, this quickly shows the lowest monthly values for the four data series you just entered.
Max (On the right of the monthly input values) For your convenience, this quickly shows the greatest monthly values for the four data series you just entered.
Minimum power stored (kWh) This shows the lowest that your stored energy reserve will drop to during the year.  We recommend you should always keep at least one full day’s power requirements in reserve, so as to give you a safety margin in case something happens to your power generating system, and also to allow for the approximations and assumptions in this model.
Number of days below min daily consumption power stored We take your minimum daily power consumption figure from the twelve months of data you entered, and then look to see how often your stored energy reserve is below this amount of energy.  Ideally this should never drop to zero days; any day when you go below the minimum day’s needs can be considered as getting into ‘danger’ territory.
Number of days with no power stored If you have no power reserve, then your total power use for the day can not exceed the total power generated during the day.  Except for the very rare cases when your power generation and consumption is exactly in balance, you can safely assume that any day with no power reserve means that you’re not generating enough power for your daily needs.  You definitely want this number to be zero, unless you like living in the cold and dark.
Number of days with max power stored Looking now at the happy other side of the equation, here’s a number that hopefully will be greater than zero.  If it is zero, that suggests you have ‘too much’ reserve energy storage and perhaps you should spend more on panels and less on storage.  If it is greater than zero, then this tells you how many days a year you have spare power.  If it is a very large number, perhaps you have either too many panels or too little storage capacity.
Net Unstored Power (kWh) This is the total amount of power you generated but did not use and did not store.  It has already been rounded down to adjust for the roundtrip inefficiency when converting from power to stored power and back to power again.  It is reassuring to see some ‘spare’ power, but if it is ‘too much’ then maybe you either don’t need so many panels or do need more storage.
Total cost for solar setup ($) If you entered a cost per kW for your solar system, this will show you the total cost of your solar setup as you vary its size/capacity.
Total cost for storage ($) If you entered a cost per kWh for your energy storage system, this will show you the total cost of your storage as you vary its size/capacity.
Total cost for both ($) This is obvious.  But it is very interesting to see that depending on how you figure the balance between power generating and power storage, the overall total system cost can vary enormously.


For more information, please return to the main page explaining the Solar Energy Calculator spreadsheet.

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