Return On Investment Calculations

With trepidation I finally added my equipment into a spreadsheet to determine the return on investment. Some comments are necessary.

Because I live in Hawaii (and there is an arcane Jones Act), shipping  costs are horrible. Some cheaper items were purchased via Amazon and there may or may not be shipping included or had free shipping due to my Prime account. These are not major items and thus are not major shipping costs. However, the controller, rack and batteries had shipping costs added and that added 30% to to cost of those items. That shipping costs is not included in my ROI although it makes a difference to me, other people on the mainland would not see that increase. Thus these numbers below more accurately reflect what others would pay.

I have added some conectors, caution tape, 100′ of wire from the solar panels to the controller, a DC circuit breaker to disconnect the panels, an emergency button to shut it all down if there is a need, and some thing which are not needed afterall but offset items I might have forgotten to add.

So panels, wires, controller, batteries, rack, and extraneous equipment and some tools to install totalled $7500. This does not include any possible tax incentive.

Even though the panels are not yet aligned and no reflective ground item is in plane and that I am only using 2 or 3 batteries, I have dropped my electric bill from $280 to $135. That means that at the moment my Return On Investment is about 4.5 years. Not too bad, but not yet low enough.

With my present setup, mostly or cloudy days causes me to have to charge the batteries to ensure that I can use batteriess to supply the house over night and make it until sunrise. I wait until 3pm and if the batteries are low enough, I change them up until 5pm when the electric rate rises. So I am waiting until the last minute to decide if I need to charge them and then take advantage of the cheap rates to charge them and stop charging minutes before the rate zooms up. In reality, I charge them to 80% or more when I really don’t need to do that. When I get the third battery installed, I will determine just how much capacity I need over night until (not sunrise) but 9am when the rate plummets.  That allows me to charge the least amount with the lowest rate based upon normal home usage and maximizes my savings.

I have all sorts of metrics from the controller such as how fast the batteries charge, at what state they are at any given time of day, how much solr I have been receiving, how much power I am using and so on.

One issue that I have to get a good grip on is this. At any given time, I receive a certain amount of electric from the sun. I also have a maximum amount of power I can store in the batteries. Finally at any given time I have a specific amount of power that the house uses. The system will accept the solar and convert it to electric. That electric first goes to power the house. If the panels generate more electric than the house needs, it will still power the house but will also charge the batteries. The batteries have a maximum rate of charge and if the panels generate more electric that the batteries and take at that time (or if the batteries are full) and the system is already supplying enough power for the house, the rest of the electric generated is just lost. If I was able to dump power to the grid, the electric company woudl pay me for that excess power. However, Hawaii like other states is starting to reduce or eliminate the ability for ‘net metering’ which allows that power to be fed back to the utility. Personally I would not want to sell power anyway as it complexity and other restrictions. I just prefer to use the grid when necessary and supply the rest of my needs from the sun.

In the past 5 months, I have dropped my usage in half. It will get less and I hope that I will soon end up paying just the minimum rate for the meter and standby charges.