Spreadsheet Heaven

Ever since my Dad demonstrated a computer spreadsheet to me, I've been in love with them. They are so damn useful, though I'm sure they frighten a lot of folks. Their more advanced abilities frighten me but with my basic knowledge of how they work, I've generated many spreadsheets for a huge range of tasks. I have one for work that I enter my start and finish hours into and it calculates total hours, overtime, pay, tax and a lot of other cool stuff. At home, we use spreadsheets to keep track of our family finances. I religiously note my book's Amazon sales rank (and the rank of some of my friends' books) to watch the trends and I keep a close eye on the sales of my photography on a plethora of microstock websites; all using spreadsheets.

It therefore should not come as a surprise that I have a sheet where I keep a note of the meter readings for my gas and electricity supply. I have this spreadsheet calculate the daily cost for both and I watch how the spend rises and falls with the seasons. One interesting aspect of this is to note how this year's consumption of gas compares to last year's. We had our walls and loft insulated during the summer and I'm happy to say that, despite us having one of the coldest winters in decades, we used 100 cubic metres less gas than we did last winter.

I just know that the arrival of a solar PV system is going to be spreadsheet heaven. Not only will I be able to count the units of electricity we use, I'll eventually be able to count what we generate, how much of our own power we use and how much is exported to the grid. I'll then be able to apply monetary values to the results and see if the system is costing more or less than expected, and even if it has started to turn a profit of any sort.

And just because I haven't got my beloved panels installed on the roof yet, don't think I haven't already created a solar PV spreadsheet because I have. I was real keen to try to model the overall costings of the system. I had accurate figures for how much power our house used in a year and at what cost. I had been told how much power my PV system was expected to generate and I knew how much it was going to cost to install. I put all this into a simple sheet and had it work out the costs and benefits over 25 years, the expected lifetime of the panels and the term of the FiT arrangement. So did it teach me anything? Well...

It seems the most crucial aspect is how much the panels will generate annually, followed by what proportion of that is exported and how efficient we can become. The matter of export is clouded by something I read today. It seems that until the introduction of smart meters, the powers-that-be are going to deem that I will use 50% of my generated power and export the rest. OK, so I can plug that into the sheet. Only smart meters are going to be able to tell when I use power and whether there was power available from the panels at that time. Assuming all goes well with the grant and the loan, the cost to me to install the system will be £10,300. Well it turns out (given that this model will include some relatively simplistic assumptions) that over 25 years, the panels will pay for all my electricity and net me £10,273 - the same as what they cost me! There's a coincidence.

OK, so what improves things? The government tables say that I will generate on average 8 units a day. What if it's 9? It's for questions like this that I almost dribble with excitement at the thought of the spreadsheet I'm going to create to find out for real. Meanwhile, my predictive model has its own answer. Now my 25-year income jumps to £14,817, a clear profit of £4,500 and my electricity paid for quarter of a century.

Now instead, let's imagine that I'm only getting my 8 per day, but that we're managing to use the vast majority of it. I suspect this will be the case. This house's electricity consumption is heavier than most. So instead of 50% exported, let's assume 10%. The income rises to £12,317, mainly because we're having to pay for less electricity from the grid. This yields a £2,000 profit over 25 years and the leccy bills covered.

I'll do one last illustration with the starting point restored to 50% exported and 8 units per day generated on average. This time, we'll see the effect of cutting our annual consumption by 2 units a day, something that ought to be easily realised, mostly by simply replacing our fridge. (Old, noisy, power hungry beast whose efficiency I've been watching with a power meter to prove my suspicions about it.) The income figure is now £12,463, a £2,100 return after 25 years for the sake of a better fridge.

I can't control how much my panels will generate though I suspect they might deliver somewhat more than the government says. But I do have some control over how much power we consume and when we consume it. The lesson is that it will pay to use less power, and that, once the smart meter is installed, we should try to use power when the Sun is out. Whether or not reality follows these predictions will only be learned when I get to grow a nice, shiny, new spreadsheet.