Can you size a PV system by just looking at a bill and analysing the peak/offpeak usage

Can you size a PV system by just looking at a bill and analysing the peak/offpeak usage

At PVsell, we often get asked whether someone can size a PV system just by looking at the bill? A similar question is “do you have to have interval metering data”?

The short answer is “In order to size a PV system for a customer, you HAVE to have an understanding of the times (and days of the week) that the customer uses energy”. Its not sufficient to know that across a month they consume a total of 1000kWh of energy during Peak tariffs and 600kWh during off peak tariffs.

That’s because that customer could be consuming all of that energy whenever the sun isn’t shining – in the night, early hours of the morning, or late afternoon. They may have minimal daytime consumption, meaning your solar system would export 100% of its generation. That example is perhaps a bit extreme – though there are customers (e.g. bakeries) with consumption profiles similar to what I described and some sites (e.g. wineries) with highly seasonal loads – but the example does illustrates a point. If you don’t know when the power is being consumed, then you don’t know what the daytime power levels will be, and therefore you can’t accurately size a PV system, let alone calculate how much it will save off a bill. And remember that some sites’ consumption levels can be very low on weekends, whereas others (e.g. cool-stores) can be highly correlated to solar radiation.

For example, here’s a sample of three days from the consumption profile of a motel that consumes 300kWh/day overlaid with a 30kW PV system. You can see on some days the solar system is exporting very little energy, but on other days its exporting a lot!


Because the value of solar generation that is consumed at the moment its produced (“self-consumption”) is so much greater than the value of solar generation that is export (“Feed-in Tariff”), its essential to know the amount of customer load AND solar generation in each and every hour of the year. PVsell handles all of those complex self-calculation/export calculations (including what the value of self-consumption is in each moment), even accounting for the orientation and tilt of the panels, which also affects how much solar power is worth in each hour.

That’s why you HAVE to have an understanding of the times (and days of the week) that the customer uses energy!

Now the easiest and most accurate way of doing this is obtaining interval metering data from your customer’s electricity retailer or distributor. Then you’ve got a perfect understanding of the customer’s consumption profile. You can even upload interval metering data files straight into PVsell!

But what if you can’t get hold of the interval metering data for your customer?

  1. You can hang a temporary clamp meter, or install a Wattwatchers device (for example) and take a month or even a week’s worth of consumption data. You can then upload this file into PVsell and extrapolate it to a full year.
  2. You can load in a consumption profile from a similar business. PVsell has a library of consumption profiles for many business types
  3. You can build up your own consumption profile or adjust one from the PVsell consumption profile library, based on your understanding of your customer’s operation hours and trading days.

None of these options will be as accurate as a full year’s worth of interval metering data, but it will be a whole lot more accurate than simply working off monthly or quarterly total energy consumption figures that occurred who-knows-when.

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