If you are contemplating new submetering for the purposes of monitoring and targeting, you will need to justify it. You might rely on the advice in the Building Regulations, although this amounts to little more than an arbitrary rule of thumb and in any case is irrelevant in the context of industrial processes. A more objective method is to identify candidates for submetering on the basis of the “risk of undetected loss” (RoUL). The RoUL method attempts to quantify the absolute amount of energy that is likely to be lost through inability to detect adverse changes in energy performance. It comprises four steps for each candidate branch or subcircuit:
- Estimate the annual cost of the supply to the branch or circuit in question.
- Decide on the level of risk and express it as a percentage factor.
- Multiply the cost in step 1 by the factor in step 2, to get an estimate of the annual average loss.
- Use the result from step 3 to set a budget limit for installing, reading and maintaining the proposed meter.
Firstly regarding step 1, there are various ways of estimating the as-yet-unmetered consumptions:
- from temporary metering
- by step testing
- using ammeter readings for electricity, condensate flow for steam, etc.
- multiplying installed capacity by assumed (or measured) load factors
- Regression analysis may sometimes have a role as well.
Next let’s explore the risk factors mentioned in step 2. They represent the average proportion of consumption that is likely to be wasted, over the long term, through our inability to monitor (and hence manage) consumption in the connected loads. We might assign risk factors as follows, although these are only suggestions:
- High: 20%
- Medium: 5%
- Low: 1%
What determines the risk category that we put something in? I would say that ‘high’ risk is usually associated with highly-intermittent or very variable loads under manual control, or under automatic control at unattended installations—the risk is that equipment is left to run continually when it should only run occasionally, or is allowed to operate ‘flat out’ when its output ought to modulate in response to changes in demand. Examples of highly-intermittent loads include wash-down systems, transfer pumps, frost protection schemes, and in general any equipment which spends significant time on standby. Typical continuous but highly-variable loads would include space heating and cooling systems. It should be borne in mind that oversized plant, or any equipment which necessarily runs at low load factor, is also at increased risk.
Medium risk is typified by variable loads and intermittently-used equipment operating at high load factor under automatic control, in manned situations where failure of the automatic controls would probably become apparent quickly.
Low risk, meanwhile, is applicable to anything which necessarily runs at high load factor (and therefore has little capacity for excessive operation) or where loss or leakage, if able to occur at all, would be immediately detected and rectified.
As I indicated earlier, multiplying each estimated consumption by its associated risk factor gives you a set of figures for long-term average likely losses and that allows you to work out the economic benefit of each possible submeter. It is not a perfect method but it is much more rational than any other approach.