How to read an ELV bill, whoever it comes from

An ELV bill for a villa can run to several pages of part numbers, and most people who receive one read the total, glance at the brand and hope the rest is right. It does not have to be taken on trust. The questions below will tell you, in an evening, whether a bill describes a designed system or an educated guess, and they work on any bill, including ours.

A good bill can explain itself

A bill is more than a price: it is the written record of a design, and every line on it should trace back to a decision about your villa. When it does, you can question any line and get a proper answer. When it does not, the total may still be right, but neither you nor anyone checking it for you has any way of knowing. A bill with no reasons behind its lines is a guess with a total, however professional the letterhead.

None of the questions that follow needs an engineering background; they need only the bill, the floor plans and a little patience, and every one of them is fair to put to any supplier.

Eight questions to ask of any ELV bill

1. Does every line have a reason?

Pick a line at random and ask why it is there. A keypad in a guest bedroom should exist because the room needs a control point at its entrance, a presence sensor because a corridor ought to light itself, a second access point because the floor area calls for one. If the answer is that this is how it is usually done, the line came from a template, not from your plans.

1. Are unknowns shown as "to confirm", or hidden as zeros?

Every design has a few facts that cannot be settled at once, such as whether a particular air-conditioning model has a verified automation gateway. A careful bill prints those items as "to confirm" and says beside the total that it excludes them. A bill that shows them at zero, or rounds them into a guess, looks complete, but it is carrying a cost it has not yet admitted to.

1. Are the power supplies there?

Devices have to be powered from somewhere, and a bill that lists them without the supplies to run them is incomplete, however attractive its total. On a KNX system the bus power should be budgeted in milliamps against the supply, with margin to spare. Wall screens deserve a separate check, because screens of 7 inches and larger are not powered by the bus and each needs its own 24 V supply in the distribution board. Count the screens, then count the supplies.

1. Is the recording storage calculated?

If the bill includes CCTV, look for the number of days the recorder is meant to hold and for some sign that the drive was sized from it. Storage depends on the cameras, their resolution and how they record, so it can be worked out rather than assumed. A drive chosen because it is the usual size may hold less than you expect, and you will find out on the day you need footage that has already been overwritten.

1. Are there spare actuator ways?

Actuators are the switching modules in the distribution board, and every controlled circuit occupies one of their channels. A sensible design leaves some channels free, about 15% in ours, so that a circuit added during the build or a few years later has somewhere to go. A board filled to its last channel will need another actuator, and possibly more board space, the first time anything changes.

1. Is there one switch series on the walls?

Walk through the bill room by room and picture each wall. Keypads from one range, sockets from another and screens in a third finish will look assembled rather than designed, whereas one brand, one series and one finish is a decision the bill should make visible. It is also worth asking whether you need new switches at all, since a retractive switch series that matches your sockets can join a KNX system through a binary input behind the switch.

1. What is in scope, and what is out?

Find the list of inclusions and exclusions, and ask for one if there is none. Check whether VAT at 5% is included, and who supplies and pays for the wiring, conduit, containment, installation, programming and commissioning. A bill that is silent on these is not necessarily cheaper than one that states them; it may simply have left them for later. Anything given to another trade, curtain motors for instance, should appear as a written exclusion rather than an absence.

1. Are these unit prices or ranges, and MSRP or trade rates?

Before comparing two bills, check what kind of price each one shows. MSRP is the manufacturer's suggested retail price, set for the public, while trade rates are what installers and fit-out companies pay, and they are usually kept private. A range or an advertised figure describes a typical house, while a unit price belongs on a bill for a specific design. Setting one supplier's trade unit prices against another's advertised range tells you very little until both stand on the same footing.

Comparing two bills fairly

Once both bills answer the same questions, set them side by side on the same scope. A shorter bill is not automatically a weaker one, since a kitchen run by four scene buttons rather than one button per circuit is a better design and a smaller bill at the same time. What matters is whether each difference has a reason. A lower total that turns out to be missing its power supplies, storage or spare ways was never really lower; it was simply less finished.

How our own bill answers the same questions

We built our bill to pass this test, so it is only fair to say how. Every line carries, in one sentence you can read, the rule that placed it. Unknowns print as "to confirm", each system carries its own subtotal, the total says what it leaves out, and every price includes 5% VAT and says so. The switch line reads AED 0, because we sell no light switches and would rather you kept the ones that match your sockets.

What to do next

Use these questions on whichever bill is in front of you. If you would like a second one to compare it with, send us the plans, and read the reasoning on every line before you decide.