What a unit really is
Every bill is built on one number: the unit, which is simply a kilowatt-hour (kWh). A kilowatt-hour is the energy used by a 1,000 watt appliance running for one hour. A 100 watt fan running for ten hours uses the same one unit.
Your meter counts these units as they flow into your home. The rest of the bill is the utility putting a price on those units and adding its fixed costs and taxes on top. For a plain-language primer on how a kilowatt-hour is defined and measured, the US Energy Information Administration is a solid reference.
Reading your meter
The bill's units are the difference between this month's meter reading and last month's. On a digital meter it is the number that ticks up over time. You can read it yourself at the start and end of a month to check the bill, or to see how much a single appliance adds by reading before and after you run it.
If your bill looks wrong, a self-reading is the fastest way to catch an estimated or mistaken reading before you pay it.
Tariff slabs and why the last units cost most
Many countries price electricity in slabs, or tiers. The first block of units each month is cheap, the next block costs more per unit, and heavy users pay the highest rate on the units at the top.
This is why two homes using similar totals can pay very different bills. The one that stays inside the lower slabs pays far less per unit than the one that spills into the top tier.
A worked example: what 350 units actually costs
Slab rates differ by country and utility, so use your own bill's numbers. The structure below is typical of tiered residential tariffs and shows the arithmetic that matters. Say the bands are 0 to 100 units, 101 to 200, 201 to 300, and 301 and above, at rising rates.
| Slab | Units in this band | Rate per unit | Cost for the band |
|---|---|---|---|
| 0 to 100 | 100 | 10 | 1,000 |
| 101 to 200 | 100 | 18 | 1,800 |
| 201 to 300 | 100 | 26 | 2,600 |
| 301 to 350 | 50 | 35 | 1,750 |
| Total | 350 | — | 7,150 |
The average works out to about 20 per unit, but that average hides the important number. The last 50 units cost 35 each, not 20. Those top-band units are the expensive ones, and they are the ones you can remove.
This is also the honest way to judge whether solar or a backup system pays for itself. Do not value the energy you displace at the average rate. Value it at your top slab rate, because that is the band solar removes first. On the bill above, every unit solar covers is worth 35 until you drop below 300, which changes the payback maths considerably.
One caution: some utilities apply the higher rate to your entire consumption once you cross a threshold, rather than only to the units above it. If your tariff works that way, crossing a slab boundary is far more expensive, and staying just below a threshold matters a great deal. Check the wording on your own bill before assuming which model applies.
Fixed charges, taxes, and surcharges
Not every line on the bill is about the units you used. Most bills add a fixed meter or service charge, government taxes, and sometimes fuel or financing surcharges that move with policy rather than your usage.
These are worth understanding because they set a floor on the bill. Even a near-empty house pays the fixed portion. When you plan savings or decide on your solar system size, focus on the part you control, which is the units, and know that the fixed charges will not shrink no matter how careful you are.
Time-of-use tariffs: when you use it can cost more than how much
Some connections are billed on a flat rate per unit, but many are on a time-of-use tariff, where the same kilowatt-hour costs different amounts depending on the clock. Peak hours, typically the evening block when everyone gets home and the grid is strained, can be priced at nearly double the off-peak rate. Your bill will show peak and off-peak units on separate lines, each with its own rate.
If you are on one of these, look at the split before you look at the total. Moving a load you can schedule, such as the washing machine, the water pump, or an electric water heater, out of the peak window changes the bill without changing how much energy you use. This is also the argument for a battery on a time-of-use tariff: you charge when units are cheap and discharge through the expensive block, which is a different reason to own one than backup during outages.
Demand charges and power factor penalties
Two more lines catch out anyone on a commercial or larger domestic connection, and neither is about total consumption.
A demand charge bills you for the highest power you drew at any one moment during the month, usually averaged over a 15 or 30 minute window, and it is priced per kW rather than per kWh. Start three large motors within the same half hour once in a month and you pay for that peak for the whole month, even if you were nearly idle the rest of the time. The fix is staggering big loads rather than reducing them.
A power factor penalty applies when your load draws more current than the real work it does. Motors, pumps, and compressors are inductive: they pull current that lags the voltage, so the utility has to deliver more apparent power (kVA) than the real power (kW) you get billed for. The ratio between the two is the power factor, and most utilities charge a surcharge below a threshold, commonly 0.90. Capacitor banks correct it, which is why you see them in workshops and factories. It is the same relationship between watts and VA that makes an inverter size calculator divide your watts by about 0.8 to get the VA rating to buy.
Finding the appliances behind your bill
A high bill almost always traces back to a few appliances that are both high-powered and run for long hours. Air conditioners, electric water heaters, irons, and old refrigerators dominate. Lights and electronics barely register by comparison.
Practical ways to cut the bill
Once you know where the units go, cutting them is straightforward. Aim at the big users and the long hours, not the small stuff.
- Attack heat and cooling first. Raising the AC by a couple of degrees, or using fans until you really need it, cuts more units than anything else you can do.
- Replace the biggest energy hogs. An old fridge or a conventional AC runs for hours daily, so an efficient inverter model pays back through lower units.
- Shift heavy use off the top slab. If your tariff is tiered, trimming your heaviest appliance can drop you into a cheaper band and save more than the units alone.
- Kill standby loads. Televisions, chargers, and set-top boxes sip power around the clock. Switch them off at the wall.
- Swap bulbs for LED. Small per bulb, but lights run for many hours, so the units add up.
Net metering: making the meter run backwards
Once you have cut what you can, the remaining lever is generating your own units. Net metering is the arrangement that lets you export surplus solar generation back to the grid and be credited for it. A bidirectional meter records import and export separately, and you are billed on the net difference. On a sunny afternoon when the panels make more than the house consumes, the surplus flows out and the credit offsets units you draw after dark.
Three details decide whether it is worth applying for. First, the export rate is often lower than the import rate, so a unit you send out may not buy back a full unit at night. Second, credits usually carry forward month to month but settle annually, so oversizing the array well beyond your own annual consumption rarely pays. Third, it requires a grid-tied or hybrid inverter and utility approval, which a plain off-grid battery inverter cannot provide.
Net metering also changes how you size a system. Without it, you size to what you can use or store at the time you generate it. With it, the grid acts as your storage, so annual consumption becomes the target instead. Our guide on how much solar you need works through both cases.
Frequently asked questions
What is one unit of electricity?
One unit is one kilowatt-hour (kWh). It is the energy a 1,000 watt appliance uses in one hour, or a 100 watt appliance uses in ten hours.
Why did my bill jump when I used only a little more power?
Most tariffs are tiered, or slabbed. When your monthly units cross a threshold, the price per unit rises for the units above it, so a small increase can push you into a more expensive band.
How do I estimate my bill before it arrives?
Work out your monthly energy in kilowatt-hours, then multiply by your tariff per unit, and add the fixed charges and taxes shown on a past bill. Our appliance wattage calculator gives you the monthly kWh figure.
Does running an inverter increase my electricity bill?
A little. Charging a battery loses some energy as heat, so an inverter adds a small amount to your bill for the same appliances. The bigger cost is usually the appliances themselves, not the inverter.
Your bill is not a mystery once you see it as units times a price, plus fixed charges. Read your meter, learn your slabs, find the appliances driving the total, and you can plan real savings instead of guessing.
