kW or kWh: the unit mistake that ruins solar quotes

Almost every confused conversation we have about solar comes back to one thing: kilowatts and kilowatt hours are not the same quantity, and half the industry writes about them as though they were.

A kilowatt is a rate. It is how fast energy moves at a given instant. A kilowatt hour is a quantity. It is what you get when that rate runs for a period of time. One kilowatt sustained for one hour delivers one kilowatt hour. The relationship is that simple and the consequences of ignoring it are not.

Where it bites on the generation side

Arrays are sold in kilowatts because that is the sum of the module nameplate ratings. A 9 kW DC array is twenty one 440 W modules, roughly. That number tells you the maximum rate the array can generate under standard test conditions, which is 1,000 watts per square metre of irradiance at a cell temperature of 25 degrees Celsius.

Your utility bill, on the other hand, is in kilowatt hours, because the utility sells you quantity. So a 9 kW array does not offset a 9 kWh bill. It produces roughly 12,000 kWh over a year in our region, and that is the number that meets your consumption.

The bridge between them is specific yield: kilowatt hours produced per kilowatt installed per year. Across our fleet in the Triangle that figure sits around 1,300 to 1,400 kWh per kW for a well oriented unshaded array, as a sample range from our own monitoring. Multiply system size by specific yield and you have annual production. Nothing else about array sizing is more important than getting that one multiplication right.

Where it bites on the storage side, harder

Batteries have two ratings and people quote only one of them. The energy rating in kilowatt hours says how much you can store. The power rating in kilowatts says how fast you can take it out.

A 13.5 kWh battery behind a 5 kW inverter can supply 5 kW for a little under three hours, or 500 W for twenty seven hours. Same battery, wildly different experience, because the loads decide which case you are in.

Now add starting surge. A submersible well pump might run at 1,200 W but demand around 2,400 W for a second or two at start. A refrigerator compressor running at 150 W can pull 1,200 W at start. Those surges are a power problem, not an energy problem. They barely register in kilowatt hours and they will absolutely trip an undersized inverter.

A big battery behind a small inverter still trips when the pump starts. Energy and power are separate purchases.

A worked example

Take a household backing up a refrigerator, a chest freezer, the internet, main room lighting, one mini split and a CPAP machine. Running watts add up to roughly 1,070 W if everything runs at once, and the realistic daily energy, once you account for the refrigerator and freezer cycling rather than running continuously, comes to about 9.5 kWh.

Energy question: 13.5 kWh usable divided by 9.5 kWh per day is about 1.4 days of runtime with no sun at all. Good.

Power question: peak simultaneous draw of 1,070 W running, with the largest surge being the refrigerator at 1,200 W, tops out near 2.3 kW. A 5 kW inverter handles that without noticing. Also good.

Now add central air conditioning. A 3 ton condenser adds 3,500 W running and around 12,000 W of locked rotor surge. The energy question gets worse, since 3.5 kW for six hours is 21 kWh on its own. The power question gets much worse, because 12 kW of surge is beyond a single 5 kW inverter with 10 kW of surge headroom. This is exactly why we back up a mini split zone rather than the central system.

How to read a quote with this in mind

• Array size in kW DC and modelled production in kWh per year should both appear. If only one is present, ask for the other.
• Specific yield should be visible or derivable. Divide modelled kWh by kW and sanity check it against 1,300 to 1,400 for this region.
• Battery quotes should state usable kWh and continuous kW separately, plus surge kW.
• Any claim of backup hours should be tied to a named load schedule. Hours without a load list are marketing.
• Inverter AC rating and array DC rating should both appear, and a DC to AC ratio between about 1.15 and 1.3 is normal and deliberate.

The short version

Kilowatts decide what you can run at the same moment. Kilowatt hours decide how long you can run it. Sizing an array is a kilowatt hour problem. Sizing an inverter is a kilowatt problem. Sizing a battery is both, and the two answers are independent.

Any proposal that blurs them is either careless or hoping you will not notice. Neither is a good sign on a system you expect to run for twenty five years.