# How to calculate maximum demand charge from appliance wattage

energy-audit

My roommate and I are planning to switch to the Terms of Use – Residential Plan that Georgia Power offers (see link below).

The one item on this plan where I'm not sure of my calculations is the Demand Charge.

The demand charge is a \$6.64 fee per KW of Maximum kW.

Where Maximum kW is defined as:

Maximum kW shall be the highest 30-minute kW measurement during the current month.

Assume my Maximum kW is caused sometime during the month by my dryer being on.

If my dryer is rated at 2790 watts, does this mean the contribution by the dryer to the maximum wattage is going to be 1.395 kW or is it going to be 2.790 kW?

If it helps, the "30-minute kW measurement" is what is confusing me.

Link to plan rate sheet: https://www.georgiapower.com/docs/rates-schedules/residential-rates/2.40_TOU-RD-3.pdf

The question asks how 30-minute demand is measured and how a 2790 W dryer contributes. There are actually quite a few number of methods for measuring demand, based on old electromechanical meter technology. Digital meters make it simpler today.

To measure 30-minute demand, the power company splits each hour into two intervals. The 30-minute demand of a interval is the number of kWh you consume in that interval times 2 (since there are two intervals in an hour). The power company takes the highest 30-minute demand over the whole bill cycle and charges you for it.

The question is how does a 2790 W dryer contribute? It depends! Let's assume that you only have that dryer running in the whole house for the whole month.

If you ran it for more than two intervals (>60 minutes), your demand would be 2.79 kW, because it ran continuously for at least one whole interval.

If you ran it for exactly 30 minutes and started it exactly when a interval started, it would also be 2.79 kW. If you started it exactly so it ran 15 minutes in one interval and 15 minutes in the next, your demand would be 1/2 of 2.79 kW.

Let's say you ran it for 2 minutes inside one interval. The demand contribution would be: 2.79 kW * 2 minutes * 1 hour/60 minutes * 2 intervals/hour = 0.186 kW demand

This means that for short peak uses (hairdryer), you won't be hit with the full wattage. But for long-running appliances (dryer), you will.

This is also why the power company says a dryer is a 2790 W appliance even though the nameplate is 6 kW+. They're right! The heating elements on a dryer cycle on and off much faster than once per 30 minutes, so it averages out to be less than the nameplate rating.

And finally, remember this is all peak. So if you take care not to run your dryer and dishwasher at the same time, or dry clothes at night when the AC isn't running, then the power company doesn't need to put as much capacity in, which means you'll save.