Should I use DC lights or run everything through an inverter?

Running small loads with an inverter greatly decreases efficiency. If you plan to leave a light on all night, charge a cellular phone, power an answering machine, or frequently use anything less than 50 watts, it makes tremendous sense to have some DC circuits rather than run everything through an inverter. A 25 watt light at 65% efficiency would actually use 34 watts or 11 watts extra. If for example this is a night light that stays on 12 hours, it would use an extra 132 watt hours - the amount of power produced by a 51 watt solar module in just less than 3 hours of sun (after deducting a 10% PV to battery inefficiency). With an average solar day of 5 hours of direct sunlight and a $325 dollar 51 watt module, the extra cost to your solar system to run that one light is an extra $195.

If you also use an answering machine or something that is on 24 hours a day and/or need to leave a light on that long, extra costs could soar over $400. And if you size your system for rainy or cloudy conditions, this number could double. In addition, many lights hum on inverter power and sometimes flicker. Frequently larger inverters will not even start when the load is small. DC lighting is a good idea!