I don’t know what part of my brain is not functioning , but I’ll be damned if I can remember what the hell volts, amps, amp hours, watts are. Its not like there aren’t a thousand very clear instructions and guides and dictionaries and diagrams on the internet. It just wont stick for more than a half an hour. And I have to refresh my brain with a cheat sheet every time I try to think of this.
Anyway, here are some hints I am picking up along the way from boat builders, rvers and . No hints on trying to remember what a watt is.

— Use small inverters, as they draw fewer amps to run.  Our Walmart 150 watt inverter draws 0.4 amps when it is turned on.  A Radio Shack 150/350 watt inverter draws 0.6 amps.  The 800 watt inverter draws 1.0 amps.  The 1100 watt true sine wave inverter draws 2.0 amps.  This “No Load Draw” is an important spec to consider when you are buying an inverter. I can’t believe it, but I am already tuning out.

— Charge more than one item when you turn the inverter on.  Since inverters use battery power just to run, you might as well charge a few things at once.  An electrical toothbrush takes 2-6 hours to charge, so you could also charge up the cordless drill or camera batteries or computer at the same time.  With the toothbrush and computer (turned off) charging at once, our draw was:  0.6 amps for the inverter, 0.1 for the toothbrush, and 1.6 for the computer, or 2.3 amps total.

— Keep the volume down on the TV/DVD and stereo.  We measured a difference of 1.5-2.0 amps on our 19″ LCD TV if we turned the volume way up.

—Switch to LED bulbs.

How much battery capacity will I need?

Design Rule: To be effective, we recommend inverters have access to a battery bank that is 20% as large in amp-hours as the inverter size in watts. This means a 1000-watt inverter should be supported by at least 200Ah of battery capacity.

The rationale behind this rule is that inverters use about 100Ah of electricity for every 1,000-watt-hours of use. So if a West Marine Slim 1000 were used at its maximum capacity for an hour, it would consume 100Ah. This would discharge a 200Ah battery by 50% if it started out fully charged. This is a recommended minimum: as usual, more battery capacity is better.

Calculating electrical loads: Many appliances and electronics have their wattage on a back panel. If only the amp rating is provided, use the following conversion formula: Volts x Amps = Watts.  A 7-amp microwave at the standard North American voltage of 115V will need 805 watts of power to run. For each appliance, its wattage, multiplied by hours run per day, divided by 10 will approximate the number of amp-hours consumed from your batteries.

If an inverter is going to run a 500-watt load, it will draw around 50A DC. The math behind this rule is as follows: 500-watts AC ÷ 85% efficiency ÷12.5V = 47A DC. Keep two things in mind: 500W is not very much power (think about a small cabin heater that draws 1500W or a large hair dryer that draws 1000W); and 50A is a heck of a lot of DC current use. Not many DC loads on boats 40′ and under would draw 50A or more for more than a few minutes. Therefore, an inverter frequently becomes the largest DC load on most boats, and may require substantial changes in the battery capacity and wiring of the boat in which it is installed.