In this series, I’ll cover several important topics surrounding a power or energy budget. What they are, the benefits of having one (especially off grid), and how to create one. Finally the series will conclude with real measurements and an online form for you to get some ideas for yours budget and save energy! In part 1 we started with power budget benefits. In this section we’ll go over the calculations and assumptions in your power budget.
What is Power?
What a loaded question. Political? Personal? No, really we’re concerned with what {en:electrical power} is as it applies to your remote home. To simplify our use of the term, we will limit the discussion to DC. (I know how complicated calculations with AC can get from work on my EE degree years ago.) Electrical power is normally measured in Watts. In our DC situation with a battery bank and inverter, its calculated simply as :
P (watts) = I (amps) x V(volts)
[editors note: The entire series can now be found on our new Power Budgets page for your convenience!]
The Time Component
Knowing how many watts a device uses does not tell us how long the battery running it will last. Time is involved also. Really when doing a power budget we are concerned about how long things will last given a certain size battery bank. Likewise, for those on the grid doing a budget, you are concerned about your power bill which is determined by what you run and how long you run it. So we want to list in our budget the power and an estimated time of daily use. The units we will use are Watt hours which are the little brother of {en:Kilowatt Hour} used by power companies for billing.
WH(watt-hours) = I (amps) x V(volts) x t(hours) or simply,
Watts x Hours
So for our power budget we take each daily appliance load multiplied by the estimated daily run time to get the daily WH. Summing up all the WH will give us the power we need (or use) each day. This number is critical in determining what size battery bank to purchase and how long it will last without charging. It also helps us understand how much average solar or wind charging is required to meet our needs.
Make it Real Please!
OK, I said simple in the post title, now lets make it physical so you can understand this stuff in your gut. Everyone thinks of batteries and electricity as having or being ‘juice’. The water analogy for our power budget is simple. Imagine a water tank on a tower.
The tank size represents the size of your battery bank, that is, how many gallons of ‘juice’ it can store. A small hose is fastened to the tank to use the ‘juice’ on your appliances. The larger the appliance power, the bigger the hose used to drain the tank. The size of the drain hose changes with the power you use. A large load will drain the tank quickly, just like an electric heater might. A small aquarium hose applies to the ‘juice’ used to power your radio. It will last a long time.
If you’re on the grid, think of a gigantic tank, even a large river on that tower. You won’t run out, but you will be billed accordingly!
Adding it all up
An example, lets do a simple power budget for a small vacation cabin. We’ll assume that the following appliances are used:
- Coffee maker, 850W used 1/2 hours/day
- lights, 3 – 15W compact florescent used 6 hours /day
- Small electric refrigerator, 150W compressor running at 50% duty cycle
- laptop computer , 75W for 3 hours/day
Lets determine the WH per day of each item:
- Coffee maker: 850W x 0.5Hr = 425 WH
- lights: 3 x 15W x 6hr = 45W x 6hr = 270 Wh
- Refrigerator: since the compressor runs 50% of the time, that’s 12hr. (this is a big simplification)
so, 150W x 12Hr = 1800 WH - Laptop: 75W x 3Hr = 225 WH
Summing up all the components we get a grand total of 2720 WH. (on gridders, thats only 2.72 KWH) How does this tell us what we need for a battery bank? Let’s assume a 24V genverter system. Deep cycle batteries are rated in Amp-Hours, so we divide our total by 24V to get our daily AH requirements. We need 133AH per day of battery only run time.
Lets say we want 2 cloudy days of storage, or 267AH. If we want the deep cycle batteries to last, we don’t want to discharge them more than 40% of their rated AH capacity. To meet this guideline we divide 267AH by 40%, or 267/0.4 = 667AH. If you were to use Trojan L16 cells you would need to purchase eight cells and have a battery bank of 700AH at 24V.
This quick thumbnail example points out the cost of a refrigerator and there are several tricks to make it work better. Next time we’ll cover real versus calculated power, and why this isn’t as bad as it looks.
Power Budgets [3] Measuring Real Power
Have you ever measured or calculated your power use? Did it help you change your power consumption habits? Please share with us in the comments below!
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You mention well power as requiring 240V and this is why you chose the Trace inverters. A better solution would have been a transformer plus a soft start module to lessen the start surge. The parasitic load involved with the two Trace inverters is twice what would be needed if you only required one inverter. Not to mention which in a split phase system you always have to cope with the unbalanced neutral load. more efficiency lost.
I don’t run both inverters all the time, just when running the well once a month or less. Nothing other than the well power is wired to the 2nd phase.
Not mentioned in this article: http://www.genverters.com/water/water-the-must-have-liquid/
was another I considered. Redundancy, since this was my first off grid move, and I had never owned a decent full size Trace inverter. I didn’t know what to expect for this critical component. Redundancy seemed good at that time. (Now I know these units are bulletproof in design and reliability.)