To wrap up the power budget series, we present it all in this one long page 
Why a Power Budget?
If you want a quality power system the place to start is with a power budget. Your experience with an emergency power genverter, or your solar powered home will be one with less hassle and cost if you get this critical step right. A detailed power budget provides 3 immediate benefits when
creating or operating your new system.
- It determines the best initial component sizes. Understanding that the system will grow in time and armed with a power budget, arms you to “future proof” the initial design. For example, getting a larger inverter initially, so it doesn’t need to be replaced when upgrading your battery bank or increasing your power needs.
- It sets your expectations. Nothing worse than moving to your off grid home and finding out that you can’t operate your <favorite appliance> as long as you like! Knowing what are the large loads on your remote power system can help make those critical first design choices better.
- It allows smart system operation. Day to day operation of your remote power system will be improved with your understanding of how much battery power is stored on those cloudy, windless days.
You will want to design in good battery monitor meters. Like our Tri-metric battery monitor, it records AH(amp hours) in and out, giving you a good idea of your battery bank charge at any time. This is critical for smart operation, and we look at ours several times a day!
One fun exception to living within your power budget is Free Power Time.
How do you go about creating this power budget? How much power does it take to make Espresso? or run my TV? Good question. The crude way to get a guestimate of the power required by your favorite appliances is to look at the UL power requirements on the packaging or device. Be warned this is often twice the actual power consumption, since it is the maximum power that will ever be used, even when the appliance is failing. Remember Underwriters Lab is a safety organization.
The best way to get a real world number is to measure it. Before we moved 10+ years ago, there were not any inexpensive AC meters to measure power. We measured appliances with our Tri-Metric meter after we moved. You, however, have the opportunity to benefit from the emerging ‘green’ industry and can get a small inexpensive device, P4400 “Kill a Watt Monitor“, that will measure the real power of anything you plug into it! At about $20 each, I just ordered two to compare with the Tri-Metric battery monitor!
With the P4400, there is a whole new way to create an accurate budget with real world numbers. Heck, even living on the grid, this could be very useful in helping increase your “power consciousness” and save some money too.
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)
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.
Measuring Real Power
The simplest way to get a quick number for the power of an appliance is found on its label. Sometimes the label will list the power in Watts, other times it may provide the AC voltage and current, other times the power supply may indicate the DC output voltage and current.
The power calculation from the label is simply the volts times the amps. The resulting number is an approximation and does not take in to account a bunch of technical details like AC {en:power factor} and {en:root mean square} power. In any case, all the numbers you use that are derived from the label are a worst case power and can be up to twice the typical power consumption
Now thanks to the mass market and the emphasis on energy efficiency there is low cost device for consumers to measure power. Before this you needed an expensive tool that clamped around the AC power cord. Its called the P4400 “Kill a Watt Monitor” for about twenty bucks and provides a reasonable way to get real power measurements. I’ve had one for awhile and it came in very handy for getting good power numbers on our electric refrigerator that we got last summer.
Using the Kill-a-Watt Monitor
Using the device is quite simple. Just unplug your appliance, plug it into the P4400, and plug the P4400 into the wall. The monitor has an easy to read LCD display and 5 buttons.
The buttons are used as follows:
- Change display to Volts AC
- Chang display to Amps AC
- Toggle display between Watts (Real RMS value) and Volt-Amps
- Toggle display between AC frequency (should be 60Hz US) and {en:Power Factor}.
- Toggle display between accumulated KWH and the Time it has been accumulated for.
All we really need for our measurements most of the time is simply the real Watts display. Having the timer and KWH display is helpful for measuring devices that can change their own power draw like my espresso machine and refrigerator.
Measurement details
When Using the monitor, you want to consider what kind of operation will be typical for you. This is especially true for electronic devices. For example, we recently got a 32″ LCD television. If I display a TV show the power is around 62W, however if I display nothing except the blue screen the power is 90W!
- Check for “Instant On” power draw by plugging in for a minute without turning the device “on”. Here is how we solve the problem.
- Check that the parameters are what you might consider “typical” like brightness on various Displays.
- Check to device/appliance instructions to see if there are any “power saver” or “energy star” modes available. We tested a Samsung LCD TV and the average power went from 60W to 40W.
Other uses
Some uses for the monitor are obvious, and others not so. You may want to consider:
- Use it to monitor the use of thermostatically controlled appliances, like refrigerators or freezers. Monitor when doors are opened too much, or when seals leak out the cold too much.
- Take your P4400 Montor shopping!! What a great way to know your power commitment before you buy an appliance! Sure it may be a little geeky, but it can save you $$ in the long run, and salespeople won’t let this measurement get in the way of a sale.
Anyway I’m busy here in our off grid “lab” collecting data you can use in a real way for your own power budget. This week we’re on backup power and I noticed that the old generator was running a low voltage of ~100Vac. That’s good for Japan’s power but not mine, or for the P4400. It would blink every 30 or 40 seconds with a power measurement, and read the line voltage as 0V!
One other note on the P4400: When you take measurements with a modified sine wave inverter, the monitor never reads 60Hz as you would expect, instead it reads lower 45-55Hz. Its nothing to worry about.
If you create a power budget based on the information you can get from appliance and device labels, you may be disappointed in the size or limitations of the power system you need to run them. Its very empowering to have a power monitor like the P4400 Kill-a-Watt to get good information about your devices under normal operating conditions. I used the monitor extensively the last week around our off grid home.
Since it’s winter and we haven’t had a sunny day for 5 weeks, we’ve been getting power in the genverter mode (no solar help), and I was careful to take measurements on inverter power. In our case the inverter outputs a Modified Sine Wave for its AC output voltage. Just to satisfy my own curiosity, I checked many devices on generator power versus inverter power and didn’t see any real power differences.
Please note the data given in the table below can be sorted by any column, or you can use the search feature to quickly find the device you want.
Power Data you can use:
| Device Type | Model / Description | Watts Measured | Max Watts (label) or Volt x Amp | Test conditions / Notes |
|---|---|---|---|---|
| Coffee | Espresso machine, Hamilton Beach | Espresso Machine | ||
| Cooker | ||||
| Computer | ASUS eeePC 1000 Netbook PC | 25W charging 15W operating | 12Vdc x 3A =36W | Charging fully discharged battery. |
| Computer | Desktop PC, Acer xx 4GB ram | 60-75W avg | 110W max measured 120Vac x 7A ~ 840W | power when off = 4W |
| Dishwasher | Kenmore | 1265W | Off Grid Dishwasher? Full load in water miser mode takes 1.37 KWH | |
| Juicer machine | Jack Lalanne JLSS power juicer | 128W no load up to 500W while jucing | 120V, 2.5A ~ 300W | Juicer power test |
| Game Console | Nintendo Wii | 17W | Has instant on feature. When 'off' takes 8W | |
| Lights | 18W Compact Florescent, 75W equivalent light | 15 W avg | 18W | 15-16W, when warm 15W. |
| Microwave | Goldstar, smaller oven | 780W | 120Vac x 8.5A ~ 1020W | Heating water for 2 min. |
| Refrigerator | Frigidaire | Refrigerator Power | ||
| Screen | CRT Monitor, 15", old Samsung | 70W | no input 60W | |
| Screen | LCD monitor, 22"Acer X223w | 26W avg | 120Vac X 1.5A ~ 180W | off when off. |
| Screen | LCD monitor, 15", AOC brand | 63W avg | not on label | no input pwr 4W. any input 52W min. Power switch does not affect power, input or not determines power. Max 100W |
| Slow Cooker | Rival Crock Pot (4qt) | 71W Low 140W High | Ran test in Rethinking Off Grid Cooking [1] | |
| TV | Visio 32" LCD flat screen | 70W avg | 120Vac x 1.6A ~ 192W | 90W with no input and blue screen. When TV screen varies from 65-75W |
| TV | Samsung LT2338 | 42W (high pwr save) 62W (auto pwr save) | 100W | Max pwr 78W, use the power saving features in setup menu. |
| USB external Disk | WD 'my book' 500G drive | 7-10W in use | 15W max measured 12Vdc x 1.5A = 18W label | 4W when USB not plugged in. |
| Wireless Router | Zyxel, 4 port | 5W | 12Vdc x 1A = 12W |
Measuring the power yielded some surprising results so far. For example, the fact that many electronic appliances had “instant on” features that drew 3-8 watts, even when turned off! I’ll try to arrange some time with friendly local merchants to take my power monitor in their store and collect even more data!
On-Line Calculator Gets You Started
I’ve created an online calculator to help you create your off grid power budget. These calculations will help you determine what your can run, and how long you can run it when operating on what I call “battery time”.By battery time I mean is that your charging sources are wind, solar, hydro, or other renewable sources. Backup generator charging often results in free power time. and is generally not “on budget” time.
When operating your system on battery time, you are ‘on budget’. To protect your battery investment do not entertain the idea that you’ll never need a backup generator source, you will. Deep cycle batteries need some TLC to keep them going beyond a few years to the payout years of 6, 7, or even 8 years for replacement. Currently, I’m looking at replacement only 5 years later, ouch!
The basic form presented below will help you get some ideas of what your off grid power needs might be. Don’t forget to consult the data you can use page for some real ideas on the actual power needed by various appliances. If you have something that might be a large load, consider getting a P4400 Kill-a-Watt monitor and do the real measurement.
The greatest value found in creating a power budget is the knowing realistically how many deep cycle batteries you’ll need. Your power budget will determine how many dollars go into batteries, and what is left for a charging system(generator), and what solar / wind / hydro equipment will fit in your first phase of the power system.
As an added bonus to those in the US, here is a map that indicates how many equivalent hours of solar energy that are on annually available. This gets kind of technical, but can give you a guide to start out with. You may want to use these numbers below for your location to get more of a worst case idea about solar contributions.
The one caution: the map below is an anual average. At my location it indicates 4-4.5 hours, while in the winter its probably 3 and in the summer it gets up to 8 hours. The wide variation is explained because we are in the Northern US and by the green road sign above.
Power Budget Calculator
Budget Calculator Instructions:
- Enter the description of your appliance, i.e.”refrigerator”, “TV”, etc
- Enter the power in Watts, that your device will require. Whenever possible enter the average power consumption instead of the peak.
- Enter your daily run time in hours per day. If you run something every other day, just take the daily average.
- Press “move that bus!” button to reveal your results!
