Solar Power and Energy Requirements – How To Calculate Your Needs

Time to sweep the solar panels
(time to sweep the solar panels)

When doing any sort of design or even “back of the envelope” calculations for alternative energy systems (e.g. solar), among the important factors to consider are these:

Solar Power and Energy requirements.

– Power equals watts.
– Energy equals power x time.

I’m going to try and keep it simple…


Power (Watts)

Anything that ‘plugs into the wall’ requires a certain amount of power (watts) to function. Some things require more power than others. When designing solar power (panel) requirements, you need to figure out the demands of the things that it will be powering.

For example a single 800 lumen LED light bulb may only require 10 watts while your toaster might require 1,000 watts (1.0 kW).

Likewise if 8 of those light bulbs are on together, the power requirement will be 80 watts.

Every single thing that ‘plugs in’ has it’s own power requirements.

But it doesn’t stop there…

Not all devices or appliances require the same amount of power at all times. In other words there may be a variability of power (watts) that a given device demands over time.

An example of this is my pellet stove. When it first goes through the igniting process, it requires nearly 400 watts. But after the pellets are burning, the fans might only require 50 watts. Not only that, but the fan speed is variable. If I crank up the stove it will begin demanding more power (watts) because the fan speed increases. Make sense?

This is true of many appliances.

That said, any given ‘thing’ will have a specification indicating it’s maximum power requirement (a sort of worst case scenario). Usually this is printed somewhere on the literature or a label on the device itself. I’ve sometimes found that there may be a fairly wide margin built into those published specs.

Whenever I’m considering the power requirements and energy demands for alternative energy (e.g. solar powered system), I need to discover the “watts” that each powered device will require and I also need to discover the anticipated energy demands of those devices (over time).

Which brings us to the next category:


Energy (watt-hours)

Especially for a solar power system that will include energy storage (a battery bank) it is critical to understand the power requirements over time so that you have enough capacity to keep things running when there’s no sun!

If I leave that 800 lumen 10 watt LED light bulb on for 6 hours a day, that bulb is consuming 60 watt-hours of energy.

10 watts x 6 hours = 60 watt-hours.

That’s pretty straight forward because the LED light bulb always draws the same amount of power – all the time when it’s on.

Similarly, A TV that draws 200 watts, when it’s turned on for 2 hours per day the energy requirement is 400 watt-hours.

200 watts x 2 hours = 400 watt-hours (0.4 kWh) (0.4 Kilowatt-hours)

The calculations for the energy demands of devices over time (watt-hours) gets tricky when you have appliances that draw varying amounts of power while they’re on. This includes a refrigerator, chest freezer, a washing machine. Many others too.

Additionally some may cycle on/off throughout the day.
Still others may not be used every day.

So it takes some thought, measurements, and a spreadsheet to help figure things out!


Calculating Energy Needs

When I designed the solar powered alt-energy system for my current home, I literally measured the power and energy requirements and demands for every single thing that ‘plugs in’ at my home. Once entered on a spreadsheet there are all sorts of helpful calculations that you can make.

How did I do that? It was easy! (although time consuming)

I used a watt meter that measures and displays Watts as well as energy over time (watt-hours).

I’ve had this one for years:
P3 P4400 Kill A Watt Electricity Usage Monitor

Note: If I were to buy a new one today, I would probably get the following, because it also keeps track of the cumulative time. But no big deal if you make a note of begin and end time (if that’s important for your test) :

Poniie PN2000 Electricity Kilowatt Power Usage Monitor

For my solar power and energy requirements I literally made an Excel spreadsheet that listed every electrical device in my home. I then measured the power and energy requirements of each over a period of time with the meter listed above. Then performed a number of calculations to get the ‘bigger picture’.


My Pellet Stove (varies with weather)
– Max Watts power requirement (360 Watts)
– Energy usage during 24/hr winter test (1.2 kWh)
– Calculate Average Running Watts per hour (50 Watts/hr)

My Chest Freezer (one of)
– Max Watts power requirement (up to 10x Running Watts)
– Energy usage during 24/hr winter test (0.96 kWh)
– Calculate Average Running Watts per hour (40 Watts/hr)

Note: Appliances like refrigerators, chest freezers, and air conditioners will require a surge of power when the compressor kicks in. This can be up to 10 times the average running watts. Factor this into your system design (inverter).

Calculate Worst Case Scenario – within reason

Power – Watts

When I figured how much power (watts) might be required at any one time throughout the day for my own home, I did not simply add up everything as though ALL devices were turned on at the same time. I knew that this would never be the real case scenario. So I simply looked at it from the perspective of realism. I made my own estimations.

Note: In a well designed system, there are circuit breakers which will trip if you did exceed the system’s capabilities. So if you did decide to run everything at once, you’re safe under that presumption ;)

I also factored the season and time of day (day / night) while determining a reasonable maximum watts requirement.

Energy – kWh

I also have a battery bank so that I can selectively run ‘off-grid’ when I choose to.

In order to determine how big a battery bank that you might need, there are lots of considerations including,

– number of hours or days of backup energy to power your system
– number of successive cloudy days without adequate recharge
– alternative backup charging methods other than solar (generator)
– maximum depth of discharge versus battery life
– system inefficiencies
– cost analysis

That said, for this discussion I’m just looking to understand how much energy (kWh) or Kilowatt hours that may be consumed in a 24 hour period, on average.

Tip: It’s cheating (sort of) but you can look at your past electricity bills to discover this information. However I looked at it from a fresh perspective – understanding the individual requirements is very helpful.

Having built a spreadsheet with the measured devices and appliances, it becomes fairly easy to calculate. Once you assign a reasonable number of hours per day that any one thing might be ‘on’, then it’s a matter of adding it all up.

Once you know your total energy demands (kWh) you can then begin to design a battery storage bank that will keep you powered up at night, and longer if you design it that way (just more $).

Hopefully this information will help some of you out there who may happen across this page while searching for answers how to get started with solar power and energy requirements.

More: The Four Essentials of Off Grid Solar


    Love the photo, obviously one of the most important things in a Solar Power System

    A Broom :-)

    1. Oh yes it is! I leave mine right by the front door…

      Snow is one important reason why I chose to mount the panels on ground mounts rather than up high!

      1. Ken;
        Quick question.
        Did you build your system as a Grid tied Battery Backup with a sell/buy back Meter? or stand alone with an integrated usage Panel?

        1. The system is not grid tied. No power/energy is sold back to the grid.

          Rather, I have integrated a panel (actually two panels) of individual transfer switches (one for each electrical circuit in the house).

          Each switch (and associated breaker) allows me to choose the power source to be either grid sourced or solar/battery-bank sourced. (Each switch has a center-off position for safety).

          Here’s a picture:

          my solar transfer switches

        2. Interesting that the Electrical Inspector did not require an ATS for full disconnect when the Grid Power goes down….

        3. NRP,

          My system is not between the grid and the house feed (like a typical whole-house backup generator would be – with it’s own master transfer switch).

          It is between each internal house load circuit and the CB panel itself.

          Whether the grid is up or down is irrelevant in that it would be physically impossible to back-feed into the grid with this setup. In essence, each house load circuit has it’s own physical transfer switch.

        4. NRP,
          I was wondering the same thing. Except that each of his individual switches seem to act as ‘isolation’ switches between the solar and gird systems. Still, I see your point. I know the State inspector over here would probably not allow this type of installation without a ‘proper transfer’ switch to protect from ‘islanding’.
          Nice Installation btw Ken!

        5. Thanks Minerjim, I enjoyed the project thoroughly.

          Every state, and county, have their own uniqueness regarding inspections. When I lived in California, it was ridiculous (at least in the region where I lived). Now that I’m living in the northern reaches of NH, it’s ‘very different’ – “Live Free or Die” (state motto). That said, I was sure to build my system to NEC standard of that time. Wouldn’t want to burn my own house down ;) Or get someone else hurt via negligence.

      2. Ken,
        I know the 2 brothers that own Midnight Solar in Arlington. They are a top notch company, I have been to their shop many times. They are both Amateur Radio operators like I am, that’s how we met. they can supply probably anything solar related. You should check them out.

        1. Doc,

          I spent $ at Midnight Solar when I put together my latest system (through one of their dealers). I’m well aware of them ;)

          I really like their charge controller. It has been functioning quite well.

  2. For those of you that want to design your own system please take note of the following:
    There are two types of power consuming devices 1) resistive – like a toaster where you can use the voltage x amperage = watts (suggested in the article). 2) Then, there are the inductive power consuming devices where you can not use the voltage x amperage = watts method for a system design. These devices include anything with an electric motor such as a water well pump, frig, clothes washer, central air conditioner or central heating, clothes dryer, or dish washer to name a few house hold appliances. This is because the start-up amperage is usually 2.25 – 2.5 times greater than the running amperage. If the start up current is not considered the inverter will usuall shut down rather than burn up. This feature is always designed into the inverter to prevent inverter fires. 3). All the wiring in the system must be designed to carry the maximum amperage that will pass thru that portion of the wiring to avoid a fire (due to wire over load). Wire size varies greatly in a system and must meet National Electric Codes or your home owners insurance will not cover you home.
    I offer this advice not to anger or discourage anyone but to help protect a fellow prepper. This comes from an electrical engineer with many years of experience.

  3. I have a small solar system that provides 500 watts 12vDC through a 50 amp charge controller. My battery bank will provide me with 100 amps per hour of 12v. I use an inverter that provides 1500 watt continuous, 3000 watt surge of 120v. The way I understand it, watts/volts=amps. Using that calculation, my inverter will provide me with 12.5 amps continuous of 120v. Can anyone out there smarter than me tell me how many amps I will be drawing from the battery bank if I were using 12.5 amps, (1500 watts) of 120v. To keep calculations simple, just assume that the inverter is 100% efficient. No need to get too fancy, I know there will be a small amount of power loss through the inverter.
    Can you show me the calculation you use to come to this answer?

    1. Texasprepper,
      something like this ?:
      12.5 amps x 120 volts = 1500 watts = (?) amps x 12 volts…( P= V x I )….therefore,
      1500 watts/12 volts = (?) amps (P/V= I) the answer is 125 amps of 12 volt power from your battery bank to the inverter, assuming no losses through the system.

  4. Texasprepper

    If you are consuming 1500 watts and your battery bank is 12 vdc then your battery bank is providing 1500watts/12 volts = 125 amps. Need to check your wire size now( you need at least a no. 2 AWG wire!

    1. Texas Boy, Texasprepper,
      TB thanks for back checking me! good to know that I still can do these calcs correctly.
      TXP- as TB pointed out, this is one heck of an amp load on a circuit. I think this is why a lot of these solar systems are now based on 24V and 48V systems. They take a smaller wire to handle the Wattage loads at the higher voltages. (Wire size is primarily based on amperage load, not so much as voltage). You are not going to be able to run that 1500 watt milk house heater off your 12 volt solar/inverter system without some very heavy wire!

  5. Texas prepper
    I am asumming THHN wire type for your application, rated for 130 amps

    1. Minerjim and Texas Boy.
      Thanks for the info. I was pretty sure that I’d be drawing 125 amps from the battery, just wasn’t sure. Yes, I am using 2 AWG wire both as jumpers between the batteries and from the batteries to the inverter. I have never pulled more than 600 to 700 watts from the inverter using it only to power some lights, ceiling fan, TV, and satellite box during power outages. Longer outages, I just pull out a generator. I am hoping to grow the system to where I can add my fridge and maybe a deep freezer if needed. From the info y’all have given me, I need to at least double my battery capacity and panel sets if I want to be able to really use the system as I’d like and if I double that, I’m gonna need a higher capacity charge controller.

      1. Another factor to consider is that if you draw a large amount of current from a single battery, or a very small bank, some power will be lost due to internal heating and resistance in the battery itself. Many batteries AH (ampere-hour) rating is based on a slow 20 hour discharge. The AH rating for the battery will be considerably lower if the battery is discharged faster. The solution is what you are proposing–add more capacity to the bank so draw from each individual battery is reduced. One advantage to a 12v system is you can use automotive/RV products (lights, radios, etc.) directly from the bank with no need to go through the inverter. Be sure to fuse protect any wiring for this! Downside to 12v system is heavier wiring is required.

  6. My solar system is going to be very small. I just wanted to be able to operate a few kitchen appliances .. Microwave, coffee pot. Tea kettle, slow cooker, hot plate and a compact fridge. I acquired the panels, stand, controller and inverter on the advice of an off grid friend. Then I went to a professional solar system installer, thank goodness. He added an inverter disconnect, and a battery level indicator for the kitchen, and a set of batteries which he will bring when he does the installation. Unfortunately the inverter is small and I will only be able to have one appliance on at a time (most of the time). A larger inverter would be $1500 so it is on the waiting list.. But, at least now I have the information.
    If in doubt, as I was, go to a professional. This is an expensive venture. Do it right the first time.

    1. Skeezix,
      Yup, wish I had talked with a professional but went ahead and pieced this system together trying to learn about solar power. This system, my first, I wanted to keep portable so that I could take it with to my deer camp to use with my RV. A permanent system will wait until I retire and move out to the boonies.

    2. Any kitchen appliance that uses heat (coffee pot, hot plate, toaster etc. are very high drain on solar. A slow cooker isn’t too bad after it heats up. I use solar for lighting, fridge, small battery recharging. I cook with propane, and use a Honda 2000 watt inverter to recharge a 600 amp battery bank when solar is not at optimum level. I run the fridge 1-2 hours 2-4 times a day, and avoid opening door very often. I also cook on my woodstove some in the winter. My system has tw0 inverters– an 800 watt and a 1800 watt. I use the small one for lighting, and only run the larger one when running high drain items like the fridge/

  7. Another question for those smarter than me. Could 4, 12v panels be hooked up in series to create 48v like you can do with 4 12v batteries?

    1. Yes–just make sure your inverter and charge controller will accept a 48v input.

  8. Texasprepper
    Yes, panels can be hooked in series but do not exceed the voltage rateing of your other equipment. Are you sure these are 12 volt panels?
    There are two voltage ratings on a solar panel, no load voltage and voltage under full load. Voltage also varies with outside temperature. For example, my full load voltage is about 26 volts while the no load voltage is over 37 volts and this varies a little with temperature. Warning: never disconnect the out power wires of your panels while they are producing power or you could/would have a fire on your hands.
    I have 14 panels in series and under no load that is 14 x 37 = 518 total volts. Do not do something like this unless you are highly experienced. Now we are talking about voltage that will kill you! At this voltage there are other things to consider as well. I hope this helps

    1. Texas Boy,

      Were you the one a while back that said he had been taught by an older fellow how to restore old acid batteries???

    2. A Blanket is nice to temporally shut down your solar panels (or maybe hide them or thick ones to protect from hail).

      A reminder folks Shade stops solar power from working. Even a branch worth of shade can greatly reduce or stop production from the solar panel.

      1. Not so sure about that. As I recall the way the individual cells used to be wired that was very problematic. I believe the wiring techniques have bee re -engineered to where that is much less of problem than before. We had an older panel on the boat that was shade sensitive but our two newer panels were not too bad with it.

      2. Regarding shading of Solar PV panels:

        Most modern PV panels are designed such that there are groups of cells within the overall solar panel that are in a series/parallel configuration.

        When a cell is shaded, it will block current flow from the other cells that are in its unique series string of cells (within the overall series/parallel arrangement of cells in the panel).

        However ‘Bypass diodes’ are wired in parallel with a string of series cells in the panel. When that string is shaded, the current from other strings that are in series with the shaded string can bypass the shaded string through that string’s bypass diode.

        In other words, only a portion of the overall panel’s cells will be affected whereas older panels would completely shut down when even a portion of it was shaded.

  9. Ken,
    Have you ever heard of anybody taking advantage of the short circuit rating (ISC) of their solar panels and shunting (shorting) to clear snow? The internal resistance of the panels would melt the snow…..

    1. not so sure that might be Bad for the lifecycle of your solar panels. A broom works just as well and the dark color of the panels will speed any sunshine melting of the rest.

  10. I have read but not quite sure if this is true. A wet leaf stuck to a solar panel or a twig will stop the operation of the panel. Any comments?

    1. Answer is it depends. At the least their will be a reduction in power production of that panel. Smaller panels may not produce much with a wet leaf on it.

      Keep them reasonably clean/unbroken and many years of service is ahead.

  11. The key to making all this work effectively is to have a good battery bank and an inverter/charger that controls everything. The inverter should be able to surge 2 or 3 times its rated output for at least 20 seconds. Determining the battery bank size will obviously be based upon the electrical usage. Tally up your electrical usage by recording your homes electrical usage several times daily into a spreadsheet. This can be used to calculate a daily and hourly average. I was surprised that for my home of over 2400 square feet in the winter time I had an average of only 1.2 KwH/hr. I do have natural gas heat however but use two units. Past electrical bills will usually show peak months and a factor can be used to determine what your average daily for that month. The battery bank needs to be sized upon a peak of power usage but also on the length of time that the battery bank will be supplying power with no charging. A lot of factors regarding battery types and other issues need to be considered. With the battery bank size then the question is how to re-charge the battery bank: solar, wind, generator, or grid power. A combination can be considered and designed for. The use of solar and/or wind is a crapshoot to really determine how much you can get from these sources over a given period of time due to environmental factors. Night time can always be calculated but sunshine during the day cannot. In summary, I think that power needs are calculated way too high because I have seen spread sheets that show all the loads of the various devices and then summing these loads up to determine what power needs are necessary. Watching the existing power usage by monitoring the power meter will give a much more accurate need over a given time. Some power meters can be monitored automatically by purchasing a device. Power utilities may even bring their equipment for monitoring usage more accurately to include peak usage for some period of time. Overall this is a very complex problem to figure due to all the variables. Make sure you identify all the variable types and then estimate highs and lows for each.

  12. We live full time off the grid and have done so for 8 years. I have a somewhat different opinion than is usually recommended on how solar panels should be set up. My opinion is based on our experience of living year round with solar off the grid.
    When we put in our first solar system in 2003, when solar panels were $5.00 a watt. I was able to purchase a used system consisting of 24, 90 watt panels with two 4Kw Trace Sine Wave inverters and two C60 charge controllers. I paid 10K for system by cashing in some vacation time.
    Shortly before moving full time to our remote off the grid ranch, solar prices began to come down. I purchased 12, 300 watt panels. As I watched the sun throughout the year I began to think that it makes no sense that when the sun comes up in the morning you have to wait hours before the sun comes around to the south to make power with your south facing solar panels. I orientated the new 12 panel array 25 degrees, east of south so I could capture some of the morning sun. This was really a game changer for us, especially in the winter when the days are short. We saw power being made as soon as the sun came up.

    I did not learn until later that on the longest day of summer you have 180 degrees of sunlight. On the shortest day of winter lose 45 degrees of sunlight, 22.5 degrees to the east and 22.5 degrees to the west. Because of the 3,600 watt array facing 25 degrees east of south, it was getting hit dead on by the sun when it comes up in December.

    Last summer I saw solar panel prices come down to .37 cents a watt. $110.00 for a 300 watt panel. Incredible! We bought 30, 300 watt panels. I replaced the 20 year old south facing 90 watt panels with 12, 300 watt panels. I welded a structure that faced due east and installed 9, 300 watt panels. I installed 3 more 300 watt panels facing south west. We have a mountain that blocks the due west sun.
    The system works great. The second the sun came up this morning over the mountains, I was seeing 1,800 watts and it went up to 2,400 watts 15 minutes later.
    Some may consider this overkill, but it is important to understand that it is all about the winter when the days are short and it is cloudy at times. We live at 37 degrees latitude but our panels are at set at 47 degrees for winter sun and so the snow slides off easy. I use a broom with plastic bristles.
    It is interesting to note that when it is overcast with no shadow, the system still makes 30-40 amps in the diffused sunlight.
    The key point is that no matter where the sun is at in the sky, we are making power, even if it is only visable for a few hours due to clouds.
    We use two FelxMax 80 MPPT charge controllers.

    I put our well on top of a hill so I would not have to pump water twice. When we fist installed the well with solar pump it used eight, 60 watt solar panels, wired for 48 volts, mounted on a tracker that follows the sun. The tracker was used because solar panels were $5.00 a watt. The tracker has failed twice. When it failed the second time last summer I had enough. I welded a structure that has 6, 300 watt panels, two facing east, two facing south, two facing west. No moving parts to worry about. The sun tracks the panels rather than the panels tracking the sun.

    We have 12, 2 volt Rolls Batteries, 2,350 amps each. They have a 20 year plus life span.
    I really like the Sundanzer freezers, we have two. They use 12 or 24 volts so they do not go through the inverter. They have 4 inches of insulation and a 8 cubic foot freezer uses 400 to 700 watts a day so you could power one with a 100 watt solar panel.

    Great article Ken!
    Amps times Volts equals Watts

    1. Excellent comment. Thanks for sharing your real world experience!

      I have been thinking about doing exactly what you’ve been doing – that is, installing some additional panels SE to advantage more of the morning sun.

      Your point is well taken that one of the very important design considerations for off-grid reliance is the winter conditions

      – low angle sun
      – shorter arc across the sky
      – fewer hours of available sunlight
      – cloudy days
      – snow removal

      The good thing is that PV panels perform best in cold weather!

      1. Are Solar trackers a dead idea nowadays? Seems they were a response to maxing the value of those 5.00 a watt solar panels.

        Fixed is easier if you have the solar exposure to use it. I have western trees that help keep my house cooler in summer so no west facing for me. :-)

        1. I looked into it and decided against it for my installation due to the cost factor of the trackers. They’re wicked expensive comparatively.

          My panels cost me about $1 per watt. Not bad… Though a tracker would have cost thousands (there are a variety of types and prices).

          My current panels are all pointing due south. It has been working out okay. However I am seriously considering adding more panels to help with morning sun (with the winter sunrise and angles in mind towards the ESE). It would be great to get an early start on charging in the morning rather than losing a few hours until it really gets going.

        2. Sounds good. Do you do angle adjustments at least seasonally to maintain proper alignment?

          BHA Solar has some good resources for this information for your latitude.

        3. I designed my ground mounts such that I could do this – although a bit of a chore… However I have been happy enough so far with a compromise angle (my latitude).

        4. It is a chore but for smaller systems the extra4 to 8 percent gain is worth while. Easier to simply buy a few more panels BUT useful to KNOW your dates and angles if hail or a storm thrown tree or a bullet destroys a few of your panels and you need to reconfigure and max out power.

          Just a thought to a friend

    2. I would all add that there are Broadband Ferrite Chokes on the solar wires at the panel junction boxes, on both sides of any MC4 connections, where the solar wires go into the combiner box, where the wires enter the charge controllers from the combiner box, where wires exit the charge controllers, where wires enter the inverters from the battery bank, where AC wires exit the inverters. Also Broadband Ferrite Chokes where DC wires enter the the solar freezers.

      The Broadband Ferrite Chokes are to prevent damage from a EMP strike or CME based of advice received from Dr. Arthur Bradley, Electrical Engineer and EMP / CME expert.

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