– By Rhys Coffin, Off Grid Solar Designer

Y our off grid load evaluation tells us how much power you’ll typically use over a 24 hour period – particularly in winter.

It’s the primary factor we use to to calculate the size of your off grid system, and is expressed as kilowatt hours (kWh).

This number could be as as low as 3-5 kWh per day in winter in the case of a tiny home, or as high as 20, 50 or even 100 kWh in the case of some mega-sized off grid solar systems we have designed and installed, in very large homes.

Most full-time family homes will use between 7 kWh and 15 kWh per day. We know this from having processed hundreds of load evaluations, system designs and installations over the years.

On our Systems and Pricing page, you can see our different sized off grid solar systems to get a quick idea of your options. It’s only a rough guideline, but you can see that the higher the daily load, the bigger the system needed.

If you get it wrong and it’s too low, the generator could run every day in winter, or even worse the lights could go out.

If you get it wrong and it’s too high, you’ll pay too much and overcapitalise.

Apart from analysing  your region’s official sunshine hours, the off grid load evaluation is the most important thing to do correctly when it comes to sizing.

This article is not a substitute for getting expert help with your estimated power use, or talking to us to check your numbers if you’ve done it yourself. Instead, you can use it as a guideline or to get some insight about the process we use.

Just a reminder, there is no charge for our consulting or design service.

But first:

The basics 

If you can get your head around these principles below, this information could save you a lot of time (and money). 

Googling how to correctly size your off grid system can lead you down a rabbit hole of no-return. There just seems to be so much information – and some of it even seems contradictory, right?

Please consider:

  • Quality matters. If you are seriously constrained by the budget, you will naturally be attracted to buying cheap DIY off grid solar online. I’m sure you have seen the ‘kits’ for sale.

    This makes sense on the face of it.

    While that may be good as gold for a bach or crib you might visit in summer, this is usually a false economy, especially if you are looking for long term value and need reliable power for 15 or even 20 years time.

    If you can only afford a low price (that’s OK), you should know that there will be risks. This could include doing it all again after a few winters, with little or no ongoing support from your online supplier or installer.

    As power consumers living ‘grid-tied’ or in town, we take it for granted, don’t we? We flick the switch and it’s on.

    Living off grid is different.  You are now dealing with a variable resource (sunlight) which isn’t helped by New Zealand’s latitude and climate which can make it tricky to get right in winter.

    That’s why, in our view, reliability should be a priority when it comes to your investment and planning.

    Unfortunately, we’ve had our share of ‘rescuing’ other people’s off grid systems each winter when they fail.

  • Cooking, hot water and heating.  By far, these are the biggest items that contribute to the size and price of your system.

    You can power anything you desire; the technology is available. It’s just a question of money!

    If you want your off grid system to run your cooking, heating and hot water (or a combination) then you should expect a sharp increase in your system sizing and price.

    For example, our double inverter PS: Medium & Large solar systems will have no problem running an electric oven, which might use 3,000W (3 kWh). But that’s a lot in one hour compared to running a tiny house that might only use 3-5 kWh in an entire day.

    Get the cooking, heating and hot water sorted with us first. Most of the other stuff like LED lights, TV, and running the jug are usually minor items in comparison.

    If you want to know what the alternatives are to electric cooking, heating and hot water, just talk to us and we can guide you.

    Your options include wetback systems, gas hot water, hot water heat pumps, wood burners, wood or diesel water boilers, infra red heaters, thermodynamic water heating, air fryers, gas ovens and hobs, and more.

  • There is no ‘one size fits all’. When it comes to sizing your system, everyone is different because we all live our lives differently and have different priorities.

    Yes, certain appliances usually have predictable power requirements, but it’s all about being as accurate as possible with what appliances and equipment you want to run.

    For example, with sewage and grey water management, you may decide to have an unpowered septic system.

    Quite often, your council will dictate what you can and can’t do depending on local regulations and the topography of your land.

    While you may ultimately install an unpowered sewage system (which keeps the load evaluation low), you may have no choice but to run a fan, pump or macerator that need electricity to operate.

    In fact, some waste treatment systems can use up to 2,500W per day, which could be a big chunk of your total daily load.

    It can be the same for internet (broadband vs radio dish vs satellite), water pumps, water filtration (U.V), desktop vs laptop computers – and lots more.

    By all means, have a stab at the off grid load evaluation yourself. Your other option is to save some time and let us help you work it out.

Make a list

load evaluation list

Your off grid load evaluation starts with a list of all the ways the household will be using power over a 24 hour period.

While we’ll usually walk you through this list over the phone (or video call), you may want to make a start yourself.

Don’t stress too much about trying to work out the exact wattage for each and every appliance you’ll use.

Be aware that there is so much variation around and it’s easy to muck it up if you try to figure it out alone, or start Googling.

One typically bad practice by some off grid suppliers, is to send you a list or a spread sheet and ask you to nut it out yourself. Not only can this be frustrating and time consuming, but the chances of making a mistake is reasonably high.

All we need you to do is note down the appliance or equipment, how frequently it’s used and for how long, and get in touch.

Unless you have a passion for appliance power use (lol) and tons of time to research, you are better off just to make the list and talk to us.

After reviewing your list and asking a series of questions, we’ll complete the off grid load evaluation together, and come up with an estimated total of the kWh that you’ll use over a 24 hr period, in winter.

We use winter because in most homes, power consumption will usually be higher than warmer months.

The goal of this list below is not to identify every item in the home that runs on electricity. Instead, it lists categories that you can use as a memory jogger.

Some items are obvious, but we have also added useful notes.

Let’s get into it:

  • Cooking. Gas and wood burner are your alternatives to solar. A gas hob with an electric oven keeps the load down, however an electric induction hob on its own could add $5-7,000 to the overall price of your system.

    The actual wattage of electric hobs varies wildly between brands and type (which can be frustrating for people). We use an average power use of 2.5 kWh per element, on high.

    An electric oven could use between 2 kWh and 4 kWh depending on the temperature set, the size and the brand. We use 2.5 to 3 kWh as an average.

    We might recommend the use of an air fryer (especially in winter) which uses only half of the power as a conventional oven.

    Often, customers will opt for a benchtop convection oven that uses about 30% less. Some people don’t like gas ovens because they are unfamiliar with how they work, while some customers swear by it.

    There are pros and cons whatever you do.

  • Hot water.  This is a biggie. In a normal grid tied house, 30% of the power bill goes directly towards water heating.

    For that reason, you’ll need to explore alternatives. These include gas, hot water heat pumps, thermodynamic water heating and wetback water heating (with a wetback you may still need an alternative to heat your water in the warmer months).

    For example, a quality hot water heat pump might use 2-4kWh of power per day for a family of three in Auckland. It will vary considerably depending on your location and habit in the home. Compare this to 10 – 12 kWh which would not be unusual for a normal, efficient, electric hot water cylinder in a family home.

    Finally, forget about investing in a separate solar hot water system using technology such as evacuated tubes. That may be fine for a grid tied house, but when you’re off the grid, your ENTIRE system becomes your hot water solar system.

    It’s usually cheaper and more efficient just to add a few more panels and more battery storage and be done with it, than it is to double-up on a separate system.

  • Heating. We’re assuming your off grid house is a new build.

    If that’s true, and you want to minimise the size (and price) of your off grid solar system, you only have a few options.

    They are: solid fuel (wood burner), gas, and diesel boiler.

    With the exception of a wood burner and gas heating, any other options could have large upfront costs.

    Everyone’s different, so what suits you will depend on your goals, priorities, funding, climate/location and attitudes.

    The majority of customers who want their off grid system to heat their home will choose an air-to-air heat pump. These deliver the ‘biggest bang for buck’ when it comes to kWh in vs energy out.

    All heat pumps need to be correctly sized for the room’s function, size (volume of space) and other factors, but this is especially true with off the grid living.

    While it won’t matter too much in a grid tied home, an undersized heat pump that chugs along all day in an off grid home, will deplete the battery fast. Likewise, an oversized system will simply consume too much energy.

    There’s a balance to get right.

    Large ducted heat pumps that feed multiple rooms are more prevalent in new builds, but they come at a significant power cost (kWh). Sometimes it is more energy-efficient to have single heat pumps strategically placed, than to run one giant system.

    All heating methods have costs and they are not always financial. For example, a wood burner may mean a smaller overall off grid system, but there’s an ongoing cost for fuel, your time, and the environment.

    A heat pump (or hot water heat pump), will mean a larger off grid system upfront, but the ongoing cost is minimal – including the cost to the environment.

    It’s your choice, and we can help you through the options.

  • Large home appliances. In recent years fridges, freezers, washing machines, dishwashers and clothes dryers have become very energy efficient.

    Most standard fridge/freezers with a 4-star energy rating will only use about 900W of energy per day which is half of what it was a decade ago.

    There’s no need to get special fridges, 12 volt fridges, gas powered fridges anymore. The exception to this is if you’re in a camping environment, and there are lots of retailers who can offer help.

    Be careful because although efficiency has improved, variability in efficiency varies wildly between brands.

    Moving your large appliances from your old grid-tied house to your new off-grid house is not necessarily a bad thing to do. It all depends on their age and efficiency (older appliances are usually less efficient).

    Talk to us about your current appliances and we’ll discuss the pros and cons of keeping or replacing them.

  • Computer use.  Standard laptop computers for office or basic internet browsing are quite efficient – at about 60W (.6 kWh).

    Where daily loads can be a problem, is when desktop computers, gaming consoles and gaming laptops are used.

    These can start at 250W per hour and go as high as 500W!

    For example, if you use a desktop computer with two monitors, and it’s turned on from 8am to 6pm (because you work from home), that could add up to 3kWh or more to your daily load!

    That could represent 25% of your power use in a medium sized home. A new laptop with LED screens (not LCD) could be a viable alternative with half the power use.

  • Water pumps and waste water. We discussed these items earlier in this article and if you’re embarking on a new build, your water pump and sewage system should be on the top of the list to get sorted early.

    What you end up installing dramatically affects the size of your off grid system – by as much a $10,000 in some cases.

    A powered system that needs to run 24 hrs (some do), can be a significant drain on your battery bank after dark. Sadly, you may have no choice in the matter  depending on your council and local bylaws.

    While there are some low-powered or non-powered systems available, these may have a high cost to buy and install – so do your research.

    Most of the time it’s a tradeoff, and we can give you the options.

  • Small appliances. All the stuff you’d normally run in a ‘normal’ house won’t be an issue for modern off grid solar systems, using the latest technology.

    These appliances include your internet router, lights (LED bulbs), LED TVs (not LCD or plasma), miscellaneous kitchen appliances, Sky TV, phone and device charging, extractor fans, DIY power tool charging, vacuum cleaner and more.

    The items that draw a lot of power, but usually are only used for minutes at time, include an electric jug, toaster, hair dryer or straightener.

    These high-draw appliances may draw up to 2.2 kWh at any one time, but because they’re usually only for a few minutes, their combined daily load can still be manageable in terms of your system’s overall sizing.

    An exception to this is if you are a tea or coffee addict (and don’t have gas), and you boil the jug or use the coffee machine 10 times a day.

    There are also ‘problem’ items to consider. These include benchtop hot plates, benchtop air fryers, benchtop grill/ovens and electric frying pans.

    The reason these items can ramp up your off grid load evaluation (and the price of your system) is that they can be used for extended periods. At 2,200W, it’s not common for an electric hot plate or electric frying pan to be on for 30 to 60 mins.

    Again, you can have what you want but it adds up fast.

    Part of our review of your off grid load evaluation is not just doing the maths, but giving you alternatives.

Off grid load evaluation tips

I f we have the opportunity to speak, there’s a lot more to share, especially when it comes to specific appliances or equipment.

That aside, here are some tips and insights below that may be useful.

  • Talk to us early. If you are building new, please chat to us before you finalise your plans, if possible. It’s not uncommon for customers’ design and purchasing decisions to be shaped by their off grid plans and mistakes can be expensive.
    • Sunshine hours. Your final system sizing is not solely a function of your daily load. Your local official sunshine hours will also be taken into account using computer modelling. In certain parts of New Zealand, you’d typically need more solar panels than other areas. This includes the Manawatu district, Central Plateau, the West Coast of the South Island and anywhere south of Canterbury. 
      • Backup battery storage. We use your final off grid load evaluation to calculate your system sizing to allow for about one day of backup battery in the event of consecutive dark days in winter. This is the correct amount, and why an approved auto-start backup generator is also required. 
        • Expandability. Advancements in technology now mean your off grid solar solution can be expanded if your power needs increase over time. There are some technical constraints, but we can explain these when we speak. Expandability makes it possible to take a phased approach to your new build. It’s common for customers to comfortably live in temporary accommodation (such as a shed) with a smaller system, then expand it later once their main dwelling is completed. This can be cost-effectively and easily done by adding extra battery modules and additional solar panels.
        • Heated towel rails. These can be a killer and can blow the load evaluation out of the water. They can range from 80W to 160W. And because we tend to run then for hours at at time (sometimes 24 hrs), that could be over 2 kWh per day. Better to have these on a timer for a few hours during daylight hours. If you want heated towel rails to run all day, and still want the lowest price, this is an unrealistic expectation.
        • Heaters. All electric fan heaters and oil type convection heaters (no matter the dimensions) are discouraged due to their high power draw and inefficiency. Please talk to us about alternatives.
        • Satellite internet.  Satellite internet such as Starlink or Gravity, are a god send if you are in a very remote area. The downside, is they will typically use about 100W and will run around the clock. That’s 2.4 kWh in one day which could be a lot considering a small family might only use 7 -9 kW per day.

        If you are not ready to get help with your off grid load evaluation, you’ll find other resources on this site including our popular guide “Off grid Solar for Dummies”. It covers a range of different off grid solar topics designed to answer your questions.

        You can also watch our video which explains how our latest systems work.

        Thank you for reading to the end.