portable backup power

WaterDogWaterDog Registered Users Posts: 3

I have a medical device that I would like to purchase a solar backup system to operate.

Kill-o-Watt readings:
120V A/C
1.35 amp
67 watts
160 VA

The device is basically a medical-grade air compressor with cooling fan. I need to operate the device for up to 1hr per day (4 x 15min sessions spaced throughout the day). My preference is an off-the-shelf system with known reliability.

I've been looking at this system, which is probably overkill:

and also this system:

If anyone has experience with these systems or can recommend a different system to look at, I'd appreciate the input.

I'm willing to build my own as well but the bottom line is that whatever I end up with has to work when the power goes out. Thank you.


  • BB.BB. Super Moderators, Administrators Posts: 31,765 admin
    Re: portable backup power

    Is this for "off grid use" or emergency backup (storm, utility failure)--Or something "in between" such as many countries where there is unreliable power (4 hour power hits in the afternoon are common)?

    Is this a "portable unit" that you will be hiking to a remote cabin or mounted on the roof of a vehicle? Where and what time of year will this (roughly) be used--The amount of sun can vary dramatically by location and season (from 1-2 hours of sun in winter to 5-7 hours in summer). If you have a 1 week stretch of stormy weather (basically almost no sun)--What happens (do you need a backup genset, do you need a battery to last 2 weeks of no sun, etc.)... Will you need to power anything else (radio, lights, etc.)?

    67 Watt load is not that much and can be done with a 100 watt panel and a smallish AGM battery (24 to 50 AH @ 12 volts may do the job nicely).

    With smaller batteries/systems--You may be looking at replacing the battery every 1-3 years anyway (for reliability and that small batteries just do not seem to last as long)--So keeping the costs down (by keeping system size small) may help too.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • WaterDogWaterDog Registered Users Posts: 3
    Re: portable backup power

    This is for emergency backup for power outages. I'd like a portable unit in that I don't want to leave it set up as a permanent fixture on the house and want the ability to take it with me should I need to evacuate in a longer term disaster. I live in southern Arizona. The capability to power other items such as small lights and recharge a cell phone would be nice but are secondary considerations. We don't have power outages very often and they have been of short duration so far. The need to power the medical compressor is really the single problem in a power outage that would force me out of the house in a matter of a few hours.

    The units I mentioned are designed to be plugged into AC power most of the time to keep topped off with the solar coming into play when the AC is not available. I don't see leaving the panels set up at all times but only when needed.
  • BB.BB. Super Moderators, Administrators Posts: 31,765 admin
    Re: portable backup power

    OK--Using PV Watts for Tuscon with a fixed panel tilted to latitude, we get:
    Month    Solar Radiation (kWh/m 2/day)
    1      5.70     
    2      6.11     
    3      7.03     
    4      7.50     
    5      7.29     
    6      7.15     
    7      6.44     
    8      6.85     
    9      7.06     
    10      6.72     
    11      5.99     
    12      5.27     
    Year      6.59

    Hours of noon-time equivalent sun per day... The minimum is 5.27 hours of sun per day. For an off grid AC system that used 67 Watt*Hours per day (because this is a small load, add ~10 watts for inverter losses for a total of 77 WH per day):
    • 77 WattHours per day * 1/0.52 system efficiency * 1/5.27 hours of sun per day = 28 watt solar panel minimum for December

    The battery bank for 1-3 days of storage and 50% maximum discharge (for long life) would give you:
    • 77 Watt*Hours per day * 1/12 volt battery bank * 3 days of no sun * 1/0.50 max discharge = 39 AH 12 volt battery bank

    To double check, we normally recommend around 5% to 13% rate of charge on the battery bank (for example, if you have a very large battery bank, you need more solar panels to properly recharge the battery even if you do not use that much power):
    • 39 AH * 14.4 volts charging * 1/0.77 panel+charge controller derating * 0.05 rate of charge = 36 watt minimum solar panel
    • 39 AH * 14.4 volts charging * 1/0.77 panel+charge controller derating * 0.10 rate of charge = 73 watt nominal solar panel
    • 39 AH * 14.4 volts charging * 1/0.77 panel+charge controller derating * 0.13 rate of charge = 95 watt "maximum cost effective" panel

    So--Assuming this is for emergency use, and you would be in a very bad way if the system failed to meet your needs (or there was a week or so of bad weather)... Such a system would supply you 3 days for normal use and close to 5-6 days if you could not recharge for some reason (battery would be dead and need replacement--but it gives you more time to figure out what to do). You should plan on replacing the battery every 2-3 years (float/storage for batteries is not good for them either).

    A ~75 watt solar panel (or several smaller panels) will give you enough power to quickly recharge the battery and enough power to run a small laptop computer (for an hour or two), LED lighting, recharge a cell phone, etc. during sunny weather. Smaller panels tend to cost more than one large panel, but are easier to store/move. You should set something to stake the panels into the ground--You don't want a gust of wind to blow them over and shatter the glass.

    A small PWM charge controller with a "AGM/Sealed" battery setting (AGMs are damaged with too high of charging voltage). A small AC charger for charging when the system is in storage (a good quality charge controller with float setting or use a lamp timer to charge the battery ~30-60 minutes per day)--again so it does not damage the battery).

    For belt and suspender backup--A small Honda eu1000i (900 watt) generator. Very clean power, does not use much gas (should run over 10 hours per gallon of fuel), and quiet/small package (get a siphon hose to draw fuel from your car, or get a 5 gallon can with fuel stabilizer, recycle once a year with winter gas--less alcohol which is bad for small engines). Use your AC battery charger to for your battery.

    Use polarized connectors for all connections (solar panel, battery, inverter, etc.). You do not want to miss-connect anything and blow up your solar panel or inverter in an emergency.

    You can adjust the numbers to your needs (smaller/larger battery; smaller/larger solar panel(s), etc.)... But try to use the basic equations so you don't end up with (for example) an over-sized battery bank and undersized solar panel--that can cause shorter battery life (under charging / over discharging is very hard on lead acid batteries).

    You should have a small digital volt meter so you can monitor the battery bank voltage (check the battery resting voltage at night--around 12.7 volts is full charge, ~12.3 volts is ~70% SOC, 11.75 volts ~30% SOC (near killing battery). You could go with a Battery Monitor (great for use with AGM/Sealed batteries), but probably overkill in this application.

    You should read the Battery FAQ / other Battery FAQ to get a good idea of how to take care of your battery.

    You could get a digital panel meter that you can check once a week to once a month when the system is in storage--so you can ensure that the battery voltage is around 12.7 volts to 13.6 volts (storage/float charging voltage range) and mount it on the outside of the storage box/container.

    Depending on your needs--You could buy the components and assemble the system yourself or some solar vendors will build such a system for you based on your needs.

    Do a paper design first (or several designs with different options--i.e., smallest system that will work, vs a larger system that will give you some extra power for other needs) and make sure that a system will meet your needs (and ask questions here) before you buy anything. There are a bunch of details to build out a balanced solar PV power system.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • WaterDogWaterDog Registered Users Posts: 3
    Re: portable backup power

    Thanks for the very helpful reply! I've got my spreadsheet humming now and I'll see what I can come up with.
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